polymer-modified plaster


These page entries are meant to be added to and usually start with general outline information, price guidance, suppliers and useful links followed by my worklog where I can put further info and photos as they come. Details of suppliers are listed in the Suppliers section.


‘Polymer-modified plaster’ is one of the terms in use to describe the combination of plaster with an acrylic polymer liquid. When plaster is mixed with this instead of the usual water it still undergoes normal setting but the resulting solid is significantly tougher and less porous .. so much so that it has been compared to resin. The invention is commonly marketed as a ‘friendlier’ alternative to resin, especially when sculptors and home-users are targeted .. non-toxic, non-flammable, no smell, easier to work with etc. Most people will know it by the brand name Jesmonite .. although this is just one of many versions, and any so-called alpha plaster can be mixed with an acrylic polymer liquid with similar results.

The usual recommended mix ratio is between 2-3 parts plaster to 1 part polymer liquid by weight . This makes it a similar proportion to the usual powder to water ratio when mixing hard casting plasters normally .. but the thickness of the polymer liquid means that the plaster can’t be just sprinkled in as one normally would. Instead the liquid needs to be added on top, as one would when making dough, followed by thorough stirring until the mixture is even.

Some acrylic polymer formulations are available which are meant to be mixed with water first, rather than added to the plaster as they are. For example SP201, available from specialplasters.co.uk is first added to water in the ratio 2:1 water to polymer ..

It is claimed that polymer-modified plaster will survive outdoor conditions, not only because of general strength but also because the surface is more sealed against atmosphere and moisture. But it is advised that surfaces should be additionally sealed to survive outdoors.

Advantages of using it

Definitely the medium is more ‘agreeable’ to use compared to plastic resins ..

Either Jesmonite or its equivalents are excellent for building strong shells i.e. as large mould jackets, layered in conjunction with jute scrim in much the same way as laminating with polyester resin and glass matting. In fact glass matting can also be used in place of scrim. Shells can be much thinner than they would have to be using regular plaster.

The polymer binder enables addition of fillers in the mix i.e. even metal fillers  etc, to change the appearance, something which is otherwise only possible with resins. There is also an advantage in that it will accept water-based pigments so i.e. strong acrylics or water-based dyes can be used.

Afterwork can be a lot easier .. i.e. refining or painting of casts, adding textures etc.


Is polymer-modified plaster anything like resin? .. no! It can be immensely useful for large casting work but there’s little comparison with plastic resins for creating a resilient version of a particularly thin or slender form. In this respect Jesmonite remains closer to hard casting plaster in terms of being too brittle for delicate forms. In any case mixed Jesmonite is far too thick to flow into slender mould forms, whereas there are resins for the purpose which are almost water-thin.

Because the polymer liquid is more viscous than water and because much more rigour is needed when mixing, a great deal of air bubbles are created which refuse to disappear! This has little consequence when using the medium as a laminating liquid or brushing into moulds as a hollow cast because the bubbles can be forced out by this action, but not so if the medium is poured to make solid casts. In this case proper degassing is essential .. beyond a lot of people’s means. The only other way to combat this is to lay down a complete detail coat inside the mould first to properly deal with the surface and, once firm, follow this up with a solid pour .. but this depends on the mould made.

The manufacturers of these plaster/polymer systems stress that large amounts can only be mixed thoroughly enough with ‘power assistance’ i.e. with a special mixing ‘blade’ attached to a power drill. When working with small amounts determined hand-mixing will work but it has to be thorough.

High cost of acrylic polymer liquid compared to water  Yes .. judging by the price of the Jesmonite constituents from some suppliers, it is much costlier to work with a polymer system than just using a strong casting plaster. But Jesmonite is the most expensive version, and there are far cheaper alternatives that .. far from doing an inferior job .. may actually do it better!

Working life

I recommend with all expendable materials, especially resins and silicones but also plasters, that they should be clearly dated when first bought but also a note made of manufacturer’s recommended shelf life (see the start of the ‘quick view’ comparisons page in this section for further advice about shelf life). It used to be fairly usual that manufacturers included batch dates on their products but this seems to be far less common now.

The range of roughly 2-3parts plaster to 1part polymer can be chosen according to whether one wants a wet paste for laminating or a pourable liquid. It’s always a good idea to keep to recommended ratios at first, until one’s gained the experience to judge ‘by eye’.

Pot-life of Jesmonite averages about 10-15mins and casts can be safely demoulded in 45-60mins i.e. similar to plaster.

Recommended 6month shelf life for Jesmonite liquid .. longer shelf life for Jesmonite powder or any other ‘alpha’ plaster if properly stored ..

There’s a short, illustrated account of mixing Jesmonite and using with jute scrim to build up a shell, towards the end of Making hollow casts in open or ‘closed’ moulds which can be found under Mouldmaking and casting in the Methods section. The same method features in Making a supported silicone mould for a life-size head and casting in fibreglass in the same section.

mixing Jesmonite

The photo above shows the set-up for working with polymer-modified plaster and jute scrim, the same whether one is making a shell casting inside a mould or building a supporting jacket around one. A good stock of scrim pieces should be cut first, because there may not be time to stop to cut them while layering. Disposable plastic cups make the most convenient mixing vessels, usually around £1 per 100. Also, because most of the containers that liquid materials are supplied in don’t make it easy to dose small amounts I usually decant them into these plastic cups while working. To keep these from falling over I’ve made a rack from foamed Pvc.

jute scrim

Above, a polymer-modified plaster shell is being made around one half of a silicone rubber head mould. First a generous layer of the mix is brushed on and left to firm up, then another layer is brushed on into which the pieces of jute scrim are pressed. Another layer of mix is brushed into the scrim .. and so on. I needed just two layers of scrim built up in this way to make a strong mould casing for this life-size head.

Strength compared to using plaster on its own

All I can say for sure is that polymer-modified plaster with the recommended ratio of polymer to plaster is bound to be more weather/water proof for outdoor work than plaster on its own, but that some people recommend the addition of a special weatherproof coating for optimum survival.

In my own small-scale work I have noticed increased strength against overall stress and increased chip-resistance of the surface when I have used polymer-modified plaster as opposed to regular hard-setting plaster, but the material is still a far cry from resin or fibreglass.

There are also some plasters which are particularly strong .. and which claim to be weatherproof .. on their own. To illustrate this mask-maker Caroline Eklund .. nymla.se .. recorded this video comparing Jesmonite with ‘Cassini’s’ plaster from Maragon UK



Additional info

In their website entry for their polymer liquid Tiranti say that it is for use with any ‘Alpha Hemi Hydrate’ plaster ..

Different types of Jesmonite liquid are available (the powder used is always the same) i.e. AC100 is the standard ..

There is a special thickener available for Jesmonite, also a retarder. Fillers can be added to Jesmonite, but then the ratio of powder to polymer liquid should be 2:1 i.e. starting more liquid.

Jesmonite refers to its powder as a ‘mineral resin’ but it can’t be anything other than a conventional plaster ..? ‘Solvent free .. no VOCs’ ..?

Can Jesmonite ‘take on any colour or surface texture’ ..? Shiny metal .. really?

What it costs and where to get it

Prices are from May 2017 and are adjusted to include VAT

Up until recently Canonbury Arts was a convenient source of Jesmonite in London but now that they have had to close the only option remaining apart from ordering online is 4D modelshop. But Tiranti also stock acrylic polymer sold separately, to be used with any ‘alpha’ plaster .. can be much more economical.

Jesmonite £27.10 for 3.5kg kit (4D modelshop)

Acrylic Polymer liquid £9.62 per 1kg, £35.34 per 5kg (Tiranti 2015) Stated mix is 3parts plaster to 1part polymer by weight.

SP201 acrylic polymer liquid £5.28 per 1litre, £21.12 per 5litre (Specialplasters 2015) SG 1.2, viscosity 500-1500 mPas. This liquid must be diluted at least 1:1 with water before being used .. recommended 2:1 water to polymer. Ratio of diluted polymer to plaster then needs to be experimented with. Note I haven’t used SP201 as much as the other options yet, but so far the results have been excellent .. and at a fraction of the price! Please see ‘Worklog’ below for further details.

In the technical data sheet for SP201 available from Special Plasters on their website it is described as ‘a fine particle size, styrene/acrylic ester copolymer dispersion. As supplied it is ammonia free enabling its use in enclosed environments. The liquefying effect of SP201 in plaster mixes allows a substantial reduction in the amount of water needed to give a pourable slurry.’

Plascrete £8.34 for 1litre polymer plus 1kg casting plaster (Mindsetsonline 2015) This appears to be a name that someone at Mindsets has simply pulled out of their party-hat because you won’t find it anywhere else .. unless you’re into sci-fi gaming that is! It’s a good price, but I can’t vouch for it because I haven’t used it. While I wholeheartedly support the Mindsets ethos, I find it completely strange that there is often so little technical info for their products .. not to mention a single MSDS sheet! Note 2017 can’t find it on the site anymore.

Compatibility of different products

In other words, I’ve already stated that any ‘standalone’ acrylic polymer liquid can be used with any alpha plaster .. but does this also apply to Jesmonite liquid and can Jesmonite powder be combined with another polymer liquid? The answer is ‘yes’ .. with varying results! See my Worklog entries for March 2016.

Further info sources


3/03/2016 Now I can vouch for the SP201 polymer liquid from Specialplasters! I was dubious before because it’s so cheap .. £5.28 for effectively 2 litres or more, compared to the £9.62 per kg from Tiranti .. not to mention how much more Jesmonite costs! But I used it today to make a thin mould jacket and I have to say that it compares very well with both the Tiranti option, and with Jesmonite .. perhaps even better!

Making a mould jacket

Above is the completed mould jacket waiting patiently to be dismantled. The prototype form underneath it all was this relatively flat and simple one below, which I made some while back and covered with a layer of spreadable silicone rubber.

base unit shaped from styrofoam

flat silicone skin mould and jacket

For the mould jacket I started with a covering coat of polymer/plaster mix over the silicone surface, prior to continuing with layers of jute scrim. I diluted the SP201 1:1 with water by volume, the minimum recommended (apparently 1:2 polymer to water is more common but I wanted even more strength). I used Crystacal R ‘alpha’ plaster and initially mixed at a ratio of 3.5-parts plaster to 1-part diluted polymer by weight. They mixed fairly effortlessly, though with all polymer-plasters mixing needs to be vigorous and thorough! After mixing the consistency was perfect for trowelling on, in order to apply a reasonably thick layer easily, but not so thick that it set too quickly. In fact, I’d often had this problem with Jesmonite setting too quickly, but the SP201 mix seemed to give much longer working time .. so much so that I wasn’t sure at first whether it was going to set at all, but it was fine.

Once the initial coat became firm enough .. this took about 40mins .. I could start layering the jute scrim. For this I used a thinner mix .. a standard 3-parts plaster to 1-part polymer by weight. This gave the mix more of a fluid ‘double cream’ consistency i.e. suitably pourable if I’d wanted to do that. I found that this wetted the jute scrim very well, but without being too runny when tackling the sides of the shape. For each layer I brushed some of the mix on first in small sections, laid some pieces of jute scrim on and pressed them so that they ‘took’, then worked more of the mix generously over the top.

detail of mould jacket using polymer-modified plaster and jute scrim

I took a risk in stopping after only two layers done this way, but I wanted to see whether this would in fact be enough .. and it certainly was! The oval is not small .. 43cm long by 30cm wide .. but when I dismantled a couple of hours after completing the second layer it ‘felt’ as strong as it could be even though it’s really only a few mm thick. I did, sensibly, overlap the jute pieces more around the rim than the centre, giving it more strength.


One mistake I did make though was making the silicone skin too thin .. mainly to save money and time. It’s also just a few mm thick .. roughly 3mm .. which means that the sides are sagging away from the rim, as shown below. This may not be an issue if the cast is to be a solid pour .. the weight of the liquid would push the skin back. But I intend to do a hollow ‘slush’ cast .. however I have another method for pulling the silicone back in place by means of ‘fish-hook’ attachments, which I’ve illustrated in Making a supported silicone mould for a life-size head .. under Mouldmaking and casting in the Methods section.

silicone 'skin mould' sitting in plaster jacket

26/03/2016 Just prior to running another Modelling, mouldmaking and casting course, I was doing my usual stock-check and found that I had amounts of ‘old’ Tiranti polymer liquid and quite a bit of leftover Jesmonite AC100 powder, but without the Jesmonite liquid. So I thought this would be a good time to test which of these might work with others and here are the results. The main points of comparison were, ease of mixing; thicknesss of mix at roughly 3:1 ratio; and setting time until ‘fingernail resistant’ .. that is, when I can’t make an impression anymore with the fingernail.

Control Test SP201 polymer with Crystacal R in each case and throughout SP201 diluted first 1:1 with water. First test 3.5parts plaster to 1part liquid. Good mixing, good trowelling consistency, but not for pouring. Very long working time/- still wet but firm at 40mins, not yet fingernail resistant. Properly hard and fingernail resistant after 1 hour. Second test 3parts plaster to 1part liquid. More fluid ‘double cream’, pourable and good wetting of jute scrim. Properly hard after 1 hour.

5yr old Tiranti polymer with Crystacal R mixed 30g plaster, 11g polymer. More forceful mixing needed than with SP201 liquid. Different consistency and colour, more ‘resinous’, glutinous ..but good, smooth mix suitable either for spreading or assisted pouring. After exactly 20mins good working time with no obvious change in consistency it firmed up quickly! After 30mins touch ‘dry’, surface fingernail resistant.

Jesmonite AC100 polymer with Crystacal R mixed 30g plaster, 10g polymer. Good mixing, eventually no granules. Thick ‘custard’, very smooth, no bubbles, not suitable for intricate pouring but good for spreading. 15-20mins working time. At 30mins, fingernail hard, strong.

SP201 polymer with Jesmonite AC100 powder mixed 30g plaster, 10g polymer. Mixed well, granules easy to eliminate, very thin mix initially, like ‘single cream’, many air bubbles. Just before 5mins sudden and rapid thickening, from ‘single’ to ‘double’ then unpourable paste within a minute. Just managed to spread on clingfilm for the hardness test, noticeably sticky consistency .. air bubbles pulled into streaks. At 15mins touch ‘dry’ firm but still a little soft, not fingernail resistant. At 25mins, already fingernail resistant.

Tiranti polymer with Jesmonite AC100 powder mixed 30g plaster, 11g polymer. Thick mix, not for pouring, difficult to eliminate granules. Very short working time, solid in 5mins, unable to spread out! After 30mins, definitely touch ‘dry’, hard, fingernail resistant.

So to conclude, firstly the Tiranti polymer liquid didn’t seem to have changed with age, working as it was supposed to. Secondly, it would seem that both Jesmonite components can be used with other brands.However, whereas Jesmonite liquid appears to be a regular acrylic polymer, Jesmonite powder is either a particularly fast-setting plaster or something has been added. I’m also thinking that this part is responsible for the profusion of air bubbles when mixing regular Jesmonite .. but this is just a hunch.






141 thoughts on “polymer-modified plaster

  1. Hey David best blog I came across and believe me I have been searching! So I want make little trinket trays and plant pots but ofcourse jesmonite is super expensive. Would the same outcome be achieved with Herculite No 2 and SP201 and be sealed off? That is all I have access to.

  2. Hello David! I’ve been scouring the internet for any info regarding how to basically make your own jesmonite and I found your finding the most helpful! I was going to use just basic DAP brand plaster of Paris (I’m not sure if your familiar) but I contacted the company and they didn’t know if it was considered an “alpha” plaster or not. Is there a different type that is better to add the polymer additives? I also read mixing regular white glue with the water can also help strengthen plaster- I may try this later just to see too haha. Thank you!

    • Use wood glue, or reg glue and acrylic paint. Its identical to jesmonite. You have to get the measurements correct though. I made several mistakes. Just play with it.. For me, I use
      (2 cups of powder) measurements-1 1/2 cup gypsum/plaster, 1/2 cup Lime.
      (Liquid 2 cups ) 1/4 cup glue, 1/4 cup acrylic paint, & 11/2 cup water. If its looking runny add more powder till its like batter..I sprinkle into the water or else your work time is very short like 10mins including mixing. Im jn the US and I had no choice but to make it myself.

      • Hi Stella,

        Trying your recipe for this today as trying to get jesmonite or even the polymer liquids here is a nightmare.
        I was wondering what you have found your demould time to be?

      • Just adding my bit here. From my experiences so far, for anything ‘plaster based’ I’ve found that a demould time of ‘1 hour plus’ is usually pretty safe, and prudent not to try sooner. Also, if the concoction looks like it’s taking longer than an hour to solidify it’s likely it’s failed.

  3. Hi David, is Acrylic Polymer and Acrylic Resin is the same ? i wonder if it is, because i cant find acrylic polymer liquid in my country

  4. I am obsessed with your site! I read it once a week at least. Have you tried creating a dupe for jesmonite pigments? I’ve tried acrylic pain but the pigment concentration is not great. Jesmonite pigments are very pigmented and I wanna recreate that. I’ve been experimenting with inks

    • Thanks Isaac! It’s always good to hear of someone really ‘using’ the site! In answer .. I’ve really only used powder pigments, and they did what I wanted from the start. As I’ve written to the previous comment ‘properly mixed into a little of the liquid first. Colour smoothness varies though .. some pigments may speckle.’ Best!

  5. Hi David,
    Did you have any other discoveries since writing this? I’m using Jesmonite at the moment, but having more options, especially cheaper would be great.
    Also, what would you recommend as alternate pigments?

    • No I haven’t worked with it for a while. Best colourant I found was simple powder pigment, properly mixed into a little of the liquid first. Colour smoothness varies though .. some pigments may speckle.

  6. Great, helpful website!
    I’m currently experimenting with making jesmonite poured table tops, with a coloured marbled effect using the Jesmonite pigments. See here: https://ibb.co/PwfpPYy
    Found it hard to get rid of lumps/bubbles in the AC100 jesmonite (even with blade mixer) and v costly, so tried Crystacal R + SP201. Better bubble situation, but it didn’t take the pigments well at all. It all just sits on the top of the cast in a glossy puddle as it sets.
    So now I’m back to Jesmonite…Wondering if anyone has diluted the AC100 base liquid down with water to any success, like you would with SP201? Feel like that might save costs and help my clump/bubble issues.

    Great to hear any thoughts.

    • Hi Sal, have you tried Acrylic One?. You will find it much easier to use and a better ratio at 2 part Powder to 1 Part Resin, and a far better end result. Visit their site and you won’t be sorry, quick delivery, always in stock, and good service, http://Www.Activecomposite’s.com.au

      • hi Celeste- could the powder be plaster powder instead of the mineral powder

    • There is also the Jesmonite AC300 which is slightly cheaper than the AC100. That has less resin in it so is basically watered down AC100.

  7. Very interesting article. I am currently trying to develop a cheap alternative to jesmonite using a high quality hemihydrate plaster, a combination of superplasticizers and a polymer. Just wondering if you have carried out any expreriments with alternative additions to your mix such as SBR.

    • Yes, I did try Cementone SBR. Nothing! .. just made unsetting plaster sludge, if I remember right. Everything else I’ve tried is on this article, maybe some in the notes at the end.

      • Right i will give the sbr a miss then! I am also looking to try a combination of potassium alum and borax in the mix as i have read this can improve the hardness of the finished plaster. I will let you know if i have any decent results.

  8. I had some AC100 liquid left in the cupboard – at least 6-7 years old, I’m testing it with ‘Prompt’ natural cement (rapid setting) so far so good, but more tests to do. I would like to reduce brittleness when using cement in thin section (approx 10mm). SP201 from special plasters presumably is ‘equivalent’ to AC100 liquid, does anyone know what these polymerisors are made from, surely not just PVA? Also, can these ‘tests’ be dried more quickly without damaging the structure ie hairdryer or heat gun? Thanks David for all the info.

    • No it’s not just Pva, but I think it’s definitely close to. It’s not like it’s a guarded secret, because a certain amount is revealed from the MSDS sheets. I think it’s more that very few people are bothered, or capable, when it gets to the chemistry. SP201 is much thinner than Jesmonite liquid and the smell is different. I don’t see how hair-drying would do any damage .. heat gun perhaps not so good.

  9. Hi David. Great page, thank you.
    Part of Jesmonite’s range is “Acrylic Modified Gypsum Composites” like the AC100-300 which I assume you’re mostly referring to here – plaster base.
    Do you have any idea what the “Acrylic Modified Cementitous Composites (PGRC)” are made of i.e. the AC630, 730 etc? i.e. cement based mixture + polymer.
    Thanks very much.

  10. Hola, un gusto saludarte David, gracias por la informacion, estoy buscando como hacer la formula del Jesmonite c100 ya que en mi pais no existe, quiero hacer laminas finas para luego partir y crear terrazo, dices que es solo una mezcla de yeso con polimero? para pigmetar con que tipo pigemento se podria dar color a esta mezcla?

    • Hi Karina, Yes it’s a mixture of good quality casting plaster and acrylic polymer liquid. I’ve had the best results creating colours using dry powder pigment, mixing the pigment directly with the dry plaster powder thoroughly, before adding and mixing the polymer liquid.

    • Hey any idea if Sp 201 is available in asian countries
      As I am looking for the plaster too and liquid binder but don’t knw their chemical names.
      Let me know if its possible to help out
      In short looking at cheaper options than Jesmonite and if possibly locally avaliable as i wanted to start a small time business in these trying times


      • I seriously doubt it, because specialplasters.co.uk who sell it are not a big company. It might be worth talking to them though. best, David

  11. Hi David, thank you so much for this detail study, I am so grateful!

    I started experimenting with acrylic modifies plaster and I have some questions maybe you can answer.
    I bought acrylic resin, in the description it says it can be use for plasters (the resin is Osacryl 1M and Osacryl 23NM). The consistency of the resin is like yogurt so I thought I can mix it with water to thin the liquid. In the description there is not info for the proportion resin:water and i mixed it just to get thinner consistency (around 2:1 resin:water). For the main mix I used ordinary Knauf gypsum/plaster. Than I made the mix plaster : thinned resin 2.5:1 and let it dry. But the drying time was very long. For a thin layer of this mixture (3mm) it took almost 24 hours to get nail-resistant and still it is not so hard, it is elastic and it does not break, it bends like a plastic. As I watched in some tutorials this thin layer should dry in about 40min and also it breaks easy and looks like chips (that is what I wanted to get, small pieces like chips to input in the main mixture and than in a mould and get terrazzo look). Can you advise me with some tips what do you think, why the curing took so long and the result is elastic and not breakable, or maybe recommend me another acrylic resigns with good price that I can find it in Europe and try?

    I am struggling so much for this is the begging of my experiments with this materials. I am planing to start small business to be able to menage this hard times so i will be very grateful if you can give me some advises and tips.

    Thank you in advance and have a good day!


    • Hello Ana, I looked on the net and there’s nothing for the ‘Osacryl’ you mention or even ‘Osakryl’ .. nothing understandable. So I can’t advise you on that .. except that it sounds like you’re trying to mix it with an ordinary plaster that’s too weak (the Knauf) .. it should be a very fine, hard casting plaster. But maybe better help is that in Europe i.e. Germany you could see whether you can get ‘Acrylic One’ from kaupo.de or ‘Acrystal’. I got this info from comments made here. best, David

      • Thanks!

        I find some link with specification about Osakryl 1M and Osakryl 23NM, the polymers that I mixed with the ordinary plaster.

        Click to access Osakryl_OSA_1M_en_ts.pdf

        Click to access osakryl-osa-23nm.pdf

        I tried mixing 1:3 – polymer : water and also 1:5 – polymer : water.

        The experiment was making coaster with diameter 10cm and 5mm tickness and I got this results:

        1. The 1:3 (polymer : water) mix added to the plaster (2.5 plaster : 1polymer ) = the result visually looked like Jesmonite products, not so easy to break but it took longer time for curing (12 hours) .
        2. The 1:5 (polymer : water) mix added to the plaster (2.5 plaster : 1 polymer) = the result was shorter time for curing 4, 5 hours but more breakable product.

        Thank you for the advice, I will try to find some alfa hard casting plasters here in Macedonia or order from EU and will be back with the results 🙂

        Have a nice day!


    • Ok. The main parameters are pH, the size of the polymer molecule and the minimum temperature for the polymerization of acrylic dispersion. The pH should be greater than 8, the size of the polymer molecule should be minimal or medium. The polymerization temperature should be closer to 0-5 degrees Celsius. If the pH is less than 7, and the polymerization temperature of acrylic is more than 15 degrees Celsius, gypsum may not freeze or solidify very slowly. The amount of polymer in dry form should not exceed 10%, and preferably 5-6%. by weight of gypsum.

    • I made a small project of facade architectural decor in Slovakia. I used Findisp 2001 acrylic dispersion. The supplier was Brentag. For façade applications, I used 1 polymer x 3 water. For the interior 1×5. For reinforcement,can be used glass mat with density 220 grams / m2 or less. The mixture will saturate 300 grams / m2 very poorly. For more details write ptmtmos @ gmail. My experience of using for the facade is 15 years, I did not use additional protection. Very high quality Scagliola is obtained from this material. To slow down the setting, you can use citric acid or borax solution (sodium tetraborate).

  12. Hi David,
    Have you come across Pebeo Resin Plaster?
    I’ve also been exprimenting with plasters (modified and unmodified), for a casting project I’m working on.
    I’ve tried your suggestions of Jesmonite AC100 and some hard plasters combined with both SP201 and an acrylic polymer sold by Tiranti.
    But my favourite at the moment is Pebeo Resin Plaster, it seems to give very hard results (as good as anything else, although I’ve not tested this scientifically). It is a powder (only) that you mix with water (i.e. no liquid additive), and this seems to help avoid the problems I’m getting using acrylic polymers with plaster.
    The problems I’ve had are: (a) getting rid of lumps, (b) air bubbles possibly caused by the stirring required to get rid of lumps, and (c) plaster setting while still stirring to get rid of lumps. The acrylic polymer thickens the water quite a bit, and it appears that this might be causing the problems I’m seeing.
    In one case (Crystacal-R with SP201), I had to stir so much that the plaster started to set right very quickly right in the middle of pouring. In another case (Tiranti Modell Plaster with Tiranti Acrylic Polymer) there were so many bubbles throughout the finished casting it looks like honeycomb; I don’t recall there being that many bubbles when mixing, so I suspect they were partly created by some kind of chemical reaction. I’m also getting these problems to a lesser degree with Jesmonite as well.
    So I was wondering if you had come across Pebeo Resin Plaster, or had any information about it. The only downside that I can see so far, is that it costs around £8-10 per 1kg, making it slightly more expensive than Jesmonite AC100.
    I’ve not yet tried a plaster retarder to extend available stirring time, but that experiment is next on my list, once I’ve got hold of some. I’m also planning other experiments, e.g. using plaster modified with PVA; and testing the various formulas outdoors over a 1-2 year period for longevity. I would like my pieces to last 100 years or more.
    I’m happy to give you more details of the experiments I’ve done, including others I’ve not mentioned here.
    Regards, Mark.

  13. Hi David,
    Thank you for your blog and sharing your knowledge.
    I am looking for an equivalent of Jesmonite to obtain the same Terrazzo/(Marmo) casting effect. I have done some researches online and I came across to your blog 😊 really useful!
    I have already tried the Jesmonite’s kit, but as you know it is really expensive and I would like to casting more terrazzo to experiment few combination of colour and shapes.
    I read that you recommend to use Tiranti’s Plaster Polymer liquid or Specialplaster’s own SP201 acrylic polymer together with a regular ‘alpha’ plaster such as Crystacal R or Basic Alpha. Are they both suitable for Terrazzo casting? I got the Jesmonite pigmento colours to mix.
    What About Snowcrete instead?

    • Sorry Elisabetta, I have no idea what’s needed for terrazzo work. But I guess if you mean something to bind the pieces, the Tiranti polymer liquid with Basic Alpha could well be ‘stickier’ perhaps also stronger than the SP201. Should take the same colours you bought with no problem. Best not to use Snowcrete because cements can be more problematic.

  14. Hi David!

    I was wondering if I could get some advice, but first I wanted to say thank you for everything so far! It’s impossible to find information like this so I feel like I’ve stumbled upon a goldmine!

    My main interest is in Acrylic Polymers as I wish to create an acrylic ‘gap filling paste’ similar to molding paste that will dry relatively flexible with a sandable surface. If I can make it lightweight that’s a bonus but it’s not crucial at this point.

    I think I’ve settled on either Talc or Marble Dust to use as the main thickening filler material that should allow it to be easily sandable with a smooth finish, However if you can think of another filler or course I am open to suggestions! The paste would be applied to either EVA or Polypropylene foam. I have considered a more foam-based powder filler but I haven’t found many options online, and I’m concerned that a foam powder may not allow a smooth sandable finish, which I am currently able to achieve with the foam that the paste is to be applied to.

    My main issue is that I’m unsure on which acrylic polymer would be best for this scenario as I have zero experience using them. I was going to order a few options to experiment but I’d love to hear your opinions on the different types of polymers before I empty my wallet!

    Essentially I’m looking for something with some flexible properties. The dried paste product doesn’t need to be fully flexible, but should be able to withstand minor flexing without any serious cracking or creasing. I suspect too much much elasticity will likely cause it to tear when heavily sanded.

    Besides that, am I missing anything? In theory the acrylic polymer (diluted if needed) mixed with either talc or marble dust should form an air-drying sandable (hopefully flexible) paste, correct?

    If there’s anything you would add to this or any advice in general I’d appreciate it hugely!

    Thanks again, I’ll continue to dig through your teachings and see if I missed anything!

    • Hi Reece!

      Sad fact is, I know the term ‘acrylic polymer’ and I’m aware of using things with that label at times but have zero proper understanding of what these actually are .. as opposed to, for example, things like PVA glue or similar! It’s all intriguing and could be valuable to investigate, but for me it’s one of those many enticing things that life’s too short for. One thought though .. Jesmonite sells different ‘acrylic polymer’ liquid for its different concoctions .. one of which they claim is ‘flexible’ ..!..! Maybe that’s a place to look.

      • Thanks for the reply David.

        I did a bit more research and while I have struggled to shed much more light on the names and properties of different acrylics polymers, I have learnt about some different filler options to the regular caulk/marble.

        Apparently if a polyurethane powder is used as the filler you can get some incredibly promising results with good flexibility, smoothing and adhesion.

        Furthermore ‘smoothing agents’ can be added to the binder which helps the mix ‘self-level’ and rid itself of any brush strokes or imperfections. It’s good information but still only half of the puzzle.

        PVA can definitely act as a binder with say chalk or marble to form a hard setting paste but it seems ‘PVA’ is a very vague term. I know very little about it and have four different types that all seem to have different properties.

        Anyway thanks for the help!

  15. Hi David.

    Great site and very informative. I’ve tried contacting you through a page before but never received a reply so hoping this might get a response.

    I’ve been using crystacal R and love it but I decpupage over parts if cast with thin paper and tissue designs for some areas. The problem is the surface of the plaster can scratch which in turn ruins the decoupage effect. I have the SP201 but never havr really needed to use it however one thing I havnt done is use it as a sealant instead of using the Matt varnish I’ve been currently using. This seems to yellow areas of the cast but at the same time does give a slight protective layer. I’m away from home for 2 weeks so hoping your reply might give me an answer to my question as I can’t test until I’m back home.

    • Hi Dave,

      I almost always reply even if it’s to say I don’t know, so here I ‘don’t know’ what happened there. You could strengthen the CrystacalR with a less diluted SP201 to make it tougher but this will make it ever so slightly darker (though not more so than any applied varnish probably). An alternative is an even harder plaster such as (I think) ‘Herculite’ (see that and others on the plasters list at Specialplasters.co.uk) or even using white cement instead (Seca71 .. I think). Otherwise I don’t know how much more scratch resistance can be got with plaster.

  16. Hi, has anyone tried any other polymer liquids with metal powders to get the ‘ flex metal coat’ that jesmonite keep tempting me with?!
    I might try some of the above but wanted to ask/check in case……



  17. Hi David, thanks for this great article. I’m living in Thailand and I cannot find any “acrylic polymer liquid” that are advertised as for using with plaster. Shipping them from overseas is insanely expensive.
    Anyway,I found it in Thai’s construction material company but it is use for sealing cracking. Are they the same?
    Any chances that this can be found in other industry’s products?

    • No I’m afraid I don’t think they are the same, because I tried with a similar product here in the UK .. called Cementone SBR .. and that was no good. So I’m afraid I don’t have any other suggestions.

  18. Hi David, im wondering if I can pick your brain a bit on enhanced plasters. Im living in Singapore, so proprietary brands like jesmonite or aqua resin aren’t available here. Im trying my best to decode the formula to something I can use for casting (sculptures like at http://www.jessethompsonart.com). so far I am experimenting with using urea+water or forton vf812 for a acrylic polymer liquid, aluminium sulphite for a hardener and fgr95, but I am missing the powdered resin component. would you be up for a chat that could maybe clear up some of my questions? thanks!

    • Hi Jesse,

      As far as I know the ‘mineral’ powder component of Jesmonite (or any other plaster-modifying system I’m guessing) is nothing more than plaster .. but a fine, hard ‘alpha’ kind. Or am I missing something?

  19. Hi David – thanks for this really useful and detailed post! It really is very helpful for a task i’m about to undertake.

    I have successfully made a series of silicone molds based on small cylinder approx 40mm diameter with a recess to hold a tall candle stick.

    I know need to cast about 150 of these to make candle stick holders for a project i’m working on.

    I have some experience in casting with normal hobby shop style plaster of paris and some jesmonite experience from work, nut i’m not overly confident in knowing which products are best for this task!

    I’m trying to get as white a finish as possible but more importantly, i’m trying complete the task as economically as possible!!

    My instinct tells me that 150 duplicates could be quite expensive in jesmonite so if you had another product reference you could advise me to use i’d be so gratelful.

    Thanks in advance for any product information you can give!

    • I’d strongly recommend using a standard fine casting plaster, such as CrystacalR or Prestia Expression (look at the range on specialplasters.co.uk). Jesmonite will not be completely ‘white’.

      • Thanks david, I’ll have a look at these now. Would they be ok with a water mix or should I use the SP301 too?

      • No, for what you’re doing a fine casting plaster just mixed as normal with water would give a very smooth, hard surface and an even white. Using a ‘modifier’ can result a slightly mottled or uneven ‘off white’ in places.

  20. Hi
    am experimenting with casting small figurines since retiring from animation, and have been using this terrific site for instructions! Now proposing to seal and paint, I wonder what is best to use? Have thought of SP201 diluted and painted on, or perhaps diluted PVA glue? Have cast using CrystacalR with good results, and will paint in acrylics.

    • There’s a problem with the painting of plaster or plaster-based materials using water-based media .. not absolutely always, but often! These need to dry properly before painting, meaning that they become highly absorbent. The water-based medium goes on, the water from it is absorbed far too quickly, and this leaves a weak layer of paint and even the risk of it detaching .. peeling off .. easily. So what you ideally need to do is prime the dry casting first with a non water-based medium i.e. auto primer (Simoniz, RustOleum or Halfords own) or thinned shellac, to seal the surface. You may be right, and thinned SP201 or PVA may work as sealant, but I reckon it’s better to do this with the casts in a ‘damp’ state or re-dampen them first if you try.

      • Hi again, and thank you for your reply. Apologies for not thanking you sooner.
        Am still experimenting, but wondered, can crystacal R mixed with SP201, and colouring pigments, be glued together once dry, and if so, with what? Have used woodworkers white wood glue for just plain Crystacal R with great success, but have been advised elsewhere that it wont’t work if polymer is added to the plaster

      • Sorry Richard, but I just don’t know about this .. never tried this, but if at all it could work better with Gorilla white glue.

  21. Hi David,

    Thanks so much for this website. So confusing to follow other sites due to the number of product names.

    I’ve made some coasters from Jesmonite AC100 and are really happy with the results. I have used some Renaissance Micro Crystalline Wax but want something to make them a little more robust.

    I phoned Jesmonite to ask which one of their sealants would be suitable. They said none of them and suggested I look elsewhere. However, they wouldn’t suggest where to start so I have no idea! Have you used something that could help me? I understand that there is no ‘perfect’ sealant and there will be compromises.

    Above someone has said they’ve used a stone floor sealant and a sealer for garden gnomes but they haven’t been specific.

    Many thanks in advance!

  22. Hi David, love your informed website! Ive been experimenting with jesmonite ac100 and also ac730 with some great results in casting vases from a custom 3 part silicone mold ive created. However, its bloody expensive and then i came across your posts concerning SP201 and im thinking this could be my more affordable casting solution! I understand you can mix the SP201 with water 1:2 , do you know if i can reduce the amount of water and replace part of the water volume with water based emulsion? Im experimenting with water based paints from farrow & ball, fired earth etc into my casts but i dont want it effecting the strength and end result.

    • It’s likely that the emulsion may interfere with the setting of the plaster/polymer i.e. it will probably set up far too quickly .. to many polymer molecules writhing around! I would give it a try! If it’s just about colouring the mixture though, it would be safer to use a powder pigment premixed into the polymer liquid.

      • Thank you David for your reply. I did try farrow and ball emulsion mixed with ac100 polymer and base and you are correct, it didnt set!!! it must have been the other chemicals in the make up of the paint itself. Ive now ordered some sp201 on the basis of your reviews and im going to do my next casting at 1:2 sp201 to water and 3 parts ac730 powder to see how that works out. im hoping this can be a much cheaper way of still working with jesmonite but using the sp201 polymer liquid instead of the jesmonite ac100 liquid which is sooo expensive!!! ill let you know how that goes!

  23. Hi David,
    What a wonderful resource! Wish I had found this many years ago.
    I’m a shoe designer and sometimes make heels and soles from PU resin and rubber composites. I have a question about using jesmonite. I would love to try and make a heel from it but wonder if it’s possible to reduce it’s hardness by blending it with something… Ideally to get it to shore D30-D50. Is this possible?

    • Have a look at the Jesmonite site. I remember there was a ‘flexible’ one .. really can’t understand how that can be, since we’re dealing with what is essentially PLASTER ..!.. but worth a look. I doubt that it would be anything like wear-resistant enough for heels though!

  24. Dear David,

    Thanks for your superb website. Do you know if SP201 will produce a polymer plaster product as water resistant as Jesmonite – without a sealer.

    • I use both SP201/water/alpha K and jesmonite and filler for garden gnomes- Jesmonite has the advantage of being faster curing (and therefore demoulding) and waterproof, though I made a batch of gnomes last year with the SP201/water (50:50) mixed with Alpha K (Very hard alpha casting plaster) and so fan nobody has come back to me saying their gnomes have disintegrated (I did seal them with a suitable outdoor varnish however). The issue with using Alpha K and a water/SP201 mix is that it cures more slowly than Jesmonite, and takes a lot longer to dry (Jesmonite becomes nail hard after about half an hour) – it also requires quite a lot of mixing as the ratio is 4 parts alpha K to one part liquid by weight, resulting in a mixture that retains air bubbles (Not a problem with the latex mould I use as I can sit there squeezing the mould with the mixture in it until the bubbles have all gone.) The finish is extremely tough, as tough as, or maybe tougher than Jesmonite, though i suspect much of this is due to the Alpha K plaster which is very hard and almost non porous even when used as a plaster/water mix


  26. Dear David,

    Thank you for your incredibly detailed site and the knowledge you share.
    Can I ask advice on degassing polymer mix plaster? Can this be done in a similar manner to resins in a chamber? I have found the plaster polymer mix bubbles continuously and does not sink. Any help much appreciated.

    Kind regards,

    • Hello Adam,

      I’m still avoiding shelling out a few hundred for any kind of ‘chamber’ so I try to solve things differently. It is certainly a right pain! I’ve solved it by using a polymer liquid called SP201 which I get in the UK from specialplasters.co.uk It is brilliant stuff .. and hardly produces any persistent foaming or bubbles! I’ve worked more with it recently and can attest that it gives the same strength as other polymer systems such as Jesmonite but without the resinous discolouration that also often occurs as well as the froth! Both Jesmonite and the other polymer I’ve used from Tiranti produce a stubborn froth. You’ve probably tried skimming off the top before pouring .. but have you thought of decanting the mixture into a cut-off washing-up liquid bottle, waiting a bit and then pouring by opening up the bottom? I thought of trying this sometime. This may help a lot in filtering out the bubbles?

      • Firstly, thank you David for this detailed test, I’d actually reached the same conclusion regarding SP201 after trying a few others, I wish I’d seen your blog first!.
        Regarding bubbles…and their elimination;
        I have tried a vacuum chamber but don’t find it really useful with this material as it seems to speed up the thickening of the mix. I’ve now constructed a vibrating table simply by building a sturdy, level frame with a thick heavy top (kitchen counter off-cut) and mounted an old jigsaw (blade removed) to the underside. If you are pouring into a mould with an open end, this is perfect as it gives a gentle vibration which allows the material to flow into all the details whilst the bubbles rise to the open surface where they can be scraped away. If a jigsaw with adjustable speeds is used, the vibration can be regulated to its most effective level.

  27. Dear David, I would like to echo Abby’s comment on your in depth research! It’s great to read although confusing to a novice so I thought I would write a post to see if you have any suggestions (should you have some free time!)

    I’ve been experimenting with jesmonite ac730 to create tables and I love the material however it’s just not cost effective for large objects. Do you have any ideas on what I could use as an alternative material? The strength of jesmonite is very impressive and I wondered if I would get the same results using alpha plasters or any other plasters you can think of? Or even any lightweight concrete alternatives? Any help or thoughts would be fantastic. All the best. Josh

    • Yes Josh, that’s basically what I’m saying in this article .. that you can get similar results with less expense using an alpha plaster such as Crystacal R and a polymer liquid such as SP201 from specialplasters.co.uk

  28. Dear David,
    Thank you very much for sharing all of this comprehensive information with us, we really appreciate the work you’ve put into your research!
    I have a few questions regarding Jesmonite (and replacements for it):
    -is there a reason why Jesmonite (we have been using AC100 liquids with the plaster base) can produce such strong, dark colors? Is this the acrylic component of the polymere liquid? We have tried their pigments in regular beta plasters, to only pastel results. We are looking into a cheaper replacement for Jesmonite, but a big reason for using the Jesmonite is the strong colors it can produce. Is it the alpha plaster?
    -can Jesmonite also be used with an aggregate filler (sand, gravel) the way cement can? Both to strengthen it and to save on material costs.
    -can Jesmonite attach to other plasters or cements? We have tried pouring 5mm slabs of Jesmonite and want to reinforce it with a 1cm+ layer of a cheaper material on the backside, but when we have tried with beta plasters they don’t stick at all. Why could this be? Is it because we need to use an alpha plaster as reinforcement? Or will a polymere plaster only adhere to another polymere? Do we need to mark up the bottom, the way one would do when attaching clay to clay?

    Any advice is greatly appreciated! Thank you very much!

    • Hello Abby,

      I think the stronger colours are perhaps, because the polymer binder makes the material (especially surface) denser, the colours aren’t as optically broken up as with straight plaster, which one would call more ‘crystalline’. That’s my guess. There’s no reason I can think of why any type of inert filler material shouldn’t work in Jesmonite (or similar) in the right proportions. I’m afraid I can’t advise very well on the last question since it’s something I haven’t yet had cause to try. I would have thought that (as with new plaster on older plaster) as long as the older surface is properly wetted all through so that it’s not going to suck water from the new plaster too soon for it, it should work. Alternatively priming/sealing the older surface with the polymer solution first might work well?

      • Hi Josh,

        Not yet, the beta plasters we tried just break right off the flat Jesmonite surface with modest amounts of pressure unfortunately. We will be running some more tests in the next month or two; perhaps scoring the Jesmonite surface first and then pouring the concrete or another alpha plaster, so there is more texture for it to stick to? Or like David suggested; priming the Jesmonite with a polymer solution to create a bond? If there is some success, I will let you know, please feel free to do the same if you find something that works!

        We will also do some tests with lower cost materials in the next few months; I am trying to source a lower cost polymer liquid in Germany. The alpha plaster should not be a problem, but I am still trying to track down a polymer liquid here. Has anyone had experience with Acrystal Prima from Lange + Ritter or a polymer component forced from Germany by chance?

        Best of luck Josh! And thank you again David, we really appreciate your support and advice!

      • I’ve been thinking (I didn’t have much time before). Re attaching a cheaper form of plaster or concrete to a Jesmonite layer .. yes I think you’re right that some kind of mechanical ‘gripping’ help is needed. My idea of ‘priming’ the top surface of the Jesmonite again with straight polymer liquid may be worth trying (who knows?) but may not be enough. But rather than have to go to the trouble of ‘scoring’ the setting surface to provide a key (after all, there’s probably only a very short time window when the material is just right to do this) what about quickly laying down a coarse fabric or mesh such as ‘jute scrim’ or something similar, just at the right moment ‘half in half out’ to act as an anchoring layer between the two. I’ve found that a very loose-weave jute scrim doesn’t draw too much water from plaster to affect curing.

        At the moment I’m finding the SP201 polymer liquid I recommend brilliant! Most of the qualities of Jesmonite, without much of the hassle with bubbles .. and a fraction of the price. I think it’s worth your getting in touch with Specialplasters.co.uk .. small firm, very helpful people .. they might help with getting it in Germany!

    • Hi Abby,
      I’ve also had the problem of trying to source a cheaper alternative to Jesmonite in Germany. There is a great product (very strong) called “Acrylic One” available from KauPo.de, it also has retarders, thickeners etc etc but its not a lot cheaper than Jesmonite. I’ve actually started to get Crystacal R plus SP201 polymer shipped to Germany by SpecialPlaster.co.uk as it still works out considerably cheaper than any similar versions available in Germany, even with the shipping costs and the VAT!

  29. Hi David,

    Thanks again for this blog.
    I’m wondering about air bubbles in Jesmonite AC100. I’m casting small amounts into small – half sphere silicone mould. Super easy shape in a ready made mould.
    I’m not so sore how to get rid of bubbles. I sprayed with 50-50 water and window spray which made it a lot better but not good enough. Still many bubbles all through the surface at the top and at the bottom.
    Do you have any suggestions?
    I read the whole post and the comments.. I guess I’m just wondering if you have any more tips..?
    I’m mixing 2.5 to 1 by hand (with a stick). And I shake the container before pouring and also when it’s in the mould. I do see the bubbles come up but I guess there are many more in there..
    Any advice will be appreciated!

    • Hello Yaarit,

      Funnily enough I’ve been thinking about this because I’ve got one of my ‘Mouldmaking and casting’ courses coming up again and I’d like to solve it! I have little to add yet though! I’ve been thinking though that since a thin Jesmonite mixture (or similar using other ‘stand-alone’ ingredients ) has a longer pot-life than plaster generally, one could take advantage by putting it aside longer before pouring it .. or leaving it on a vibrating surface for a while (however that is achieved! ) .. or in addition if the mix is heated gently for a while i.e. in a tub of hot water? These may help further, I have to see. Another possibility is either if there’s a stronger form of surfactant than the window spray? .. or, if the inside of the mould is lightly greased and a dusting of AC100 blown onto it .. that might prevent the bubbles from forming? Of course, a surer way is most probably investing in a de-gassing chamber, but you sound as reluctant as I am to fork out that kind of money?

  30. Thanks David for your really helpful and inspiring blog.

    Is there a blog post somewhere – or have you ever tried – combining Jesmonite AC100 (or a similar polymer plaster) with fillers such as crushed stone, crushed glass or powdered metal? I’m aiming to experiment with all of these (not only surface effects but also how fillers affect the ratio of powder to polymer), and but wondered if you’ve had any particular successes or otherwise?

    Thank you

  31. Hi David,

    really helpful blog! Quick question: I have been working with Jesmonite for an art project and would like to try it as a 2D medium on canvas – mixed with metal powder. However I’m concerned that it would crack if the surface is not rigid.

    Would Acrylic Polymer SP201 be an alternative? – could it be used just diluted with water as a cheaper alternative to clear acrylic gesso. I really like the quality of the jesmonite with the powder but fear it would just crack at some point.

    Basically need a matt (ish) medium for metal powder. Do you think the SP201 has a degree of flexibility?

    Thanks again for the blog!

    • Hello Matt,

      Give it would crack, definitely, if it’s just on stretched canvas and SP201 would not be any different .. not flexible, and too heavy! In the past I read that there was a more ‘flexible’ version of Jesmonite ..?.. worth looking on the Jesmonite site. I haven’t used it. But my feeling is that you probably need to use a much LIGHTER impasto paste (the artists’ paint companies produce them, but often expensive!) and create the metallic effect as a separate ‘paint’ coat on top. Nowadays there are ultra-light fillers (B&Q, Wickes etc) which have a bit of flexibility (i.e. one brand was called Red Devil I think). It might even be possible to mix one of these cheap fillers with latex to make it more flexible and more adhesive?

  32. Hi great blog! I am investigating casting plaster polymer into plaster polymer – do you know if it will stick to itself? And if a release agent is needed what is best to not compromise any of the surface detail? Many thanks.

  33. There are a lot of dispersions available around the world that can be used to modify gypsum. The whole set is: in the US is available Forton MG and DuoMatrix. Europe is Acrylic One and Acrystal. In GB Jesmonite. Dow Chemicals has offered the Primal AC33 dispersion and now SF016ER. I suppose BASF also offers such dispersions. The wholesale price of such dispersion does not exceed $ 3 – $ 4 / kg. It is important that the copolymer reacts in the acidic environment of the gypsum. Therefore, not every dispersion is suitable for such application. In addition, dispersions offered in the kit are not homogeneous. These are mixtures of defoamers, dispersants and rheology modifiers. Alpha plaster also can not be ordinary. This must be alpha plaster with a very high degree of grinding. A very good recipe is at https://www.smooth-on.com/products/forton-fmg/. VF 812 is an acrylic dispersion and MF 415 is a melamine.

  34. Hi David! Thank you SO much for this great blog!
    I have used Jesmonite many times and I like it a lot, but I am thinking of trying something cheaper this time. I also have a big bag of Cassinis plaster, so I was wondering if you have tried mixing Cassinis plaster with an acrylic polymer – for example the one from Specialplasters? Would Cassinis be a good plaster to use for this you think?

    I tried searching for some info on it and someone said that Cassinis already have polymer powder mixed in it? I wonder if using only Cassinis and layer it with jute scrim to make a thin mold would be as durable as Jesmonite! Or if I should get some acrylic polymer to be sure.. Don’t want to risk breaking this mold. I might have to do a test with only Cassinis..
    If you have any advice it would be much appreciated!
    Thank you /Caroline

    • Hello Caroline,

      I don’t know, I’ve never used ‘Cassini’ plaster .. I’m a bit suspicious of materials that can only be found on Maragon, without any available tech or MSDS sheet as back-up. But if what they say is true .. yes, it sounds like a polymer strengthener may not be needed. Scrim reinforcement essential though. All I can say is .. extra thorough mixing; 3:1 plaster to water ratio; and wait longer i.e. a good couple of days before subjecting to any strain.

      • Thanks for the reply! I made two test pieces yesterday (one jesmonite and one cassinis) and gonna see how strong they are after the weekend I think, but so far it seems great. I was wondering also, do you know if there are any difference between using the “Jesmonite Quadaxial Cloth” and jute scrim with open weave? I’ve only used the Jesmonite cloth so far but maybe jute scrim is as strong?

      • I use the jute scrim a lot and have never had any problems with mould ‘cases’ up to about 2ft. The ‘Quadaxial cloth’ is really meant for much larger structures .. like boats .. when working in fibreglass! Just a ‘money-spinner’ by the Jesmonite firm .. really unnecessary for smaller forms.

      • That is what I suspected, that jute scrim works fine as well. I did a little test with Jesmonite compared to Cassinis, I think Cassinis is a good moldmaking material but perhaps Jesmonite is a little stronger but Cassinis is probably strong enough for most stuff and soo cheap. Gonna use up the Cassinis I have and next time probably test one of the polymer liquids + plaster that you wrote about.
        Made a little video with the tests 🙂 https://youtu.be/mRK_YryCWGM
        Thanks again!

      • Brilliant! Can I link to the video in the article? Impressed that you stood on, threw and attacked with a hammer .. and they were still only chipped! Love the latest masks, especially the fox .. and great idea using the brown cowries as eyes!

      • Yees I was impressed too! Also wanted to make some test with just flat pieces and weights but no time right now. ^^ I just posted this at my Patreon, that is why the video is unlisted, but yes you can post it in the article if you want! I added a bit of info in the video description too 🙂
        Thank you so muuch, happy you like my masks 😀 Working on a new mask sculpt now, kinda excited to mold it, its fun to improve the molds every time I make a new one! ^^ Soo, thank you again for all the info on your blog!

      • What do you mean by “scrim”? Would that involve somehow getting the plaster to stay on the walls of your mould to create a shell reinforced by fabric? How would you stop the plaster just running down to the bottom of the mould?

        I have some Cassini’s and some Crystacast. I haven’t tried the Cassini’s yet as it takes so long to cure and it’s much easier to mess up but an issue I’m worried about with both is the weight. Casts in either are very heavy even with quite small sculptures and if there was a way to somehow make hollow casts that would be great. I just don’t have a clue how and I can’t find any info anywhere. Do you have any tips/advice?

      • Hi, ‘scrim’ is made from jute .. so a kind of jute webbing. What’s normally done with hollow casts in this way is to apply a layer of whatever one’s using i.e. Jesmonite or plaster first (a few ml thick). Let that firm up a bit, then apply another wet layer and press pieces of the scrim into it, then another layer of plaster to cover the scrim, working it in .. and so-on building up a proper thickness. The scrim should hold the plaster in place as it goes on, even on vertical surfaces.

    • I took David’s advice and used the SP201 acrylic polymer from special plaster and mixed it with Cristacal Alpha K, available from the same stockists- I use latex moulds (For cheapness!) to make 6 inch igurines and the mix of 4 parts Alpha K to 1 part SP201 (with 50% water added) has produced some very strong results- it gets less bubbles than the Jesmonite AC100 mix, and can be used on hollow casts by jiggling it about for about 15 minutes. Not sure how weatherproof it is but I have made stuff with Alpha K before and sealed it with stone floor sealant!

  35. Pingback: Polymer-Modified Plaster – Creating Complexity

  36. Hi David

    Thank you so much for sharing your knowledge and expertise, this is such an invaluable blog!!

    I noticed while reading through these comments that you said that the polymer liquid can be used with cement. I have been trying to cast in white portland cement (Snowcrete) but haven’t been able to get the mix right and my work is cracking in the sun (after letting it dry/cure for a week indoors). Anyway, I am now experimenting with plaster and polymer liquid as it seems to be a good alternative although more expensive. After seeing that you mentioned it can be mixed with cement I now wonder if this might be a good option for me. What would the main difference in the final result be apart from price? Would just white cement and polymer liquid give a strong result? I’m making pots which aren’t thin or delicate.

    Thanks again


    • Hello Chloe, I haven’t used the ‘Snowcrete’ you mention .. could you tell me who supplies or makes it? Did you do what usually needs to be done with cement (as different from plaster) that is .. covering the mould top with clingfilm or similar so that no water evaporates (for at least 24hrs while setting). It sounds like maybe you didn’t so it didn’t set properly?

      • Hi David

        Thanks for the reply. Snowcrete is a blue circle product. You are right I didn’t cover the mould and will do that next time. I also didn’t mention that I was adding a ‘frost proofer & rapid hardener’ and wonder if I was adding too much. I will try again with less of the additive and will cover my mould to retain the water. Thank you very much!

  37. Hi David, thanks for this. I’m having a bit of trouble editing a cast (evening out seams etc) since the surface of the new filler is much rougher than the original cast (I used a silicone mold). I’m using the Jesmonite sandstone mix, it almost seems like it needs gravity or the pressure of a mold for smoothness. Any tips? I’m thinking of sandblasting it to a more uniform rough finish since I just can’t seem to achieve the same smooth surface with other sanding/polishing techniques. Thank you!

    • One thing you could try is filling the seam with the new mix (a little proud) then stretching cling-film over it as smoothly as possible, so that it’s ‘encapsulated’ as it sets. Worth a go?

  38. Hi- I found your blog in the web and I am impressed by your knowledge- I have used Jesmonite in the past and found it a bit slow when used to make hollow figures- the demould time took ages for the jesmonite to stiffen up (I found that even after 24 hours the finished pieces broke when demoulding, even from fairly thin latex moulds- what sort of time woukld you leave a 5-6mm thick hollow cast in the mould, if I were to use a Crystacast and SP201 mix?

    • Hello Andy,

      Something must have gone very wrong! I just can’t imagine what though .. there’s quite a bit of leeway with the mix and even with old materials (a few years) I’ve never had this happening. Under normal conditions demoulding can be fairly quick, similar to when using Crystacal R on it’s own for example .. just one hour! .. and I’ve always found the set pretty durable by this point. Certainly never had to wait longer than 2 hours however thin.

      • I bought the Special Plasters Polymer and cristacast – so far no problems with demoulding shell casts (5mm thick!) from small (fist size) latex moulds- thanks for the heads up! I am casting garden gnome (15-18cm) next with this- I will be interested to see how strong and weatherproof this stuff is compared to the Maragon Cassini outdoor plaster I had been using- how long would you recommend leaving a hollow cast in SP201/cristacast to dry before painting?

      • I’ve found getting acrylic paint to a strong intensity needs a lot of paint, this can affect the curing substantially. Small amounts for mid to lighter colours work pretty safely. I also picked up jesmonite pigment, that stuff is intense. It doesn’t take much to get say a deep black or even a nice clean white. They are big tubs that will probably last some time compared to an acrylic paint/ink.

        What’s interesting here is these are all acrylic polymers of some sort, I was very surprised to see them affect the cure time and some failed entirely. With the jesmonite pigments no failed casts.
        They do cost have to say, but they should last given the intensity and bottle size.

  39. Hi David!
    I think your blog is fantastic, thank you for being so generous with your knowledge.
    I have problems with air bubbles in Jesmonite as I often cut/carve to shape after casting and setting. The bubbles appear in the intersection of the material. Quick setting time is also a problem. Do you think SP201 would be a better option (not only price wise)? I guess my concern would be if the SP201 cuts and carves as easily as Jesmonite or if it, like plaster, could have more brittle tendencies and crack irregularly?
    Thank you again!

    • Sorry, so little time at the moment. Re bubbled you could try spraying with water+detergent (or diluted Windolene) as a ‘surfactant’ .. gets rid of bubbles if they can be got to. Certainly using the SP201 creates less bubbles but the set material is different from Jesmonite .. yes, more like stronger plaster only. Can’t compare re carving as I haven’t done that with either.

  40. Hello David,
    I’m planning to start a small company for manufacturing Alpha Stone Plaster. While researching on Jesmonite I found your website. Actually I thought Jesmonite would make the Alpha Plaster more water resistant and make surface like ‘plastic finish’.
    Is this true? Can you please help me more on this? Is there any alternative for Jesmonite? I would be very grateful for your help.

    • Hello Nikita,

      If ‘Alpha Stone Plaster’ is a regular alpha plaster then any acrylic polymer medium designed for adding to plaster or cement should make more waterproof .. but it doesn’t give it a ‘plastic finish’ just tougher and more impervious.

  41. Jesmonite is just “Gypsum Plaster” and “P.V.A. Glue” and that is all.. no big secret.
    they say its a resin.. But its not.. Easy enough to make this for yourself.

    • My thoughts exactly, Sammy .. ridiculous to refer to it as a resin! .. and as I’ve indicated here, a good ‘alpha’ plaster and liquid polymer bought separately can work just as well. But the liquid is not the same as Pva glue, it’s a different polymer .. I imagine it could affect the proper setting of the plaster if you use it.

    • I’m not convinced that’s true. You get a much nicer, stronger finish from jesmonite than you do from plaster mixed with pva… At least that’s what I’ve found experimenting with it. I tried a few different ratios of Herculite 2 and PVA as well as regular plaster of paris and PVA and found that I got nicer outcomes with no PVA than any ratio of PVA to plaster. Jesmonite I experimented with a little bit as a student and got quite a nice finish.

    • hey Guys,
      I’m actually curious about this as I’m looking to make stronger sculptures but can’t really work with epoxy or polyester in my shop.

      Do either of you have any suggestions or links into DIY type solutions for this type of project? Much thanks.

    • No I don’t, but just Google ‘Jesmonite’ or ‘polymer plaster’ with ‘US’ .. there’s got to be some version of acrylic polymer liquid for strengthening plaster/cement in use there!

      • I tried with “polymer plaster us”, now I will try with “Jesmonite US”.
        Thanks for your reply.


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