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 and 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 ..

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.

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’ ..? ‘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 March 2016 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 £29.00 for 4kg kit (3kg powder, 1kg AC100 liquid) 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!

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.





21 thoughts on “polymer-modified plaster

  1. 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?

  2. 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.

  3. 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.

  4. 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.

    • 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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