‘Beginner’s Basics’ – mouldmaking and casting explained

using meltable vinyl

Photo Astrid Baerndal

What you can achieve if you know how to make moulds and casts

Everyone I’ve tutored so far has experienced a strong sense of achievement in making a successful casting, even if the original form is ‘found’ i.e. not of their own making. It’s curiously exciting, unpacking a mould for the first time to see how a cast has turned out. Even though the form itself will be no surprise, it feels like making something new.. it becomes one’s own creation!

With more practise it’s not hard to produce your own sculpture edition i.e. a series of casts, or it may open up ways of manufacturing your own functional product. It’s probably fair to estimate that at least 90% of the things we use in our daily lives have relied upon some form of casting for their manufacture. If you are, or intending to be, a prop-maker or model-maker, it is a fairly essential skill to have. Model-making often involves repetition of forms which make up the whole, whether the columns of a Greek temple or a set of replacement hands for an animation puppet.

Repetition is one thing, but mouldmaking/casting is not all about being able to repeat. It has been an inseparable part of sculptural or form-making methods for thousands of years and its importance hasn’t diminished with the development of new materials! It means for example that a form can be modelled in a material which makes modelling easy, such as clay or modelling wax, but which can then be transformed into something permanent such as metal or concrete. Also, during the process of creation, sculptors may wish for a way of ‘saving’ an important stage in their work, rather like one can on the computer. Although more time-consuming, being able to make a cast will achieve just that! It’s also worth bearing in mind that the same can apply to forms created by 3D printing. Once a form has been digitally modelled and a one-off prototype printed and refined by hand, it’s worth considering whether traditional mouldmaking and casting might offer a quicker and cheaper means of reproducing the object in the long run?

It’s true that mouldmaking/casting can require quite a time-investment; also that it needs patience, planning, a methodical approach and a certain amount of prior knowledge. One needs these things if one wants consistently good results. But it’s also true that ‘trial and error’ are the best teachers; that there is room for spontaneity and invention, and that often a sequence of simple steps will achieve more than one complicated one!

fossil trilobite

If it can happen accidentally in nature, as illustrated by the fossil trilobite above, can it be so hard to achieve on purpose? All that nature needed was the right conditions .. and plenty of time!

What is involved?

Mouldmaking/casting involves covering the object you want to copy in a material which will then become firm enough to be detached from it and keep its shape, so that a hollow space or negative of the object is left .. the mould .. which can then be filled with a casting material to make an exact replica of the shape. Either the mouldmaking material needs to be flexible to be easily released from the original object and any casts made in it; or a hard, inflexible mould can be made if instead both the original object and the casts are themselves flexible.

The most effective and most used flexible mouldmaking material today is silicone rubber which can either be poured as a liquid or (with a special thickener) brushed as a paste on the surface of the original object. If it is poured as a liquid, temporary walls of cardboard or clay need to be set up around the object to contain the liquid rubber while it sets .. or cures, as the proper term is. Many different forms of object can be reproduced in this simple way by just creating a block mould of silicone around them as long as they have one flat side (the side that’s secured to a board first and therefore not covered by the silicone) which then becomes the entry or pouring hole of the mould. If the flat side happens to also be the largest area of the form (for example, a rounded paperweight) when the cured block is detached from the board and turned over to take it out, it will be pushed out fairly easily by flexing the silicone. Often though that flat side will not be the largest part of the form, for example in the case of a modelled head with part of the neck. This form involves undercuts.

puppet head

This is what ‘undercutting’ means.. imagine trying to pull someone’s head through a hole the size of their neck. The space around the neck ‘undercuts’ the size of the head so even if this space was filled with flexible rubber it would have to stretch a great deal to get the head out in one piece. ‘Undercutting’ is the commonly used term, but it’s really not a very helpful one! It’s the space around that undercuts or ‘underfills’ whereas it’s clearer if one thinks of the form itself as overhanging its base. These ‘undercuts’ are likely to occur not only in the overall shape but often in the details, in this case the nose, or the ears for example. Many silicone rubbers can flex a great deal .. but not so much that a form like this can be simply pulled out!

However, silicone rubber has the valuable property not only of flexing easily but returning exactly to its original shape without distortion. This means that if you do make a block mould form around a head shape and the sides of the mould are thick enough (i.e. around what will become the negative void of the head and neck) the silicone can be split with a sharp knife just enough to be able to take the form out. Afterwards you will hardly have to coax the cut surfaces back into the right position .. the silicone should ‘marry’ again perfectly if it has set properly in the first place, so much so that you probably won’t see the cut anymore! We refer to this as the material having the memory of the shape it was formed in, a property which most rubbers and some plastics share. With the right gentle support i.e. rubber bands or tape binding the outside (but not too tightly), the mould can be filled as if it were uncut.

splitting a mould

A more ‘advanced’ method, often necessary for more complicated or larger forms, is to make a detachable plaster jacket (also known as the mother mould) to fit around the silicone part which ensures that it keeps its shape under handling. For an example of this method see Making a small mould for a four-legged animal .. in this section.

Another important aspect I haven’t yet mentioned is whether the original form can be covered in silicone just as it is or whether a barrier or release agent may be needed. Silicone will hardly stick to anything except itself (the other reason for its suitability) so usually if the original form has a sealed or dense surface (tight-grained or varnished wood .. or stone, plastic, soft natural clay, modelling wax, polymer clay, etc.) there is no need for a separate barrier. One exception is glass, because silicone will bond with this because both materials share a silica base. But also if the surface of the original form is fragile or porous it will either need sealing, by varnishing if possible (or a coat of Pva wood glue can work well), or by greasing with Vaseline just prior to covering with silicone. Care must be taken to work the Vaseline into the surface but not use too much in case it fills surface detail.

An average silicone will need a recommended 24hrs to properly cure, though there are some special fast-curing ones which usually cost a little more. Once cured the mould can be used. The most common way of making a cast is to fill the mould with a liquid which changes into a solid, as is the case with resins or plaster. Resins for casting are supplied in two liquid parts which when combined in the right proportion start to harden. These two parts need to be thoroughly mixed before being poured into a mould. For polyurethane resin these parts are mixed in equal amounts whereas polyester resin consists of the resin itself and a hardener or catalyst which is added in a very small proportion. Plaster is supplied as a powder which first needs to be mixed with water and this is done by shaking the plaster gradually into the water .. never the reverse! Resins are often the best options for casting small, delicate or highly detailed forms for which plaster would be too brittle. Plaster is a much better option for bulkier forms, such as life-sized heads, since casting these in solid resin would be very expensive. Resin sets on the whole within 30mins and some plasters can be almost as quick .

polyurethane resin and Fillite

Here above, polyurethane resin is being portioned out in equal amounts using disposable plastic cups. Parts ‘A’ and ‘B’ of the resin are different in appearance. The third cup contains an equal amount of grey ash filler known as Fillite. It is not essential to add this other ingredient to resin but various fillers are often used to increase the volume of the resin (making it cheaper), to make casts lighter or to change the surface appearance.

Normally when you pour a liquid into a container you assume that the liquid will fill the whole of that container evenly, at least up to the level that you stop at. With casting, because the ‘container’ one wants to fill is rarely a straightforward shape, it can be rather different in practice. When liquid fills a shape it will push the lighter air upwards and out easily, but only if the air can escape. Air can become trapped in parts of a complicated shape, meaning that the casting material will not be able to fill those parts. Sometimes air can be helped out by tipping/rocking/tapping the mould while filling it but often this isn’t enough. The simplest and often the most effective solution is to give air extra means of escape by cutting little channels in the mould, leading from the problem parts to the outside. Because air is ‘thin’ these channels only need to be very small.

That may be one major challenge solved but unfortunately there are other ones standing between you and a perfect cast. Casting materials are very pourable, but they’re not like water .. even the thinnest polyurethane resins are somewhat thicker. So they may bring air with them in the form of bubbles. Most of these air bubbles will rise during the filling of the mould but there are always a stubborn few that manage to lodge themselves where they can’t rise out. Again, a good deal of tapping, rocking etc. can help a lot! It also helps a lot if you can manage pouring in quick stages, especially with deep moulds, interspersed with the above. Resins will fill a mould very uniformly because they don’t separate out, whereas with plaster any excess water in the mix will be forced upwards and may collect in the same places that trap the air making ‘rivulet’ lines in the cast surface. The solution is .. if you want perfect casts in plaster you have to get the mix right so that there’s the minimum of excess water.

air and water damage

The photo above illustrates what sometimes happens when casting a head shape in plaster. The overhang of the chin can trap both air and excess water, causing damage to the cast surface.

How expensive are the materials?

Unless you’re planning to cast in metal the most expensive materials you’re likely to use are silicone rubber which averages around £22 per kilo and polyurethane resin which averages £13 per litre. Think of a litre in volume roughly as a block 10x10x10cm. A lot of small forms can be made from this amount, especially if a filler is used to extend it as mentioned above. Moulds will always be much bigger in surface area than the object itself so the main expense is silicone if this is used. Here I am confining this basic overview to the making of simple block moulds (which can be rather uneconomical in the amount of silicone used) but there are more ‘advanced’ ways of building up a layer of silicone rather than a block, meaning a great deal less is used. After trying out small block moulds if you are interested to learn more about these further methods look at the posts ‘Making a hollow 2-piece cast in fibreglass’ parts 1-3 from August 2012.

silicone 'skin'

For example, above I am coating a modelled head with silicone rubber which has been made thixotropic or ‘non-slump’ by adding a special ingredient. A thick layer is built up in this way and left to cure. Before cutting the cured silicone layer into halves I made a plaster shell around the form, also in two interlocking halves so that it could be easily separated. In progress below is the mould being filled, showing another method of saving expense (and weight). Here I am making a hollow cast in filled resin by first building up a shell manually in two pieces which will later be joined together. The above mentioned post also deals with how to do this.

making a hollow shell casting

There is also a range of much cheaper alternatives to both silicone rubber and polyurethane resin which can work just as well dependent on your purposes and the quality needed. Polyester resin can cost half as much especially in larger quantities; Vinamold can be a third of the price of silicone and it is also reusable, by melting it down again; hard casting plasters are a small fraction of the cost of resin, and just as suitable for many larger forms.

Silicone cannot be made liquid again for re-use once cured but old silicone moulds can be granulated using a traditional meat grinder. The granules can then be added to fill- out the mix when fresh silicone is made up. If this is done, the first layer of silicone coating the prototype object should be unfilled though, so that all of the surface detail is caught.

recycling silicone

Where to get the materials?

If you get your mouldmaking and casting materials from a regular art or hobby shop you’ll get discouraged pretty quickly because of the expense. Here you will find versions of them at ridiculously high prices for impractically small amounts! If you live in London you will pay much less for better quality materials .. and get reliable advice! .. if you go either to Tiranti’s in Warren St or the 4D modelshop near Tower Bridge (details in the Suppliers list). If you live somewhere else both of these specialist suppliers have an efficient online-ordering service, and both conveniently list their prices including VAT. I have included a number of other nationwide suppliers in the suppliers list.

Is expensive equipment needed?

The most expensive piece of equipment I use is a digital kitchen scales which cost around £20. If you want to mix materials properly it’s fairly essential to have these. Professionals who provide mouldmaking/casting services for a living may well benefit from special ‘degassing’ or pressure chambers to eliminate air bubbles etc. but one can often achieve perfectly good results without them.


What equipment do you need apart from the materials?

This list is long because it’s thorough! None of these accessories are expensive and some may not be needed dependent on what you’re working with:

notebook and calculator (for example volumes and quantities, especially when mixing in ratio need to be worked out and noted down, often a little too complicated to work out in the head)

a means of taking photos (especially with this kind of work the process is just as much a source of personal satisfaction as the outcome, and it deserves to be recorded)

some form of covering for the work surface (i.e. newspaper or polythene) because it can get messy

another level area (check with spirit level), out of the way, where moulds can be placed while curing

baseboards (size depends on the size of form you are working with. The baseboard should be around 5cm larger on all sides. I use offcuts of Palight foamed Pvc in various thicknesses, offcuts of smooth 5mm MDF or sometimes strong cardboard)

something to make containment walls with (I use either scrap cardboard, any thickness or type strong enough to stay upright; foamed Pvc sheet; Lego bricks; plasticine, modelling wax or natural clay)

mixing cups or pots of various sizes (I use disposable plastic party cups £1 per 100 for mixing resin, and plastic milk bottles cut down for mixing plaster)

reusable clear plastic measuring beakers (these are essential for mixing silicone rubber. Available in different sizes, but I use medium-sized ones which hold 200ml, calibrated in 25ml stages. Available from Tiranti). Leftover silicone should be left in the pot and once cured can be easily peeled off

mixing sticks (‘coffee stirrers’ i.e. from Starbuck’s, Costa’s etc. are fine for small amounts of resin. Disposable chopsticks are excellent! Larger, broader sticks are better for stirring plaster .. better than using spoons). Mixing sticks should either be thoroughly cleaned or kept separate (i.e. those used for resin, those used for silicone etc.) to avoid possible contamination

digital weighing scales, as mentioned (mine is a Salter brand ‘Aquatronic’ which takes up to 5kg measuring in 1g increments, available online c. £20)

disposable plastic pipettes may be needed (dependent on the brand of silicone used) for dosing small amounts of additive. These can be found on ebay for 4p-10p each

a spirit level to check that moulds are left curing on a level surface (see below)

a small sharp knife, preferably Swann Morton surgical scalpel for slitting and trimming moulds, cutting cardboard for containment walls, etc.

a cutting mat to cut on (A3 should be sufficient)

UHU glue or similar will be needed to temporarily fix the original object to the baseboard and to fix containment walls if cardboard is used

Vaseline petroleum jelly is essential as a barrier between silicone and any porous surfaces such as soft wood or cardboard

white spirit and small brushes (you will need white spirit in case of silicone spillage. Uncured silicone rubber dissolves in white spirit. You will also need it to clean brushes afterwards if you use them for brushing a first ‘detail coat’ of thin silicone on the original form)

acetone (uncured polyurethane, polyester and epoxy resins can be cleaned up with acetone)

What sort of space is needed? Is it possible to work in the corner of a lounge or kitchen?

Yes, if you’re content to work on small things and you can keep children at a respectful distance. But often it can be difficult to simply clear things away at short notice. Some extra space is needed to leave moulds undisturbed for a day while curing; the room should at least have possibilities for ventilation (some materials needing much more than others); table-top and floor should ideally be covered in case of spillages

What are the health & safety issues?

There are relatively few materials commonly used for mouldmaking and casting that pose serious health & safety issues, but those that do need special measures. Polyester resin for example should never be used in the home because firstly the build-up of styrene emissions is harmful and secondly the catalyst (MEKP methyl ethyl ketone peroxide) is highly flammable and even explosive!. The clear version of polyurethane resin (different from the standard opaque versions) should not even be considered! Dust-masks should be worn whenever handling large amounts of plaster or any other substance, such as a filler, which becomes easily airborne. Good ventilation is essential to dissipate the vapour from solvents such as white spirit or acetone, which are of course also flammable. It is essential to read and act upon the MSDS (Material Safety Data Sheet) for any material you are using. Nowadays one never receives this sheet automatically with the product, but it can be downloaded from either the manufacturer’s or the supplier’s website.

How should you start?

I’m often asked for advice from people wanting to make the most complicated moulds or casts before they’ve experienced even the simplest handling of the materials. It’s far better to start simple, building up an idea of what can be achieved by the simplest of means first and then, if one needs or wants, extend these means little by little.

For example, start by making the simplest kind of flat, 1-piece, open mould. Make or choose a prototype (the original form to be reproduced) which can be fixed down to a flat board, has an interesting amount of detail (to make the effort worthwhile) but fairly minimal undercutting. Start by exploring what’s possible by making simple block moulds first. Many complicated forms can be more achievable by making them in easily mouldable parts (each requiring just a simple mould) which can then be easily filled and the parts then assembled. The following is an example:

chair prototype parts

The prototype pieces for this chair were cut and smoothed from 2mm Palight foamed Pvc and fixed to a flat Pvc board using small spots of superglue. The upholstered parts are cut and sanded foam from Kapa-line foamboard also superglued in place. The Pvc needs no barrier against the silicone rubber but the unpainted foam needs a light greasing of Vaseline because otherwise the silicone would grab into the porous surface too much. Catalysed silicone rubber (without addition of a thickener) will reach every detail when poured over the prototype. Standard silicones can remain fluid for some hours and during this time most trapped air rises to the surface away from the prototype, but often as an extra precaution a first thin layer of the mix can be brushed on and left to settle a bit before the rest is poured. This is generally known as the detail coat. Because silicone ‘travels’ as far as it can before starting to harden it is important to ensure that the prototype pieces are secured without any gaps underneath them.

Below shows the cardboard containment walls for the mould block, fixed to the Pvc base using UHU. It is important to ensure that there are no gaps in the containment walls. These should also be Vaselined inside to prevent the silicone from sticking to the cardboard. When the box is filled it should be put on a level surface out of the way i.e. not just flat, but checked with a spirit level. This is important especially with very flat moulds because when they’re cast into they should also lie level, otherwise the liquid casting material will set at a slant.

preparing for the mould

This mould took 24hrs to cure and parted easily from the prototype pieces. The mould is shown below alongside some castings in polyurethane resin (plaster would never be remotely strong enough for the chair legs and arms). It would be very difficult to mix up polyurethane resin and then pour it exactly into these small, shallow shapes. Instead the resin is poured over the main parts, a little overflowing, and teased into the finer ones with a cocktail stick. Polyurethane resin is usually clear until it starts setting so air bubbles can be seen and teased away in the process. It’s best to fill generously and then, taking a straight edge of plastic or card, draw it steadily over the mould surface to remove the excess. Polyurethane resin can generally be demoulded (taken out of the mould) after 30mins, but small parts may still be pliable. This is useful because trimming them is easy at this stage. It’s usually better to wait a further few hours at least before the resin can be sanded. I needed to do this to get a perfectly flat finish on both sides, but this was not much work because polyurethane resin sands easily. Although the resin is fit to work on in this way after a few hours, complete curing actually can take a few days.

mould and cast parts

Polyurethane resin also bonds extremely well with superglue. Below are the assembled chairs which have been primed with Simoniz acrylic car primer, ready for further painting.

assembled chairs with primed surface

Some things I wish I’d understood better from the beginning

The original form can be made of anything which will hold together long enough for the mould material to set. The possibilities are endless!

One should always aim for exactness in dosing chemicals together as a general rule, but many are fairly forgiving. For example if by mistake too little catalyst is mixed with the silicone rubber, i.e. 75% of what it should be, the silicone will still cure but just take much longer (perhaps a few days instead of one).

catalyzing silicone

Photo Astrid Baerndal

It’s generally much easier to divide up a complicated original form into separate pieces that can be easily and perfectly cast, than it is to achieve a perfectly filled casting in a complicated mould! There’s virtue in designing a prototype from the beginning with that in mind.

Polyurethane resin needs to be mixed quickly ..but thoroughly! It’s difficult to judge how long one can risk continuing to mix before it’s too late to pour because when it changes this is not gradual but sudden. One reason why I use thin, disposable plastic cups for mixing is that I can then feel the slightest warmth through the bottom of the cup. At this point it should be poured! If polyurethane resin is not completely mixed, most will still set but there will be softer patches ‘bleeding’ unmixed resin which may remain like that.

When plaster is sprinkled or shaken into water a good ‘rule of thumb’ is to continue until pretty much the whole of the water volume is filled with settling plaster and there is little or no residual water ‘swimming around’ on the surface. The mix can be stirred at this point .. but one can wait! The plaster won’t start setting until stirring begins. Waiting a few minutes will release more air, help to dissipate lumps and therefore give a better mix.

Another ‘rule of thumb’ when wanting to mix the right amount of plaster for the job is that the volume of plaster mix will roughly double the volume of water you start with.

Small ‘pinholes’ in the surface of a plaster cast are often caused by bubbles of air attaching themselves to the mould surface during casting. This can be reduced by breaking the surface tension at the mould surface by using what is known as a surfactant. The easiest method is to use a detergent such as Windowlene, diluted with water and sprayed lightly into the mould prior to casting. There is no liquid surfactant suitable for resin casting, but many say that dusting the mould with talc before filling it will achieve the same thing.


40 thoughts on “‘Beginner’s Basics’ – mouldmaking and casting explained

  1. I was wondering if you have any information on “wood paste”. I have seen religious statues molded of wood paste. I am not too sure what it is and how I can get it. Many statues made in Spain use this material. I am assuming it is compatible with silicone molds. It would be nice to try it.

    • I remember looking into this before because I’d seen similar religious work in museums. You can’t buy it as far as I know, and I could find no recorded recipes online .. only hints that it was a mixture of fine wood-dust and glue (possibly rabbit-skin, possibly a drying oil binder like linseed oil ..) with other ingredients added (possibly chalk or marble dust). Such a mixture would take an awfully long time to ‘set’ within a mould .. days, maybe weeks .. and it’s hard to get hold of very finely ground wood-dust. So it’s not the cheap and simple option one thinks it might be! It is however possible to mix a faster setting resin (thin polyurethane probably best) with enough wood dust to make a paste, and paste quickly into a mould in stages .. but you’d have to work very fast! .. and you’d be lucky to find a good source of fine wood-dust! .. regular ‘sawdust’ i.e. like those sold for pet bedding or cat litter, is too coarse.

  2. Hello David,
    Is it possible to seal a dry plaster mould with ordinary wood varnish rather than shellac? I sealed a large plaster mould with shellac before it was quite dry and the shellac has disappeared into the plaster in patches. Rather than buy more shellac, I was wondering if I could use other wood varnish already purchased? The mould will be used with paper pulp to create large ceiling tiles.

    • I haven’t had good experiences using water-based sealers or varnishes on plaster. These are all a form of acrylic, and what happens is that it paints on easily enough and dries quickly, but can damage or peel off later. The acrylic just forms a skin, not properly anchored in the plaster. It’s different with wax/oil or spirit-based varnishes though .. these will fix much better (hence the usual recommendation re shellac).

      • Thankyou David for your response. I will stick to using shellac. The wood varnish I have is oil or spirit based but of course, the drying time is longer than shellac. I have worked out what happened to the previous coats of shellac. I don’t think it had anything to do with the plaster not being quite dry. I read somewhere to finish with a coat or two of thinned shellac, which I did, but of course, the extra Methylated spirits just soaked through the previous layers of shellac, thinning them and they then soaked into the plaster. I have since read instructions to START with thinned shellac so that it soaks into the plaster a bit and end with full strength shellac to seal properly. It all makes sense now!
        I look forward to reading all your posts from now on.

  3. Hi David, thanks for all your information. I have a specific question though; we are filling shoes with a mixture of plaster (to fill the tip of the shoe) and a flexible polyurethane foam. We then scan the result to get a good idea of the post-production quality of the shoe in 3D. However, not to destroy the shoe we now use a very thin plastic liner before we poor the plaster in. It seems however we are losing too much detail. Can you suggest any filler (i was thinking of silicone perhaps? but there are so many and I am not even sure it is the right material) that would go easily into the deepest point of the shoe and also not need a liner because it would be easy to release and not stick to the shoe? thanks for your help and I would be willing to share any result with you obviously!

      • Yes we know it’s a tricky one, that’s why I am reaching out 😉 Looking forward to any suggestions you have. Thanks anyway for your prompt reply.

        kind regards

      • I’ve been thinking. The flexible polyurethane foam is the best option I think (i.e. ‘Self Skinning Flexible Foam’ from tiranti.co.k for example) but poured in stages. The other thing I’m thinking is that instead of creating a barrier with thin plastic liner (which is bound to interfere with detail) maybe a thorough rubbing/dusting with talc inside will be enough? Least harmful to the shoe for later. Then pouring a small amount of the PU foam mixture into the toe part, waiting until this has stopped rising and becomes firmer, then pouring again, and so on in stages. I can’t guarantee because I’ve never done this inside a shoe myself, but it’s my best guess!

      • So, what do you think? If you do try the talc-ing method I’d be grateful for some feedback. Nine times out of ten I never even get a reply .. it makes me wonder why I’m doing this?

  4. Hi David,

    I did a prop making course last year that included making a silicone mould and a plaster mould so was looking up how to do it again which led me here.

    I was hoping for some advice, I am trying to make a mould for a clock for “Three sisters”, where it needs to break like porcelain. Which makes me think it needs to be a slip casting? I have plaster so I am tempted to make a plaster mould from a clock I form in clay (not sure which type would be best?). Does this sound the right way to go about it or would you suggest a silicone mould? I am less confident in silicone but could give it a go.


    • Hi Lucy,

      I’d say that’s exactly the right way to go about it! Are you clear about how to slip cast? Ideally the plaster should be a more absorbent ‘pottery plaster’.


  5. Hello David,
    I am trying to mould and cast one of my dog’s favorite toys that he seems to go through super quickly! I’m hoping for the final product to made from a food safe silicone, he never eats the destroyed toy, but I do want it to be safe for him. The problem I’m running into is that the toy is hollow in the middle so I’m having to cast the two halves separately and then join them together to have that hollowness in the final product. Do you have any recommendations for how to join the two halves together that would be pretty strong? Thanks!

    • Yes, the only way in fact is to use the same silicone as a glue and it is a very effective one .. as strong as the rest because it will bond totally, as long as the edges of the two halves are totally clean.

  6. Hello David,
    I’m try to make hollow tubes/pipes using a mixture of powdered sulphur and hide/bone glue. The mixture of this material hardens into something akin to unfired clay in terms of structure/friability. I did one experiment in which i basically applied the sulphur/glue mixture as a kind of paste around a piece of narrow plastic piping (d. 5 cm approx). Not really a suitable method especially if i want to make pipes that are larger in dim. and length. So I was wondering if you could advice on how to make a reusable mould. Would it be wise to buy a piece of plastic piping of an appropriate size, cut in two and make a mould from the two halves? I’ve no experience with this kind of mould making I’m afraid, so don’t how to begin, and what would be the appropriate materials to use. Would appreciate your advice. Very best wishes, Kim

    • Ok Kim, so now I’ve looked on your portfolio/website and if it is all just a cover for an incendiary career it’s a very convincing one! Can this ‘mixture’ be poured and still ‘set’? Does it have to be ‘pure’ or can it be cut with something which will turn it into a pourable and curable material?

      • Ha! no I wouldn’t wish this particular mixture of materials on anybody! It’s pretty smelly. Thanks for getting back to me David. Why am I using these materials? Well I suppose they fall into a category of materials that I’m interested – which I guess you will have spotted from previous work I’ve made. I haven’t done too much pouring with the mixture but it is possible – although I’m not yet certain whether the results are too fragile. I’d have to play around with different strengths of bone glue. It does have to be pure – am a total purest (according to my own logic probably) when it comes to materials. Am not interested in creating an illusion, I’m more interested in the push and pull of materials, finding both its and my limits.
        What I’m trying to experiment with at the moment is making the sulphur/animal glue pipes which I would then pour a mixture of oil-shale waste/bone oil (which is a rich black mixture). I guess I don’t even mind if the pipes go through some kind of partial collapse when the oil-shale waste mixture is forced down it. I hope some of that makes sense? Am happy to send pics-you’ll be able to see some of the sulphur etc experiments on the slow air blog (although it needs updating for sure). The sulphur and animal glue mixture can certainly be used to make substantial structures. I’ve made a coil built structure in the studio which is really stable – although it may not be moveable! k

      • Thanks Kim! That’s much clearer to judge now. My feeling is that .. yes, a possible way would be to slice a PVC drainage pipe in half, tape back together on a board for a base, Vaseline inside then coat inside a thick wall of the mixture .. but I think it would still be quite difficult to detach the ‘mould’. What would be much easier to detach would be a long strip of plastic coiled into a tube shape! .. would still need to be Vaselined inside though, but once the stuff is set you just carefully uncoil it! You might have to shape it around a cut disc attached (temporarily) to the board .. but this disc could be kept as a base for the receptacle perhaps? When I say ‘strip of plastic’ I’m thinking of thin sheet plastic like 0.25-0.5mm styrene sheet (4D modelshop) or thin polypropylene, or a stable (not too thin) sheet acetate. Best, David

    • I’ve always used Vaseline .. or similar petroleum jelly .. thin coating, but you have to be thorough! Never had any problems, but it helps if you can colour the Vaseline (a little powder pigment will) so you can see what you’re doing.

  7. Hello

    Thank you for sharing experience.
    I wonder how could I mould thin layer of silicone, like haloween rubber mask ?


    • I’m not sure. The only way I can think of off-hand is to model the mask in clay, make a plaster mould of it, then paint on a layer of silicone which you’ve thickened (by adding a thixotropic agent). You’d get the right surface outside but inside wouldn’t be totally smooth. A better way perhaps would be to use latex in the plaster mould rather than silicone.

  8. Hello. I have a rather difficult mould to make; I would like to ask you how you would proceed.
    I am making a maquette of a luxury toilet unit. I need to make 4 toilets and 3 urinals. My scale is 1/10. How can I make a mould of a toilet bowl? The outside form is not a problem. It is the inside of the bowl. I made a plaster model, and now I need to reproduce it 3 times more. I am not even thinking about the urinals.

    • Hello Lisette,

      There is a way of making a mould in one piece for this. I can’t go into every detail in the time I can spare .. but basically you need to set the ‘toilet unit’ you’ve made on a small ‘stalk’ (guessing 2cm high) underneath, which will become the pouring hole. I’m assuming you’ve made your toilet bowl to look as if it goes ‘down’ but doesn’t actually i.e. filled in? .. you have to do this. You will need to coat bare plaster (Vaseline will do) before pouring silicone on it. Surround the whole setup with a containment wall (guessing 1.5-2cm distance from object all round) and c. 2cm higher than its top. You can then fill the whole thing with silicone right to the top (could take half a kilo). When the silicone is cured you need to turn the form over and start splitting the silicone enough so that you can remove the model (may take one cut, or one either side, not sure). The silicone ‘block mould’ can be ‘resealed’ i.e. cut edges eased back into place, and should be strong enough to be cast into (may need gentle rubber bands). Doing it this way means that both outside and inside shapes are ‘connected’ in one continuous mould .. quick, but the most expensive way in terms of silicone, that’s all.

  9. Pingback: Mould making research. – farsya ahmad

  10. I hopefully will be studying special effects makeup at university and they have sent us a project to do over the summer but I am struggling to understand what types of sculptures are suitable for silicone flat moulds, as he has asked us to do this “Practice and document evidence of your own sculptures, appropriate for flat moulds in an appropriate oil – based clay medium (chavant, plasticine, etc.)” Would you perhaps know what sculptures would be appropriate to make for flat moulds?

    • I guess they just mean what’s possible with an open 1-piece mould, rather than a 2-piece or multi-piece mould for a more completely 3D form. You can get the idea from this or the other articles. So for example a ‘relief’ form, but the relief could be quite deep as long as it doesn’t involve too much undercutting.

  11. hi,
    I did the casting with easyflo 60, but the paint is peeling off.
    do you have any idea which primer and paint shall I use?
    thank you

    • You have to wait a few days, then scrub the casting with warmish water and slightly abrasive detergent e.g. Cif, then when fully dried prime with Simoniz auto primer .. that’s always worked for me. Somewhere here, maybe in ‘Polyurethane resin’ in ‘Materials’, I’ve written about preparations for painting.

    • I would avoid the one containing ammonia. The standard Windolene which was recommended to me, and which I’ve been using, is either clear or blue tinted. In the directions for use it says that it contains vinegar, but underneath in ‘Ingredients’ it only states ‘5% Anionic Surfactants’. If you go to rbeuroinfo.com and select ‘Windolene Trigger’ you’ll see a fuller ingredients list for comparison with what’s available where you are.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s