Making hollow casts in open or ‘closed’ moulds – Part 2

This follows on from the previous post in which I mentioned that hollow casts can be made in ‘closed’ moulds i.e. without having to set up a pouring hole. The cast is achieved in exactly the same way as the puppet head .. by building up a sufficiently thick layer in both halves of the mould, then joining them together. In fact it can even be a little easier since the two mould halves often have a consistent rim to work up to. This method of casting is a big advantage when the prototype form offers no convenient area for setting up a pouring hole, as is the case with the form below.

Chloe Allen moulded this lightbulb while participating in our Modelling, mouldmaking and casting course in 2012. She wanted to preserve the distinctive shape of the contacts at the base, which would have to be remodelled if this area were cut out to form a pouring hole.

The cast on the right, which came out near-perfectly, was a thin shell casting using polyurethane resin (Biresin G26 in this case) and Fillite. Note how finely the silicone rubber and resin have captured the smoothness of the glass. When silicone rubber is used on glass the surface must be very thinly greased with Vaseline to prevent the silicone from sticking. The distinctly different result on the left was obtained using Sculptamold as a casting material. Sculptamold is a mix of casting plaster and cellulose fibres, bought ready-mixed in dry form, making a thick paste when water is added. It feels and looks very similar to papier-mâché pulp, except that it sets hard in about the same time as regular casting plaster i.e. around 30mins. One has to work fairly quickly and paste the mix into both halves of the mould to form a thick shell. The paste is very workable and has good thixotropic properties, meaning that it is non-slump. When working with polyurethane resin the shell can be left fairly thin at the rim, because this will be strengthened when more resin is rotated around the closed mould. But Sculptamold is too thick to do this with .. instead the walls of the cast need to be built up to a good thickness right to the rim, but preferably with the top edge sloping down towards the centre of the mould, so that the mould halves will close properly when they’re put together. Usually once this is done the Sculptamold has already set firm, the top edge can be trimmed with a knife if need be and excess Sculptamold cleaned away from the mould surfaces. A little more needs to be mixed and then ‘piped’ or troweled on one or both of those edges before the two mould halves are joined together. Since the edges were sloping downwards most of the fresh Sculptamold will be pushed towards the centre of the mould, though a little will be squeezed the other way and will form a thin flashing on the cast which can be easily removed.

Because of its mix of plaster and fibres Sculptamold traps a lot of air, and this is impossible to get rid of, resulting in the surface effect shown above. Although the plaster component becomes firm very quickly the fibres retain moisture so, like traditional papier-mâché pulp, the material needs many days (weeks even! .. for thicknesses over half an inch) to dry out completely. After the two mould halves have been pressed together the setup should be left at least for a couple of hours before the cast can be safely demoulded. Even then the damp Sculptamold surface is somewhat fragile, rather like slip-cast clay, but this can be an advantage because it means that the mould seam can be more easily cleaned up, even using sponge and water to blend it a little if need be.

A while ago I wanted to reproduce two interesting fragments of driftwood I’d found on the Thames shore (we live only a couple of hundred metres away). The one below was a fairly complicated form which I knew would be difficult as a poured cast because of air entrapment, plus the fact that I didn’t want to compromise any detail of the form by cutting out a pouring hole.

In each case the original driftwood is on the right, the resin copy on the left. The paintwork isn’t complete in these photos .. just a basecoat with the first, lighter dry-brushed colour over it. I’d scrubbed the casts with warm water and Cif to give a slight ‘key’ to the resin surface and to remove any remaining greasiness. I used Rosco SuperSaturated acrylic, a theatre/film scenic paint which dries especially matt and has a strong binder.

I made the silicone moulds in the usual way .. by embedding the form up to a half-way point all round in plasticine or wax; coating the first silicone half over that, followed by the first-half plaster jacket once the silicone was cured .. etc. See previous articles e.g. Making a supported silicone mould for a life-size head .. for details of this method. Again as usual I coated the two mould halves with a polyurethane resin/Fillite mix, building up a strong shell.

As with the mould above for the smaller of the two forms, it can get tricky to determine in some places where the object surface ends and the mould seam surface begins, especially if some parts of the object are flat and thin. Although I’m only thinking of this now, and am yet to try it .. it could help if the very first, thin layer of silicone on the object, the detail coat, is coloured differently from the rest to make the border of the form clearer. Silicone rubber will accept a small amount of powder pigment to colour it without affecting its properties, as I illustrate in the next example.

As always with this method, the edge of the resin shell needs to be cleaned up so that there’s nothing preventing the two silicone mould edges from fitting together. If in doubt or if the mould halves no longer meet properly when testing them together, it’s better to shave away a bit more than necessary. Usually the resulting gap in the shell is filled when the final batch of liquid resin is rotated around inside the closed mould. In the failed example below, either the resin/Fillite mix was a little too thick with Fillite to begin with or I’d waited too long before pouring into, closing or rotating the mould. The resin hadn’t travelled enough along the whole seam line.

Here’s a brief account of making a similar mould .. at least, done for the same reasons .. but with some differences in the method. Once again, I used these found objects mainly as test pieces .. this time tackling a dryer ball. The original below is pink and the cream coloured one is the cast. Fairly obviously this form presents only one option for making a poured cast .. setting up a pouring hole in the space where the writing is. But I wanted to keep the writing, and in any case .. managing to fill all these little ‘horns’ without trapping air would be impossible without the assistance of a vacuum chamber to pull the air out. I don’t deal with these more commercial methods because most people, like myself, are unlikely to have one.

I also wanted to try covering with a complete silicone layer first, without embedding the form and applying it in two halves as I’d mostly done before. I intended to cover it and then split the silicone skin with a scalpel. The problem though would be finding the right place to cut once the form is covered .. ideally I needed to cut around the middle exactly in between the rows, where the manufacturer’s seam line is. I thought I’d solved the problem with the following, but it didn’t work out as cleanly as hoped.

I applied the silicone rubber layer in two stages .. the first one above, and once this had cured, I completed with the second half below. It doesn’t make any difference if it is applied in sections like this .. the second section will fuse completely with the first. My idea here was to colour the second half to make the cutting line along the mid-point clearer. This is ordinary, not especially finely ground powder pigment, used in theatre scenic painting. The best way to mix with the silicone is to combine it thoroughly with a very small amount of silicone first .. to wet it in other words .. before adding more silicone. I’ve found that powder pigment blends very readily with silicone rubber. Apparently up to 10% powder pigment by weight can be added to silicone without affecting its properties. For this I used roughly 3g Ultramarine for 40g silicone rubber.

Below is the containment setup I made around the mould using modelling wax, in order to make the first half of the rigid mould jacket, also called the mother mould. This enclosing jacket is necessary, especially when making larger moulds, to keep the flexible silicone skin in its proper shape. There are more, step-by-step photos showing how to model this containment at the end of Modelling wax in the Materials section.

The mould jacket can easily be made using a hard casting plaster, especially if this is left for a while to thicken up so that it can be troweled on over the form. For this though I decided to use Jesmonite .. which is basically the same as plaster but using an acrylic polymer liquid in place of water. This makes the material much tougher, and if a reinforcement such as jute scrim is also used, a stronger but thinner shell can be made.

Jesmonite powder and polymer liquid can be mixed together in a ratio of anything between 3:1 to 2:1 powder to liquid dependant on the pouring consistency needed. 3:1 gives a thicker mix and is more economical since the powder (basically just a fine casting plaster) is by far the cheaper of the two. Contrary to the way plaster is normally mixed .. the polymer liquid should be added to the powder. This should be thoroughly and vigorously mixed until the consistency is even. This is possible by hand for small amounts but the manufacturer of Jesmonite recommends using a special power drill attachment for mixing larger quantities.

Above, I have poured the first small batch of Jesmonite over the mould form and to fill the ring around it. Below, I’ve started pasting small pieces of jute scrim into the wet Jesmonite. More can then be mixed up to cover the scrim .. and the procedure can be repeated to build up a strong shell.  I used two layers of scrim for this small form, but one would probably have been enough .. even for much larger mould jackets such as the one featured in Making a supported silicone mould for a life-size head .. I only used 2-3 layers.

The Jesmonite took very little time, less than 40mins, to set hard and shortly afterwards it was safe to remove all the wax and turn the form over. The silicone will eventually be sliced using the Jesmonite rim as a guide.

But first, shown below, I’ve set up a wax wall for making the second half of the Jesmonite mould jacket. This is exactly the same procedure as before except that the Jesmonite rim needs to be thoroughly Vaselined to prevent the second half from sticking to it!

Here is the completed mould being dismantled. I had thought that applying the silicone in two colours would indicate the line I had to cut in the silicone (i.e. between the rows of ‘horns’) clearly enough. But it wasn’t accurate enough, and in the end I got some parts of the horns on the seam line.

It meant that these along the seam were much more difficult to fill, and my first tryout using polyurethane resin and Fillite didn’t work perfectly.

But for the second attempt I used a very thin, unfilled resin .. Tomps Fast Cast .. manually filling each ‘horn’ bit-by-bit, including a lot of jiggling around with cocktail sticks to dislodge trapped air. It was quite a lot of painstaking work .. but here again is the perfect cast, just to prove that it’s possible!

‘Beginner’s Basics’ – Mouldmaking and casting

The following is a copy of an introduction I’ve put together to go in the Methods/- mouldmaking&casting section, for people who have never done any mouldmaking or casting before but would like to try. It describes the basic processes, lists all that is needed, but above all encourages starting simple! I may add to it or improve it over time.

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 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!

If it can happen accidentally in nature, as illustrated by the fossil trilobite above, can it be so hard to achieve on purpose? What 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.

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 ears for example.

But 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 have to coax the cut surfaces back into the right position, but 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! 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.

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 the posts ‘Two legs good..’ parts 1-3 from January 2012.

Another important aspect that 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, stone, plastic, soft or hard 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 as both 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 24hrs to properly cure, though there are some special fast-curing ones which will 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 first shaking the plaster 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 plaster can take a touch longer.

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 add to the volume of the resin (making it cheaper), to make casts lighter or to change the surface appearance.

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. In 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.

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 a little 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.

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 litre and polyurethane resin which averages £13 per litre. Think of a litre in volume 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 above. Moulds will always be much bigger in volume 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 layers of silicone rather than blocks 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.

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.

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 the mix when fresh silicone is made up

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. 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:

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 5-7mm 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 pealed 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 is more convenient)

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)

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; table-top and floor should 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.

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:

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 remain fluid for some hours and during this time usually all 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.

This mould took 24hrs to cure and parted easily from the prototype pieces. The mould is shown below along with some castings in polyurethane resin (plaster would never be 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 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.

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.

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).

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!

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.

Making a hollow 2-piece cast in fibreglass – Part 1: Making the mould

Please note first of all before you start reading this older post that I have long since included an updated/expanded version in the Mouldmaking and casting section of my Methods pages which can be accessed above.

This part takes the process up to the completion of the mould. ‘Part 2: Making the Cast’ will follow soon. What’s new here is that the words in bold type are given a brief explanation in the alphabetical vocabularly in the page menu above, which will grow as new posts are added. Perhaps eventually I’ll invest the time in making these bold type words into active ‘jump to’ links, but for the moment the vocabulary page needs to be consulted ‘manually’. It will take me a while to do the same with older posts, but at least I’ve started!

I’ve also finally updated most old information on my SkyDrive open site                   http://cid-eaecd1e7a26821ec.skydrive.live.com/browse.aspx/.Public                        which makes available a large selection of my teaching resources (technical info sheets, slideshows, ‘how to’ guidance etc).

I’ve had to take leave of WordPress for quite a while .. not ideal in terms of my plan to post articles here regularly, but the actual teaching and making inevitably takes precedance. There’s been a lot of college teaching, and the short courses I run (with my partner Astrid Baerndal), but equally a number of practical projects such as the following.

I was asked by the sculptor Andrew Logan to make mould and casts from a lifesize head sculpture in clay. Andrew models his subjects fairly realistically but then paints and embellishes them. He also likes to produce two copies; one for the sitter and one to keep himself. The clay sculpture was approximately 570mm high by 200x200mm at it’s fullest point, much larger than the size I’m used to working in. A lightweight, hollow fibreglass cast was really the only reasonable option in terms of cost and the final treatment the surface would receive.

The form was relatively simple, offering a clear dividing line up the sides of the neck-piece and head. If a silicone mould is intended, allowing for some ‘local’ undercutting (principally the ears), the whole can be made in just two parts. Another option, especially when producing fibreglass casts from a straightforward form, would have been to make the mould in plaster. But this usually involves making more than just two mould sections, because with a plaster mould there is no ‘give’, so there can be strictly no undercutting. In addition the inside mould surfaces would have to be specially sealed and coated to allow the fibreglass to release from them. In the end it’s simpler, though more expensive, to rely on the versatility of silicone.

A common option for mouldmaking from a simple bust, i.e. head and shoulders, is to completely cover with a thick skin of silicone which, when cured, can be slit along one side (usually up the back to the top of the head) when it’s time to remove the prototype. This is similar to the first stage of the rhino mould in ‘Two legs good..’ posted January 29. The plaster jacket needed to keep this silicone skin in the right shape later is made in interlocking pieces over it. With this method the silicone part of the mould is effectively a ‘1-piece’ and seam lines are kept to a minimum. This is fine if the cast will be poured either as a solid fill or a so-called slush casting hollow build-up. But fibreglass is a lengthy, manual process in which layers of fibreglass matting and polyester resin are applied to the inside of the mould in sections which are then joined together. The mould itself needs to be made in separate sections which allow complete access to the inside.

Andrew had modelled the prototype in natural grey clay which had been allowed to become firm, going towards ‘leather-hard’. No special treatment is needed before covering with silicone, as long as it hasn’t dried out. Some clay will stick to the silicone when the mould is removed and this can be easily washed away. It’s always best to assume that a soft prototype will be defaced or irretrievably destroyed in some cases during the mouldmaking process. Having decided upon a dividing line which follows the form and creates two roughly equal halves the first step was to set up a dividing wall. With natural clay the two options are either to use more of the same to model a temporary wall, or to use pieces of very thin metal pushed carefully into the surface to form a fence. Using thin metal, in this case 0.1mm brass shim, is cleaner. I’ve cut wedge-shaped pieces of the shim to make pushing them into the clay easier. Shim this thin can be easily scored using a scalpel and snapped to make the pieces. These should be positioned tightly against each other following the intended dividing line, overlapping slightly, and the joins sealed with sellotape.

While this preparation work is being done care should be taken to prevent the clay surface from drying out too much. With leather-hard clay small cracks will almost certainly appear on the surface and these need to be smoothed over before the silicone is applied. In the photo above I’ve covered the back portion with clingfilm, pressed carefully against the surface, while the front half is being prepared. The lower part of the form had been modelled against a wood support which wouldn’t take the brass shim so I had to complete the wall at the base with fresh clay (below). Instead of sticking these parts of clay wall directly to the clay prototype I found that lightly Vaselining these areas still provided enough adhesion for the new clay wall, which later could be removed quite cleanly.

I chose to use a medium-hardness (Shore A 25) silicone from Tiranti called T28. The choice of a softer/harder silicone depends mainly on the amount of ‘give’ or flex required to cope with areas of undercutting balanced against the rigidity needed for the silicone skin to keep its shape within the mould. Both are also determined by the thickness of silicone built up. In order to ensure an even thickness throughout the silicone needs to be applied in succesive layers, beginning with the most important first covering which needs to fill every detail of the prototype surface. Subsequent layers can almost be trowelled on but this first layer should be applied quite thinly using a brush to work the silicone into detail. Most silicones come in a liquid form meant for pouring and a special additive called thixotropic agent is needed to convert them to a paste thick enough to stay put on vertical surfaces. I measured out a starting amount of silicone (150ml) and thoroughly mixed in the recommended amount of catalyst (in this case 5% by weight) before adding the thickener. In the case of this brand, 0.5-2% of thickener can be used according to the supplier so I guessed that 1% would probably be enough to make the mixture reasonably ‘non-slump’ without making it too thick to brush smoothly into surface detail. When applying silicone to a surface that is still relatively impressionable both the choice of brush and the action are important. The brush needs to be rigid enough to push the silicone into detail but not so hard that it damages the surface. I usually use a small hogshair brush with longish bristles, at least for this first coat. It goes without saying that the brush then needs to be sensitively controlled, trying to avoid too much jabbing. I’ve drawn a border on the shim in permanent marker c.1cm away from the clay edge as a guide for building up the silicone (below).

Another way to ensure an even build-up is to estimate in the first place how much silicone is needed in total (for the mould half being covered), divide this into batches and colour each alternate batch. Most silicones can be coloured (I’ve used dry powder pigment mixed thoroughly with a little silicone first before being mixed into the batch) without affecting the properties of the silicone, if not more than 10% by weight. I usually add the pigment at the same time as the catalyst which has the advantage of indicating (when the colour is completely uniform) that both have been thoroughly distributed. In this case I estimated that the surface area to be covered for the first half was 1,200 square cm, and that 1cm would be sufficiently thick for a strong skin. So 1,200 cubic cm (or ml) would be needed in total, built up in 4 layers comprising 300ml each. In practice I found that I needed to mix up the silicone for each layer in two 150ml stages because although catalysed T28 has a working-time of over an hour the thickener shortens this noticeably. Also one should wait until a silicone layer has cured to firmness but still slightly tacky on the surface before applying the next coat (in the case of this brand an interval of about 6 hours) so the overall process takes some time!

When silicone is used to make moulds of this size it is almost always applied in this way, as a thick and even skin, rather than poured into a solid block shape around the form. This would involve using an excessive amount of silicone and be ridiculously expensive! But however thick one chooses to make the skin the silicone is, by nature, not rigid enough to maintain its shape on its own. A second, hard shell is needed, often called the jacket or the mother mould, which encloses and supports the silicone. The pieces of this jacket must interlock as securely as the silicone parts but it’s also important that the jacket is easily detachable from the silicone. For example later in the casting process, once the mould has been filled, the rigid jacket must be removed before the silicone part can be flexed and peeled away from the cast. The silicone is meant to cope with undercutting, whereas the jacket cannot.

There are a number of ways of making the hard jacket: building up a shell in hard-setting plaster; layering plaster-bandage (Modroc); layering fibreglass and resin etc. Here I chose to use Jesmonite which is basically a polymer-modified plaster. It comes in two parts; an alpha plaster and an acrylic polymer liquid. These are mixed together, normally 2.5-3parts powder to 1part liquid, to produce a hard-setting compound which achieves some of the qualities of resin but without any of the health issues. It is certainly strong when reinforced with either fibreglass matting or jute scrim and the shell doesn’t have to be especially thick. It also works out much cheaper than resin at c. £3 per kg if bought in substantial amounts i.e. 20kg (15kg powder and 5kg liquid) for around £60. One can cut the price a little further still, by just buying the Jesmonite liquid and using it with any other hard-setting alpha plaster .. but personally I feel the Jesmonite company is well worth supporting, so I don’t mind either the little extra or the harmless charade in buying their ‘mineral powder’, as I believe they refer to it. Above, a first coat of Jesmonite is brushed onto the silicone and allowed to harden a little before a layer of jute scrim (cut into c.10cm squares) and more Jesmonite is added. No release agent is needed between the silicone and the Jesmonite. The setting time for Jesmonite is very similar to plaster, in that the pot-life of a mix averages about 10-15mins and can be safely demoulded in 40-60mins.

I was careful to fill the fibres of the jute scrim fairly generously with the Jesmonite, using a 1inch stiff decorator’s brush. Because of this I found that I needed only 2 layers of scrim altogether plus a good final covering coat of Jesmonite to create (what turned out to be) a very strong shell. Above is the completed jacket for the first half. I’ve extended it to form a rim around the outside, partly for strength and partly to locate the two halves of the mould better. Normally natches would be built in (that is, stud shapes with corresponding depressions between the two mould halves to locate them firmly. But here I felt that the facetted surface produced by the individual shim pieces would probably do that job anyway. The photo below shows the beginning of the repeat process for the second half of the mould. The brass shim pieces have been carefully pulled out and any damage to the clay surface smoothed over. The blue and white striations of the silicone layers can be faintly seen. The only major difference in the process for the second half (and a vital one that mustn’t be forgotten) is that the silicone/Jesmonite rim must be thoroughly Vaselined to prevent both silicone skin and Jesmonite jacket from sticking to their counterparts.

But I also took advantage of the fact that I could now attach a containment strip of plasticine round the edge of the previous rim which helped a little while building up its conterpart (below).

This isn’t quite the end of the story with the mould halves. Part 2 will see them finished off before starting the fibreglass work.