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

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                                 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.

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