This step-by-step account was first posted during August 2012. As usual words marked in bold type can be followed up in the Lexicon. I have adjusted certain words or terms, but where I have made major additions or changes since 2012 I have noted them.
I was asked by the sculptor Andrew Logan to make mould and casts from a life-size 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 choice of 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 for the inner mould layer.
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 Making a small mould for a four-legged animal .. in this section. 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. Constant spraying with water is needed (plant spray bottles or empty cleaning-fluid bottles are perfect for this). In the photo above I’ve also 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 successive layers, beginning with the most important first covering which needs to fill every detail of the prototype surface. Subsequent layers can almost be troweled 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’s directions, 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 .. 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.
Note 2014 Tiranti’s T28 has a colourless catalyst. I would recommend always using a silicone brand which comes with a coloured catalyst, whenever possible, until one gets used to the amount of stirring (and scraping of the sides and bottom of the mixing pot) necessary to distribute the catalyst properly. It’s just one less uncertainty to deal with! Otherwise, or if you haven’t got a choice .. at least 3mins continual mixing should be allowed even for small batches of silicone. Nowadays I’ve become accustomed to using Lukasil 429 silicone from specialplasters.co.uk which has a red-pigmented catalyst. This is softer, Shore A 20, but if I want to toughen it I’ve found that adding some Fillite to the mix (after the initial detail coat) or adding recycled silicone granules will do this very well.
In this case I estimated that the surface area to be covered for the first half (that is, one half of the form) 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! As indicated by the photos below, I alternated straight mixes of the white silicone with coloured.
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.
It’s very important during demoulding that the jacket can be separated easily from the silicone first, so that this can be flexed and peeled away from the cast without damaging it. For this reason the silicone skin needs to be built up around features that would otherwise cause undercutting. In the last photo above, the ears for example are completely buried and I even had to be careful to fill out the area under the chin.
What looks like silicone above is actually the beginning of the ‘plaster’ jacket. This can be applied as soon as the topmost layer of silicone has cured. 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, or there are other cheaper polymer liquids available .. 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.
Note 2014 It’s wrong to think of Jesmonite as a viable alternative to resin in every respect though, even though the health and environmental benefits are clear! A shell like this is certainly stronger than plaster, but needs to be thicker and heavier than one for the same purpose in fibreglass. Neither would it be anything like as resilient! Similarly although smaller, more delicate forms are possible with a plaster/polymer mix than would be practical using casting plaster, they’re still much more fragile than those in resin, and the viscosity of the liquid compared to most polyurethane resins limits the range of forms achievable.
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 faceted 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 on this side first 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 counterpart (below).
Jumping ahead a little now .. the second silicone half and its rigid jacket have been completed .. here are the two mould halves on separation from the clay prototype, below. A little of the clay has stuck to the silicone and to the jacket (where a clay wall was added at the base) and this needs to be cleaned off first. With large forms this is easiest to do in the bath using a shower hose, warm water and detergent.
Natural clay is generally one of the easiest materials to clean off, though it can occasionally be rather stubborn in which case an old toothbrush is useful. Deep detail needs to be especially checked. I’ve also found that absorbent ‘micro-fibre’ cloths are ideal for initial drying because there’s no danger of leaving snagged fibres on the mould.
A little more clean-up may be necessary, this time concerning the silicone itself. When a thickener is used to convert the silicone to a paste it makes it easily spreadable for a while but towards the end of its pot life it becomes increasingly sticky and rubbery. It is still usable but it often means that the outer surface of the silicone skin becomes quite bumpy, as below. This is the surface which will receive the fitting jacket, so either Jesmonite, plaster or fibreglass resin will reproduce this surface when applied to it. If there are particularly raised strokes such as the one in the centre of the photo these may catch when it comes later to re-assembling the mould parts ready for casting. These should be sliced off using either scalpel or scissors, and can be done either at this point or earlier before the jacket is made. Another clever technique for smoothing out the silicone surface while it is still wet is to apply clingfilm to it and smooth that out with the hand, leaving the clingfilm there until the silicone has cured. The clingfilm can then be peeled off.
More importantly, the same check should be made on the silicone edges which will meet each other. Air pockets are almost inevitable when working with thickened silicone and these will be filled when the second, corresponding half is applied causing protrusions, illustrated below, which could also not quite ‘marry’ again.
Silicone rubber does shrink a little when it cures, but generally the linear shrinkage rate is so minimal that that this doesn’t noticeably affect the overall size of casts, at least not for normal sculptural purposes. Condensation cure silicones (see ‘Vocabulary’, the moulding silicones most commonly used including this one) on the whole have a shrink rate of between 0.5 and 1%, whereas with addition cure silicones the rate is usually much smaller at between 0.1 and 0.2%. As I said, this doesn’t really have any impact on size, but especially with large skin moulds like this one it is at least one of the reasons why the edge of the silicone skin piece starts pulling away very slightly from its containment jacket, as illustrated below. If this contraction occurred to the same degree and in exactly the same places on both halves of the mould it may not pose a problem but of course this is rarely the case. It means that the two mould halves won’t line up perfectly, the seam on the cast will be more pronounced in places and there will be some more work filling and correcting this. One common solution I’ve seen is to cut dovetail shapes at an earlier stage around the edge of the silicone skin before the containment jacket is made over it. These will lock the silicone in place at least round the edge. I didn’t do that in this case, thinking that I could try another idea.
Here I’ve bent a large-headed pin into a fish-hook shape and pushed it through a strip of ‘gaffer’ or duct tape. The hook was embedded firmly in the silicone, pulled just enough to close the gap and then taped in place. The tape stuck well to the Jesmonite (about a week had passed since making the jacket, allowing it to dry out thoroughly). Since most of the casting work will be done in the open mould halves the hooks can stay in position until these shells are finished.
Below is the front half of the mould set up, ready for starting the fibreglass shell. The standard procedure for fibreglass lamination is to first apply a thin coat of polyester resin (see Lexicon for more general details, including the various types) which is often thickened and known as the gel coat. The purpose of the gel coat, which is carefully brushed on, is to properly capture all the surface detail without air bubbles etc. and to act as a ground layer upon which further layers of resin and matting can be applied. Without this the matting structure would most likely show through the surface. The gel coat is also normally filled or coloured, according to whatever surface is needed on the cast.
However I decided to approach this slightly differently, rather than buying or mixing up a separate gel coat for the initial covering. The casts needed a white surface because they would be painted, so I started by brushing on an unthickened layer of GP (general purpose) polyester resin with an amount of white pigment mixed in.
Good digital kitchen weighing scales, measuring in increments of 1g are necessary for portioning the pigment, especially when working with small batches. Even then it is a somewhat rough measure, but at least I could ensure that I wasn’t adding too much. The initial surface coat is the most crucial part of the casting and if something happens to this due to an incorrect mix one may not discover that until all the rest of the work has been done and the cast is demoulded! I portioned 2g of white pigment per 25ml resin, and since 25ml of resin weighs 28g (the SG is 1.12) this was safely below the maximum. Resins will generally take up to 10% pigment by weight without becoming affected.
I thoroughly mixed the measured amount of pigment with a very small amount of resin (just a teaspoonful) first in a separate cup and put this aside. This, rather than tipping the powder pigment directly into the catalysed batch, is known as wetting and ensures that pigment and resin are properly combined. In actual fact I’ve found that powder pigments generally combine more readily and smoothly with polyester resin than they do with water. I then measured out 25ml of resin in another cup, added catalyst and mixed this thoroughly before combining with the pigment. It is usually better to mix resin and catalyst thoroughly before adding any filler .. at least in the case of polyester resin. Polyester resins need only a small amount of catalyst (normally between 1% and 3% by weight). The reason for this range is that larger volumes of mixed resin will become very hot while curing (the resin is exothermic) needing only the minimum amount of catalyst because the heat itself will accelerate the reaction, whereas a small batch .. such as 25ml .. may therefore need more. In the case of the brand I am using, Tiranti’s ‘Multi-purpose’ polyester resin, the catalyst is supplied in small dropper bottles with directions for dosage. Once catalyst is mixed with the resin one has on average 15-20mins to work with it under normal temperatures. I found during the course of this assignment that I could comfortably work with just 25ml at a time .. carefully fibreglassing an area of up to about 20cm square with each before the resin became too gel-like.
Here, above and below, I first tipped the catalysed mix of resin and pigment into the mould and spread this around both by rocking and using a cheap paintbrush. This coat didn’t need to be thick, but it did need to cover well, evenly and right up to the mould edges. At least with this first coat, it is important to let it cure firmly enough before the next coating is applied. I waited a little more than an hour in this case. If this is done too soon the action of the brush could break up the thin coat and cause fissures on the outer surface of the cast.
For the second, thicker coat I mixed up the same proportion of resin and white pigment but added an amount of Fillite, which is an industrially produced ash commonly used to bulk out resins or other media. After some testing I kept with a mix of roughly 6 teaspoonfuls of Fillite per 25ml resin, which gave a ‘smoothly spreadable sludge’. This is different to thickening the resin with a thixotropic additive, which would normally convert the resin to a non-slump, spreadable but gel-like paste. The Fillite is inert, meaning that although it bulks out the mixture physically it doesn’t chemically change the resin itself. So in this case it needs to be coaxed back up the sides for a while, at least until the resin itself starts to cure and thicken.
But I chose to use this kind of mixture because I could pour it more easily into the nose and ear cavities while spreading the rest over other areas. The Fillite gives thickness and strength while keeping the weight down, but my other reason for using it was that it makes the resin easier to cut into in the event that alterations are later needed to the surface.
There are a number of different types of fibreglass matting. One’s choice depends mainly on the undulations of the surface to be covered. As it comes fibreglass matting appears rather thick and stiff, but when ‘wetted’ with resin it becomes much more mouldable (the binder holding the strands in place dissolves allowing them to move around more). So even a heavy matting (450g per sq metre) can be coaxed into curves .. but there is a definite limit to how easy this is. For this piece I bought a few sq metres of 300g chopped strand mat and did some sample test pieces mainly to find out how many layers I might need for strength. I cut the matting into small (about 10x5cm pieces, ordinary scissors working just fine) but it was difficult to mould these into the sharper details such as the eyes or around the chin. So in the end I used the 300g matting wherever possible but with multiple layers of fibreglass tissue (which is gossamer thin and very mouldable) on the more complicated parts. Below, the chopped strand matting on the left and fibreglass tissue to the right. The strands in the matting are not woven, merely compressed together .. hence the name ‘chopped strand’.
For the test pieces, below, I followed all the intended steps; a thin pigmented coat, a second layer thickened with Fillite (with the aim of filling parts like the ears almost completely) and then just 2 layers of fibreglass matting fairly generously wetted with resin. The result was very strong so I decided that the 2 layers would be sufficient for a form of this size provided that the seam was properly strengthened.
Any work involving polyester resin, but especially fibreglass lamination, can only be carried out inside if the room is equipped with a strong extractor fan .. and not even if one has all windows and doors open! In fact even the extractor fan may not be enough if the work is prolonged. The styrene vapour emitted by the resin is harmful if breathed in.
The best working procedure is, firstly in terms of preparation: to cut up a good stock of small pieces of matting (as I said I cut most pieces roughly 5x10cm but I had some larger ones for the flatter areas and either longer/thinner strips or small squares for more involved parts); to have a number of suitable brushes on hand together with at least 100 disposable plastic beakers; a bundle of wooden stirrers; a supply of paper towels and more than just a few pairs of latex gloves. I also had acetone, for cleaning up any uncured resin and for cleaning the brushes as I went. I had around 5 cheap natural bristle brushes which could be used and cleaned up in rotation. The acetone evaporates quickly so it will not usually affect the resin when the brush is used again.
First a batch of resin needs to be catalysed (as I said, I worked in 25ml batches all catalysed at 2%) and brushed generously onto a smallish area of the mould, a piece of matting laid on top and gently stippled down with the brush then quickly dabbing more resin on top. The stippling action is important (as opposed to stroking) because it forces resin into the fibres better. Also a stroking action is likely to slide the matting around too much. Once one piece has been thoroughly wetted the next piece can be applied, slightly overlapping, and so on until the resin remaining in the cup starts to get a little like treacle. If one has established the right rhythm (and not mixed up a far too ambitious amount!) one should be left with just a brushful or two at this point and these can be smeared anywhere in the mould to make use of them. The mixture will turn from treacle to gel almost immediately at this point so one can’t lose any time in cleaning the brush now. Below, at the mould edge where the two halves will meet there’s no sense in trying to position the matting painstakingly against the line. It’s better to overlap a little, because this can be fairly easily trimmed or filed down later and it will give a stronger, thicker edge.
For the second cast (I had to make two, and most of the photos here were taken during the first casting) I decided to make things a little easier by doing some extra filling of the nose, ears etc. before applying the regular matting. I mixed a fairly thick batch of resin plus Fillite, filled these hollows, and found that if I layered some fibreglass tissue over them I could do all at once without the mixture slumping.
It’s best to trim down this edge within a few days of laminating, preferably the next day, because although the resin will seem quite hard it’s still at its so-called green stage and filing it down will be relatively easy. I found that the type of file shown here, for filing ceramic tiles, worked very well. The edge of the fibreglass needs to be filed down exactly level with(or a touch under) the silicone edge and better if the edge thickness is made to slope inwards slightly. The black gaffer tape is to prevent debris collecting between the silicone mould and the jacket since these should not be separated until later once the parts are joined.
The next step, below, was to protect the jacket edge (on both halves of the mould) with a greasing of Vaseline. This was to prevent any resin from sticking to the Jesmonite surface. It would be impossible to remove if allowed to harden completely on it. The Vaseline shouldn’t come too near the fibreglass edge because Vaseline can inhibit the proper curing of the resin applied as a seal.
The two halves need a sealing line of resin applied to both edges which will glue them together when the two mould halves are re-assembled. This needs to be made the same colour as the surface coat because it will show. Straight polyester resin would be too runny to stay enough on the edge so for this I had to buy some thixotropic paste (the same resin in a stiff, gel-like form) designed to be mixed in with the resin to thicken it up. The resin can be progressively added to the paste (better this way than the other way round) until the desired thickness is achieved and then the mixture can be catalysed as per normal, before adding the pigment. Once mixed, I used a brush to dab it thickly onto the edges, but equally a small hole could be cut on the corner of a polythene bag and the mixture piped on.
As I mentioned before, I dispensed with creating the usual mould natches to locate the two mould halves together because I felt that the edges were contoured enough to lock together correctly. As a guide though I’d traced the edge line of one of the mould halves (which was slightly smaller) on the other half. I placed one on the other securely and the weight of the mould itself pressed them together. There was no need in this case to clamp the halves together.
I waited a day before demoulding, to be safe. Catalysing at 2% often means that it’s safe to demould within 2 hours but full curing and hardness isn’t usually reached before 3 days/one week. I didn’t want to wait this long because I wanted to take advantage of the green stage for trimming and cleaning up. The correct procedure is to remove the rigid jackets first before peeling away the silicone parts of the mould. In any case this is the only possible way when casting polyester resin into silicone because the resin has a tight ‘grab’ on the surface of the silicone.
Also, care needs to be taken to ease the silicone gradually from protruding parts such as ears. Whereas, given the strength of fibreglass and resin, it’s unlikely that the ears would break it’s also a precaution against damaging the mould. The seam line is marked by a thin, ragged wall where the surplus from the sealing resin was squeezed outwards .. this is known as flashing.
I was surprised by the number of small imperfections, by which I mean the small, raised grains which peppered the surface. These were caused I think by minute enclosures of air when applying the first thickened layer of silicone on the prototype surface, which the polyester then fills but which are largely invisible to the eye on the silicone mould itself. I thought at the time (when applying the silicone skin) that I should have used a little less thickener for this coat because it was getting sticky too soon, but it was too late to change it. As with many things .. I’ll do it differently next time! The surface needs to be sanded a little anyway and these blemishes are easy to get rid of. I don’t know what caused the scratch-like indentation on the cheek in the photo above. It wasn’t on the silicone and I can only guess that it might have been caused by a hard particle of something in the resin surface coat which left a trail while brushing. Again, this is a simple thing to repair.
Before cleaning up the surface there was one last and very important step in the casting process; to strengthen the seam inside. So far the two halves are only being held together by a thin edge of resin with no fibreglass reinforcement crossing over and with an almost enclosed form like this it is not possible to apply any to the inside. It is at least possible however to pour resin into the hollow form to run along the seam (from the small opening left at the base). I mixed up some further batches in succession (adding 4 teaspoonfuls of Fillite this time to give some substance without affecting strength) and poured the first straight down to collect in the head area, rocking gently to and fro along the seam axis for a while. I poured the other two, each in turn, from the base opening to run down the side seams. As a precaution I clad the seam on the outside with plasticine, shown below, just in case there were any gaps in the seal I couldn’t see. This was also the reason why I’d left the ragged seam edge as it was for the moment. For this technique it’s quite a good idea to leave a small amount of resin each time in the mixing beaker. When the resin in the beaker gels one knows it’s safe to stop rocking the mould.
So that’s it, basically. Below is the finished cast. Imperfections can be filed or sanded off easily, especially if this is done within a few days before the resin achieves full hardness.
Note 2014 This isn’t the end of the story. There was some alteration to do on the column of ‘hair’ which supports the head, and the fixing of a long metal screw-thread bar so that the sculpture could be mounted on a base-block. I took photos of this work and intend to write this up .. at some point!