SCULPTURE MOULDING & CASTING
by Christine Charlesworth
CREATING A PORTRAIT HEAD
After spending some time with the subject, to enable us to get to know each other, I will decide how best to depict their looks and personality. First I take a number of photographs, in a selected position, from all angles, but these photographs may not be the ones I work with as initially the sitter may not be completely relaxed. I will then take some measurements all over the face and head, including measurements of shoulders and chest if I am making a head and shoulders portrait.
Back at my studio/workshop I make an armature out of aluminium lengths and wire, fixed to wood on a board, to the required internal size to make the head and shoulders in clay. Using the measurements taken from the sitter I start to block in the basic shape of the head with clay ready for the first sitting.
At the first visit to the studio I take further photographs and measurements. I will begin to block in the main facial details in clay during this sitting. Between each sitting I am able to work on the clay using the photographs and measurements, but will probably require approximately three or four sittings of around an hour and a half before the sculpture is complete.
Once the finished clay has been approved I will sign the sculpture and begin to make the mould.
MAKING A MOULD
The mould is made by painting on layer upon layer of silicon rubber. The first layer is very important and is quite thin and runny as this layer has to be carefully blown into all the details, such as eyes and all the undercuts. During the layering process ‘shims’ (thin walls) are created to form the edge of the mould. The number of walls depends on how complicated the mould will be. Usually a head is only a two-piece mould, which opens out, with the face in the one section and back of the head in the second.
The rubber layers are built up the shims and further layers are added to the main part so that there are no undercuts showing and the whole of the surface is coated in a final thick layer of rubber, which is fairly smooth. In total there are usually approximately 6 coatings of rubber, in varying thickness. Next, the rubber covering the shim-walls is cut neatly to form a clean edge for the resin jacket to cover. Round holes of approximately half an inch are cut at two to four inch intervals along the shim wall, just deep enough to meet with the shim beneath the rubber. Once the rubber mould is prepared a hard support ‘jacket’ is made using resin/fibreglass and special packing-fillers. Again adding layer upon layer. This resin is painted up and over the rubber, to the outer edges of the shims, making sure that it is pushed into all the round key-holes that have been cut in the rubber. These will form studs, which will clip into the holes in the rubber and keep it in place when the finished mould is being filled. When this jacket has completely cured and hardened, usually within 12/24 hours, the thick shim wall is drilled through, at intervals, to fit the nuts and bolts that will hold the mould together when it is filled. The edges of the mould have to be filed down to display where the shims finish and the finished jacket is then prised open. The rubber is carefully cut with a scalpel all along the seams, cutting down into the clay surface. These rubber sections are then carefully pulled off and inserted into their jackets.
The clay is returned to the clay buckets and the rubber is cleaned of all clay residue and prepared for casting. This mould is suitable for casting in either Bronze Resin or in Bronze at a foundry using the lost-wax process.
BRONZE RESIN CASTING
Once I have created the mould I can then start the Bronze Resin casting process.
The first layer painted into the mould is called the 'gel-coat'. This is the part that you see on the finished sculpture and is made by mixing bronze powder (in a high percentage) with resin and a special catalyst. When this gel coat has dried, layers of resin with slate powder, followed by layers of resin with fibreglass strands, then layers of sheet fibreglass with resin, then resin with slate powder are painted or stubbled into the mould. It is important that each of these layers is completely dry before the next layer is introduced to eliminate heat build-up in the resin. A figure would have metal bars fixed where necessary to add strength to the finished sculpture. The mould is then seam-cleared, seam-packed and then fitted and bolted together and left to cure for 48 hours.
Once cured the mould is un-bolted. It is split open, taking the outer jacket sections off first. The rubber is then gently peeled away to display the sculpture. The rubber is fitted back inside the jacket and the mould allowed to ‘breath’ in open sections for several hours before fitting back together, empty. The sculpture looks quite strange at this stage with resin protruding out in thin walls all round the seams where they join together. This all has to be filed away, cleaned, prepared and small areas filled, then a thin layer of the outer 'gel-coat' applied so that there is no indication of the join. When this has cured, the whole sculpture has to be completely rubbed over with wire-wool to cut through the resin and display the bronze surface.
At this stage the sculpture is hollow. Large standing figures have their feet and part of the legs filled with a resin compound, then special nuts are fitted to take stainless steel rods for fixing the sculpture to a plinth or ground. Seated figures and portrait heads with shoulders are packed inside to a certain level then a resin compound is poured in to a depth of approximately two or three inches and a nut is fitted for fixing the sculpture. The underside of the sculpture is filed round the edge and then has a thick resin-compound added all round this edge before being placed onto a flat surface to make the underside smooth and suitable to fit onto a flat surface. Once this is cured it is lifted off the smooth surface, filed and any final gel coat added if needed.
The prepared surface of the sculpture can now be patinated with acid to make it darker, if required, or changed in colour using various other chemicals. It is then washed ready for waxing. Once waxed the finished sculpture is suitable for displaying inside the home or outside in the garden.