MAKING A BRONZE SCULPTURE
1/ Making a silicone rubber mould
The following is an explanation of how I made the Agony bronze sculpture. A bronze sculpture is produced by first making a silicone rubber mould from the clay form. I apply silicone rubber using a paint scraper. I begin by positioning each of the undercuts horizontally so that the silicone rubber can be scooped into the cavity. It is important that the undercuts are filled so as to make the silicone easy to remove. Sufficient thixo is added to prevent the silicone from being too runny. Four layers of 2-3mm thick silicone are built up allowing each layer to set before adding the next.
2/ Making an outer support mould
In order to support the flexible rubber mould, a rigid outer mould has to be made. Plaster reinforced with fibreglass is used for small to medium sized sculptures. For larger sculptures a fibreglass mould is produced, making it much lighter and easier to handle.
Before making the outer mould, the position of the parting lines need to be determined. With less complicated pieces I use a two piece outer mould. In the case of this complex sculpture I needed to produce a three piece mould. A combination of sewing pins and packing tape are used to produce the parting lines, allowing the outer mould to be easily separated.
I add a layer of clear gel coat first, followed by two layers of fibreglass soaked in polyester resin and then gel coat for the outer layer. The gel coat gives the mould more rigidity and provides a smooth surface.
After all parts of the fibreglass outer mould have been created, the pins and packing tape are removed. The edges are trimmed using either a hacksaw or coping saw. A sequence of holes is drilled into the flanges of the mould so that nuts and bolts can be used to hold the mould together.
Finally, the outer mould is pried apart. The rubber mould is carefully cut with a scalpel and peeled away from the clay sculpture.3/ Making a wax
The ideal thickness for the wax for bronze casting is 4mm. If the wax were much thinner, it could constrict the flow in the mould and if it were thicker than 7mm, it would increase the weight of the sculpture and could have hairline fractures.
In producing the first wax coat in the silicone mould, it is important to reduce the amount of air bubbles. This achieved by pouring hot wax so that it fills one quarter to a third of the mould, and rotating the mould smoothly so that all areas are coated with wax. It is important not to let the wax linger in one place before pouring the wax out of the mould, as this causes uneven thickness. Straight after pouring this coat, a second layer of cooler wax is poured. Very little of the wax collects on protrusions inside the mould. In order to address this strips of 2mm warm thick wax are placed over these protrusions. A third layer of wax is then poured. Depending on the thickness of the wax, this is usually the final layer. I weigh the wax and use this as reference for making future waxes of this sculpture.
4/ Adding sprues and risers to the wax
I then courier the wax sculpture to the foundry where the following steps are carried out. A wax cup connected to wax tubes (sprues) are attached to the wax sculpture. The sprues will deliver the molten bronze. Risers are also added, which allow air to escape from the mould. There are two methods to allow casting of a hollow sculpture. Some foundries fill the interior of the wax sculpture with core material and hold it in place using nails pushed through the wax. An alternative method is to gain access to the interior of the wax sculpture by cutting out several sections of the wax wall, allowing a ceramic shell to be made of the interior.
5/ Making a ceramic shell
The wax is then covered with several layers of a ceramic refractory coating. This is achieved by dipping the wax into a silica slurry followed by coats of silica sand in increasing grit sizes allowing enough time for each layer to dry. After several days a ceramic shell of least 2.5cm is produced.
The sculpture is now inverted and heated in an oven to melt the wax so that it can be recycled. Any remaining wax is burnt off at high temperature. The heat also helps the ceramic shell to strengthen.
6/ Pouring the molten bronze into the ceramic mould
After melting the bronze in a furnace, the crucible is lifted and the molten bronze is poured into the mould.
7/ Removing the bronze from the ceramic mould
Once the sculpture has cooled, the ceramic shell is removed using a hammer and cold chisel followed by sand blasting. Afterwards, the nails are removed from the bronze followed by the core. The sprues and risers are cut off using an angle grinder, along with the flashings (the areas where bronze has seeped through fractures in the ceramic shell). The nail holes and any large pits in the bronze are welded.
8/ Fettling and chasing the bronze
I carry out the following steps in my studio. The stubs from the sprues and risers are ground and sanded to match the surrounding surface. Once finished, the sculpture is polished using a buffing wheel and buffing compound.
9/ Applying a patina to the bronze sculpture
The purpose of the patina is to provide contrast and colour to the sculpture. I use three colours: brown (ferric nitrate), black (potassium sulphide) and green (cupric nitrate). I start by heating the sculpture using a blow torch and then spray several coats of the chemical compounds onto the surface. Fine steel wool is used to remove the patina from highlighted areas. By building up layers it gives a graded change in colour. Once the desired colour is achieved, I add wax to the sculpture whilst it is warm and then polish the surface.
The bronze sculpture is bolted to a stone base. Underneath the sculpture bronze plates have been cast that are used to bolt the sculpture to a stone base. This involves drilling and tapping the plate so that a bronze countersunk bolt can be screwed in. A hole is then drilled into the stone base and counter sunk. After the sculpture is bolted on to the base, the underneath is covered in felt and the sculpture is finished.