LOST WAX CASTING
HOW TO MAKE A BRONZE SCULPTURE
In all his work Professor Pavesi has used the lost-wax casting technique, an ancient method of casting already in use in 400 B.C., and used to make the Riace Bronzes.
ALL THE STAGES IN THE CASTING PROCESS
For the purposes of this description, a bronze head is being cast.
The work is modeled in clay to the desired size, usually also making use of an internal armature to reinforce medium and large works.
While the work is still damp, it is divided into sections using metal spatulas. In the case of a head, a circular motion is used to insert the spatulas, starting at the top so as to divide the head into two halves, front and back.
Being very careful to leave no part uncovered, a first layer of very liquid plaster is applied to the entire head. This layer is gradually made thicker by applying a thicker plaster mixture, until it is 2-3 centimetres thick. When the plaster has hardened, the spatulas previously inserted in the clay are removed. This leaves a gap in the plaster that allows it to be separated into two parts, front and back.
We have therefore created two negative moulds of the head, the front negative and the back negative. You then clean the mould and lubricate the inside of it with a mould release agent, before once again pouring liquid plaster into it.
The mould releasing agent makes it easier to get a positive form in plaster from the mould, and we will have two, the front and back halves, which when joined together will have the original dimensions of the clay model. At this point in the process, you can correct any imperfections arising from the various casts and recasts in plaster, and thus we come to the casting process.
We now have two positive moulds of our head. Using a new mould releasing agent we now create two new negative moulds, this time, however, instead of plaster, the moulds are created using a special type of silicone.
For the second time, after cleaning and lubricating the negative silicone moulds, liquid wax is poured into them. As with the plaster used earlier, once the wax has set it will give two positive moulds of the head. When put together they are the same size as the original clay model.
The two parts in wax will be united by gently heating the inner edges, having first filled the hollow part with fire-resistant material. Our head is therefore made of wax and full of material that can withstand high temperatures. Plaster is then applied as before: the head is first covered with a very liquid layer, until the layer is sufficiently thick, which can vary quite a lot depending on the work.
A large block of cement is made to house the wax head. The block will be put in a furnace at a medium temperature, until the fire-resistant material gradually becomes a solid mass.
The temperature of the furnace is then increased until it is hot enough to melt the wax inside the block.
Without going into every tiny detail, the reader must imagine that the block has an entrance and an exit connected to the work on the inside. Funnels known as pour channels are attached for the molten metal and to expel wax, gas and air. Once all the wax has come out of the block, a cavity will remain inside it that has the exact shape of the head. The exit funnel is closed and liquid bronze is poured into the entrance funnel until the gap has been completely filled.
By controlling the temperature inside the oven, the block is gradually cooled. It is then broken and the bronze head, which has taken shape by solidifying in the cavity, can be extracted. It is then smoothed off and given its final polishing.
At the end of the casting process, the head will be shiny with a golden colour typical of bronze, and ready to be given a patina.
There are several different types of patination and Professor Pavesi uses a method that dates from antiquity and requires a great deal of skill. After the final polishing, or in the case of large pieces, after the various parts have been soldered together, the bronze is heated to 400° Centigrade. At this temperature the bronze molecules spread out and can absorb the mineral oxides that are rubbed over it, retaining their different colours. It is a bit like adding colour to the bronze mixture when still in a liquid state, but by doing it at a later point different colours can be obtained, or it can also be done at this stage to highlight specific details of the sculpture.
The process of patination described above is the best method, because instead of covering the work in colour, it colours the very material as if it were still in a liquid state, allowing incredible gradations of colour to be achieved, which will always express the beauty of the minerals used.