Bronze
The Lost Wax Process
Bronze has been used for artistic sculptures for at least 4,500 years (2500 BCE) and used for tools and weaponry for over 6,500 years. Although traditionally thought of as a Copper and Tin alloy in a ratio of 88%:12%, over time and in different regions of the world, the constituent minority elements of bronze alloy vary widely. Most modern sculptures are made from Silicon Bronze, as it is preferred by artists for its excellent fluidity when molten, its strength and superior resistance to corrosion. This durable copper alloy makes it the perfect material for creating long lasting works of art.
The lost wax method of casing bronze is a traditional method that is still used today because of the ability to reproduce the very fine detail intended by the artist in the final bronze sculpture.
The original model is first created by the artist in their preferred medium (often clay or a sculpting wax over a wire armature). When the artist has finished the original, a negative mould is taken from it using a flexible silicone. The mould will be made in multiple parts, depending on both the size and complexity of the sculpture. A hard removeable fibreglass or plaster shell is then added to the mould, to support the more delicate silicon.
Melted wax is poured into the silicone mould to create a hollow wax positive of the original sculpture – a wax pattern than will eventually be ‘lost’. The thickness of the wax will determine the final thickness of the bronze (usually 6-8 mm).
The artist carries out any final work required to the wax to ensure the original model is authentically reproduced.
The wax pattern then has ‘runners’ and ‘risers’ attached (traditionally called sprues), which are also made from wax. These channels will later allowed molten bronze to flow freely through the mould and for air to escape preventing imperfections in the final bronze.
The wax pattern is then dipped into a ceramic slurry liquid and sprinkled with fused silica up to twelve times to build up a strong ceramic shell investment. When fully cured the investment mould is ready for wax removal.
The wax is melted out (or ‘lost’) from the ceramic investment (sometimes referred to as a shell), by heating in a kiln. Whilst still hot in order to prevent thermal shock, the ceramic investments are positioned ready for the bronze pour.
Bronze which has been melted in a crucible inside a furnace at 1,100 degrees Celsius is then carefully poured into the ceramic investment.
After cooling, the ceramic investment is broken away. The bronze casting will have bronze runners and risers attached to the bronze artwork, which need to be cut away. Skilled metalworkers chase and fettle to restore the fine surface texture and detail back into the work to match the artist’s original.
The final stage is patination. A variety of acids, metal salts and dioxides can be applied to the surface of the fresh bronze to give the desired finish. Patinas can range from an almost black, traditional dark Victorian style bronze to more vibrant and creative red, green, blue and white tones. A final protective lacquer or polish wax is often applied once patination is complete to slow down further natural oxidation that occurs with all metals over time.