Bronze is a golden-brown metal alloy comprised mostly of copper with a small amount of tin or other metal. A high quality bronze piece often contains as much as 90 to 95 per cent copper. And just like with copper, patination occurs on the outer layer of bronze as it oxidises, which changes the original colour of the bronze. The oxidation process and colour change can happen in several ways, which effect the outcome of the patina. Since bronze is a favoured metal used by artists for sculptures, the patination process is often detailed and precise, for a specific intended result for the patina.
Natural Bronze Patina
Natural patina occurs as the bronze piece is left to oxidise from environmental factors. This patination process depends entirely on the bronze's surroundings, whether air, land or sea. The patina is not controlled by humans with the use of chemicals. Weather conditions and even air quality in the physical location of the bronze also determine the patina colour. Natural patinas differ in colour if the bronze piece is covered in earth or water and for the length of time the piece is covered. Since naturally occurring chemicals can't be controlled during the patination process, the end patina colour can vary. The colours are typically limited to blues, greens, black and sometimes a reddish colour.
Hot Bronze Patina
Hot bronze chemical patina gives the fastest results, so it's the most popular among artists for bronze sculptures. The bronze piece is heated by a torch or other method, then chemicals are applied while the surface is hot and porous, allowing for a deeper chemical reaction in the bronze. Specific chemical recipes are sprayed or brushed on the hot bronze to reach desired patina colours. Even with this technique, the patina colour can vary slightly on the same bronze piece, since some areas of the bronze might cool sooner and reheating might be required. There is a wide variety of chemical formulas available for purchase by artists to get the patina of choice. Options include black, several shades of brown, several green or blue shades, rusty reds, white or a combination of any of these.
Cold Bronze Patina
The bronze patination process is similar to a the hot bronze technique, except the chemicals are applied to an unheated piece of bronze. The patina colour takes hours or days to show results and can require several applications of chemicals to reach the desired colour. The process is still favoured by artists for the depth of colours achieved that appear to reach deeper layers of the bronze, much like a natural patina does. This patination process takes additional time but doesn't require the use of a blow torch or other special heating elements on the bronze piece. Chemicals are rinsed from the bronze when the desired colour is reached.
Buried Bronze Patina
An ancient patination technique consists of burying the bronze in a variety of mediums, such as dirt, sand, straw or sawdust. The chemicals used to aid in the patina process often meant using readily available sources such as mammal urine, sour milk or aged wine. Some of these techniques have been borrowed by the current patination process for bronze, although the piece is often wrapped in plastic or buried in sawdust to surround the bronze piece after applying a purchased chemical solution. The chemicals, whether natural or purchased, can be rinsed from the bronze at any time the desired patina is achieved.