The natural patina formation in bronze alloys
The air contains, among other things, salts, carbon dioxide and, of course, a wide variety of particles. Together with the weather, these affect metals. Copper and copper alloys such as bronze are therefore given a so-called patina. This ranges from an intense, malachite-like green to various reddish-brown shades to brown and black dyeings. Grüne The green patina is colloquially often referred to as verdigris, although it is none (verdigris is poisonous, the natural green patina of a copper alloy is harmless). The coloring of the patina depends on numerous factors. In addition to the concrete atmospheric and weather influences, the concrete composition of the present bronze alloy is decisive.
Factors affecting the color of the patina
Towards the end of the 20th century, the equestrian statue in the Deutsches Eck in Koblenz was restored. Parts of the figure were replaced. At first, the patina could be adapted exactly to the color of the old patina. However, the restored parts were made of an alloy whose composition is different from the original alloy. Therefore, the new patina today differs significantly from the older one.
Bronze can be patinated in an extremely versatile way
It is not possible to discuss all procedures for patina- ting copper alloys or bronze here. This could be a small book filled. Therefore, we limit ourselves to the hobbyists, hobbyists and home improvement often applied patinating techniques. The most important processes for patinating bronze are:
using hydrochloric acid and marble to create a carbon dioxide atmosphere
the workpiece is dabbed with an aqueous solution of sal ammoniac and clover salt
- the bronze alloy is alternately immersed in an ammonium sulphate solution and dried
- the workpiece is brought along An aqueous solution coated with sulfuric acid (potassium polysulfide)
- Patination of bronze with sulfuric (potassium polysulfide)
- In particular, the patination with sulfuric acid is used by do-it-yourselfers because sulfuric acid is relatively easy to produce. In addition comes on two parts potash a part of sulfur blossom (Sublimatschwefel). In a sufficiently large tin can (which will later be destroyed), both are mixed together and heated to 250 degrees with a gas burner.
The container must be sufficiently large as the mixture foams up when heated. Due to the enormous development of an unpleasant and intense odor, the process should be carried out outdoors or in a room with sufficient deduction. There remains a hard, stone-like lump in a dark yellow to black color, the sulfur liver. By destroying the can this can be removed.
Application of the solution for patination
Later, water is added to a container, which is diluted with 10 percent sulfuric acid. Then the solution is painted on. The patina ranges from light brown to black, depending on how often you repeat the brushing. In addition, you can partially polish the patinated bronze to create a contrast.