Bronze – Patina, Bronze Alloy, Cleaning, Waxing, Bronze Disease

Patina

For thousands of years, bronze’s rich warm patina has made it the fine art sculptor’s preferred medium. Its beauty is more than skin deep, however; the moment freshly cast bronze is exposed to the atmosphere, carbon dioxide begins to react chemically with the copper, creating a micron-thin airtight barrier against the corrosive effects of further oxidation (note 1).

Colour can be any tone from light green to dark brown to mahogany to black, depending upon the purity of the alloy composition and its atmospheric environment. (For example, in Victorian London, monuments erected in public places became blackened due to sulphuric acid in the coal smoke).

Final colouration may take decades to reach maturity, however, the creation of a custom patina is completed in a matter of days by our expert patineurs (note 2).

Bronze Alloy

When casting sacred sculpture, Chola bronzesmiths used the highest quality bronze alloy their technology allowed. A thousand years later we feel the same commitment and prefer to use Everdur, a premium sculptural bronze (95% copper, 4% silicon and 1% manganese). Everdur is formulated for purity, strength and predictable patination, whether faux or natural.

Panchaloha, literally “five metals’ was traditionally composed of 8 portions of copper, 8 portions of brass (an alloy of copper and zinc), 4 portions of silver, 1 portion of gold, and a small amount of iron. The formula was deemed auspicious and in the distant past was used extensively when casting sacred sculpture and much later, jewelry. Today’s panchaloha, while still containing some small portion of copper and brass, substitutes lead for silver and gold. Unfortunately, the term is used to describe almost any metal object exported from India associated with spirituality, trading on panchaloha’s ancient prestige to market inexpensive metal objects of poor quality.

Cleaning

Regardless of which protection you choose, your sculpture will need cleaning from time to time. We recommend you dedicate cleaning tools and material for use exclusively on your sculpture. Residue from brushes and cloths used for any other purpose may chemically react with the bronze. To prevent scratches when dusting with a brush, we recommend applying a protective layer of tape around your brush’s metal ferrules.

If your sculpture is installed outdoors, in the spring and fall wash with very mild soap and a soft brush, rinse thoroughly with tap water to remove all soap residue and allow to dry completely. This is an excellent time to apply a fresh wax layer, as even mild soap will strip old wax. Never use cleaning sprays or furniture polishes. They often contain chemicals which chemically react with elements within the bronze.

Waxing

To wax or not to wax is a matter of personal preference. Unwaxed bronze’s colour will age naturally from a warm, bright shine to shades of black, red or pale green, depending upon the proportions of copper, tin or lead in the alloy. If at any point, you wish to preserve the current colour of your patina, applying a coat of wax acts as a barrier to further oxidation. In a dry climate, waxing once a year is sufficient; twice a year in areas of high humidity. If your sculpture is outdoors, it requires two coats. In any case, we recommend applying was outdoors at the height of summer, when the heat ensures the wax is absorbed deep into the pores. We also recommend using a premium clear paste wax such as Renaissance micro-crystalline wax polish, available from either; Picreator Enterprises Ltd http://www.sculpturedepot.net/clay-wax-tools/product.asp?Patina_Sealers, or Amazon.

An alternative to wax alone that’s been gaining popularity in recent years is a base coat application of a lacquer specially formulated for bronze sculpture. It comes in gloss or matte finish and has the advantage of permanent, thorough protection from oxidation, as well as oils from handling, from moisture, and even ultraviolet radiation. The disadvantage to lacquer is that it will ‘freeze’ its current patina colouration. These products are known as patina sealers and available from the same sculptor’s supply stores as the waxes.

When cleaning an older bronze, remember the patina is only a micron or two in thickness, so delicately apply just the thinnest wax coating. Ideally avoid polishing, handling or rubbing altogether.

Bronze Disease

When bronze is protected by its patina it is one of the most durable of metals. A pair of Greek bronzes, The Riace Warriors, were found on the Mediterranean seabed where they’d lain for over 2500 years https://en.wikipedia.org/wiki/Riace_bronzes. The high quality of the bronze alloy used in their casting ensured their patina protected them. 

Ideally, patina covers the surface of a sculpture more or less uniformly and its lovely colouration is permanent protection against heat, humidity and atmospheric pollutants. However, if the bronze alloy or the casting is of poor quality, the oxidation process may spread beneath pitting and other surface imperfections to produce acid (normally hydrochloric or hydrosulfuric acid) which eats deeper into the bronze. Bronze Disease may initially present as a small furry ‘bloom’ of bright green or turquoise and may spread to other nearby bronzes.

Unfortunately, there is no permanent cure for the disease and many methods only halt the spread for a time until the metal and air interact again. Various treatments, such as baking at high temperatures, soaking in distilled water, and application of chemicals such as benzotriazole (BTA) are treatments. The course we recommend is to remove as much of the loose bloom as possible with a wooden implement to avoid damaging the patina surrounding the area, heating the affected area with a blow dryer or direct sunlight and finally, an application of either a premium clear paste wax such as Renaissance, or a specialty lacquer (see above). The goal here is to maintain an impermeable barrier between the atmosphere and the metal, a barrier that should be renewed regularly to avoid re-occurence.

Note 1 – this layer is two to three thousandth of an inch in thickness, or 0.05 to 0.07 mm. 

Note 2 – it’s a time-honoured practice. The lustrous red-gold patinas of Italian Renaissance bronzes are the result of a coat of special lacquer.