There is a general misconception among those who appreciate the beauty of a sculpture in bronze that the surface colorations are an integral part of the bronze itself. The truth is that the magnificent tones and visual textures are a result of skillful treatment of the surfaces with a variety of chemicals, usually applied with heat. "Beauty is skin deep" applies here as the color or "patina" merely resides on the surface and does not penetrate the bronze, itself. It scratches as easily as a painted surface...maybe easier...and is tougher to repair.
1. Avoid direct sunlight in placing your sculpture.
2. Avoid fluorescent lighting over long periods of time.
3. Be sure your guests or gallery goers keep their distance from the piece. Hands are a source of oils that can mark the surface. Rings on fingers can do real harm to the patina.
4. Clean outdoor sculpture to avoid the damage of pollution. If it's gone too long, get a pro.
5. Never use "spray can" waxes. Hand-applied, thin coats of Carnuba-based waxes are acceptable.
6. Remember that patinas provide no real protection for the sculpture.
7. Be aware of what effect natural and not-so-natural forces are having on the work over time. "Silent Destroyers" (dramatic, isn't that?) can turn your "Beauty" into a "Beast".
What is one to do? First of all, let's make a point of protecting the delicate surface of the patina. Tell your friends, "Hands Off!!" Repairing a scratched patina can be very costly. Ten minutes after one of my shows opened, I found a long scratch on the peice made by an admiring visitor. I was required to remove and re-apply the patina at several hundred dollars cost to me.
Bad news. Patinas get better with age, but only to a point. Then they can get really 'grungy'. Outdoor pieces should be cleaned by a professional whenever they show signs of weathering. I always wipe a thin film of Carnuba paste wax over the sculpture before it leaves my studio, hand-buffing it to its orignal glow. Maybe once or twice a year, an additional application helps, especially if the piece is outside, but don't overdo it. Oh yeah, remove bird "poop" with water as quickly as you can.
Obviously, the worst enemy of steel is rust. I treat exposed steel elements in my sculpture (beaks, feet, legs, etc.) with clear lacquers or urethanes. When I apply just enough heat to the bare steel it brings out beautiful blues and oranges. To preserve the effect, I try to include ultraviolet inhibitors with the clear coat. I'm just as protective if the steel has been overlaid with copper or bronze. With pieces in the out-of-doors, the problem is that extremes of cold or heat can develop minute cracks in the coatings that may let moisture in. In my work it is not unusual that the steel surface may have areas of openwork that expose the inside structure and defy sealing. In those cases, vigilance is a virtue.
For the most part, the resin aspects of my sculpture have a heat-cured, urethane coating that includes an ultraviolet inhibitor. Color should remain stable for years to come. I lightly wax each piece, again with a Carnuba-based product, at its completion. My cast pieces are usually of polyurethane resin and have more resilience than polyester products which, because they are more brittle, can develop surface crazing. Works 'built up' with polyester-based resin will have additives and steel structure that provide more stability. All that should be needed is an occasional dusting with a soft, slightly damp cloth. Never use spray-waxes and keep sticky fingers away.