By David J. Marks
A patina is the thin layer that develops on the outer surface of a metal after it has been exposed to the elements. Acids react with the metal, creating a variety of colors and patterns, depending upon the chemicals used and how they are introduced to the metal.
Ancient metal workers used numerous methods, ranging from burying bronzes in soil, to exposing them to fire, to urinating on them in attempts to gain control over the colors. In the nineteenth century, metal workers in France devised methods of applying heat and chemicals to produce patinas with predictable outcomes. These artisans were called patineurs.
Patinas can be divided into categories according to the methods used for application. Hot patinas, which are applied after the metal has been heated by a torch, are by far the most widely used and they are the fastest method of creating patinas. This is the technique that most foundries use. Cold patinas work much more slowly, taking up to two or three weeks to produce results and consequently are not utilized as often. Fuming is another patina technique which involves placing the object in a tent (for a large piece) or a container (for a small piece). If you are looking for soft color tones, this would be a technique to use.
One of my favorite patinas is a blue green crusty patina which works well on copper, brass, or bronze. The following is the process that I used on the base of my contemporary bench. The base is made from two boxes constructed of half inch thick Baltic birch plywood, which I laminated with 20 oz sheet copper. After roughing the surface of the copper with 60 grit sand paper on a disk sander, I glued the copper one side at a time using slow set epoxy. After all the edges were filed smooth, I sanded the copper with 220 grit using a random orbit sander.
Sanding the metal etches the surface and creates a “tooth” which aids in bonding the patina to the metal.
The formula I use to achieve the blue green crusty patina is:
Ammonium Chloride ½ oz
Copper Sulfate ½ oz
Household Salt ¼ oz
Hot Water 8 oz
Zinc Chloride 1/16 oz
Saw Dust ½ dozen handfuls or enough to cover surface
Acetic Acid or Vinegar 1/8 oz
Anytime that you are working with potentially toxic chemicals be sure to exercise all safety precautions such as wearing an apron, rubber gloves, goggles, and a charcoal respirator.
Here are the instructions on how to mix it:
Always add the chemicals to the water. Never add the water to the chemicals.
Stir until dissolved. Use a spray bottle with water and dampen sawdust. Then, mix the chemical solution into the sawdust saving about 15%- 20% of the solution (to apply later). Spray water onto copper, press sawdust onto surface and wrap with cheesecloth. Spray water so cheesecloth and sawdust cling to copper. Drip remaining chemical solution onto cheesecloth, sawdust, and copper. Wrap with 4 mil plastic. Seal with duct tape, allow to set for 3 weeks.
After the piece has set for two to three weeks, unwrap it, and carefully remove the cheesecloth. With a soft brush lightly knock off some of the loose sawdust, and then set the piece in the sun until it has dried completely. Now take a soft brush and using a stippling action lightly jab at the surface knocking off more of the loose sawdust. While the piece is still warm from the sun I apply a microcrystalline wax, again using a stippling action with a soft brush. Some of the crusty surface will come off, but at this point you will find that you have a nice blue green crusty patina mottled with some soft copper colors in the background. Buffing the surface with a lamb’s wool bonnet on an electric polisher will create a nice, soft sheen.