Introduction: Hot Patina on Bronze
Here are some basics on applying a hot patina to a bronze sculpture. For this piece, three different chemicals were used: bismuth nitrate, ferric nitrate, and cupric nitrate. All of these chemicals are hazardous and care should be taken when handling them. The chemicals can be applied with spray bottles or natural hair brushes (synthetic brushes will melt when they touch the hot bronze). Use clean containers to hold the chemical solutions and don't use the same brush with different chemicals. There are literally thousands of patina recipes. Here is a link to a site that will give you measurements for mixing your patina formulas: http://www.sciencecompany.com/Patina-Formulas-for-Brass-Bronze-and-Copper-W160.aspx
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Step 1: Clean Bronze Casting
Make sure your object that you're applying a patina to is free of dirt and grease. You can scrub it with a solvent (xylene or acetone works well for removing grease), or a bit of soap and water. Rinse it well with clean water and you're ready to go.
Step 2: Heat the Bronze and Apply Undercoat
Heat the bronze evenly. All steps are important but nothing can ruin a patina faster than having hot spots. Heat the entire piece to a straw yellow or honey gold color and then let it cool a bit. Bronze is a good conductor and the heat will dissipate quickly to eliminate hot areas. The metal should be hot enough so water sizzles but does not ball up and run off (sort of like when you used to spit on a clothes iron when you were a kid). You can test the temperature by brushing or spraying a little clean water on the surface. Bismuth nitrate is applied first with a spray bottle to give an opaque white coat. Keep in mind that many layers of chemicals will be applied so build up the bismuth just enough so that the bronze is covered but no more. If the bismuth gets too thick, the chemicals applied on top of it will tend to flake off because it is not able to create a chemical bond to the bronze.
Step 3: Apply Ferric Nitrate
After the undercoat is applied, begin to brush on ferric nitrate. Keep the piece warm by periodically applying heat with your torch. For this piece, the first layer of ferric nitrate was stippled on to create a modeled effect. If the sculpture is textured and you want to take advantage of it, don't worry about making the patina too even.
Step 4: Build Up the Patina
Continue heating and applying ferric nitrate to the entire piece. When stippling, be sure to rotate your brush so that patterns don't become evident.
Step 5: Apply Cupric Nitrate
After you're done with the ferric nitrate, begin applying cupric nitrate to add complexity. This layer is less even than previous steps to accentuate certain parts of the piece. You can focus more heat on certain areas to achieve darker tones or even put the torch flame directly on the cupric just after it's applied. This will make the cupric go opaque bluish-green. If working cool, be careful that you don't "drag" the chemicals underneath as this will lead to a muddy looking patina. Some of this layer can be done by dry brushing. Shake most of the chemical off the brush and very light brush the high layers of the piece. This is especially effective in bringing out the texture of a piece.
Step 6: Add Complexity
Continue working the entire piece. Vary the amount and type of chemical to bring up the texture and add dynamism to the sculpture. Don't try to make it perfect or you will drive yourself crazy. Keep in mind that you'll be applying a couple coats of wax at the end which tends to even out the piece and add surface depth. Darker patinas can be waxed when the piece is warm to add richness, while opaque patinas like the one in this instructable are better off being waxed when the piece has cooled.
Step 7: Wax It
After the piece has cooled to room temperature, apply two coats of carnuba based wax. Stipple the wax over the entire piece, let it set for a couple of minutes, and rub it off with a clean, soft cloth (micro fiber works well for this). Apply a thin second coat and buff it out to give it a little sheen. Don't let the wax sit on the surface too long or it gets difficult to rub off. Sculptures should be waxed at least once a year to maintain the patina. If the piece is outside, waxing two or three times a year is necessary to keep it looking good. The patina described here is moderately durable and will change considerably if not maintained.