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My earliest experiences with applying patinas to bronze involved using materials like cupric nitrate, ferric nitrate, and liver of sulphur, that were relatively hazardous to breathe or touch. As time went on, I was introduced to some more user friendly options and some creative variations. I decided it would be advantageous to create a supply of bronze test strips that I could use to try out potential patina choices, so I made some textured clay strips that I then used to make a plaster mold for casting waxes. After pouring a couple of dozen of these in bronze, I was ready to experiment.
The test strips allow me to view and document the effect of variations of application - hot or cold , number of coats, exposure time, brushed on or fumed, flat surface or textured. When using a manufactured product like Birchwood Casey's Antique Black or Brown patina that can be applied either cold or hot, it's handy to be able to compare test set times and colors of both options before actually applying patina to your final piece. (*Note: smaller containers of BC patinas are available through Sculpt Nouveau.) When I tested the hot application of a variety of oils, I found that I could get much darker colors with Seseme oil that peanut oil. (Olive oil and linseed oil can also be used.) I also found that motor oil should be tested outdoors if you do not have a well ventilated studio (duh!). Another application that can be somewhat problematic (but not toxic) in terms of smell, is urine on heated bronze. I understand that urine can also be used to fume on a patina by mixing with sawdust or wood chips in an airtight container with your bronze. In either case, I would recommend sealing with clear satin spray lacquer.
Fuming is another technique that benefits from testing. The amount of time your piece is exposed to fumes definitely influences the shade and intensity of the patina. Ammonia will give you varying degrees of blue and ammonia mixed with vinegar (and a bit of salt) will produce greens - depending on the proportions mixed. Another way to achieve a green patina is to apply a 50/50 solution of Miracle-Gro with water to a heated sculpture. These patinas tend to be somewhat unstable and should be sealed with clear satin lacquer. Sealing will alter the color somewhat - yet another good reason for utilizing test strips!
Cynthia Smith, Maine artist, originally from Connecticut. Taught art at secondary level for 35 years, retired in 2004. Sculpts in bronze, wood, stone, clay & plaster. Her work can be seen at several mid-coast Maine galleries and shows.