Sunday, March 4, 2012

White Coating in Jewelry

Silver has well-known properties in jewelry art. For example, it blackens easily from exposure to the environment. Sulfur and nitrogen dioxide and trioxide gases, which are present in the air as pollutants, react with the surface of the silver to form black compounds such as silver nitrate and silver sulfide. This happens especially fast on unpolished or matte surfaces. From my earliest work, I very much wanted to use this property of silver as part of the design process – using the contrasting white and black surfaces. This is made easier by covering a surface with oxide, a liquid sold in shops for jewelers that allows us to blacken the silver. The metal can be made as dark as you like, while you are watching it. The untreated surface (sealed from exposure to the liquid oxide) remains silver-white. 
 “Leaves” earrings with black and white silver surface. I found there was a problem in keeping this contrast fresh and vibrant, because white silver naturally darkens faster than blackened silver. If the design in a jewelry piece
depends on the black and white surfaces, the visual effect can be lost over time. I have tried a number of strategies over the years: varnish, wax coating, cold enamel, and a chemical preservative called benzotriosol. One or two layers only gave the silver a fragile protection. On the other hand, if the coating was too thick, it changed the surface texture – which always changed the design. Having learned that it’s possible to plate silver with rhodium (a hard protective coating that looks lovely when plated on gold), I wondered if I had found my solution. Rhodium can be finished to any texture. I love and often use what’s called a “sateen” texture, which is gotten by polishing and then scratching the surface with a thin needle. My “Leaves” jewelry set was made with this combination of dark and light surface and a sateen finish. I took a “Leaves” ring to a chemist who does rhodium plating, and I asked him to cover only the white silver, leaving the black exposed. When I received the ring back, it looked no different; I was pleased that the texture had not changed at all. I then dipped the ring into oxide – and it turned completely black! There was no difference between the rhodium-covered and uncovered surfaces. Certainly, no client will dip a silver ring into oxide. But I proved that even rhodium will not protect silver from exposure to the atmosphere. In fact, the air around the Dead Sea (full of bromide) darkened another test ring and gave it a yellowish patina. I went back to the chemist, showing him a completely black ring.
 A silver jewelry piece covered with rhodium, after partial immersion in oxide. Because the part that had been treated was darkened, the chemist refunded my money. As far as he was concerned, my problem remained unsolved. Then I found out from another chemist that rhodium plating is a porous covering. Possessing high mechanical strength, it protects the basic metal from friction, but not from oxidation. That’s why it’s used much more often for gold – gold itself does not oxidize. By the way, rhodium plating is nice when applied to white gold, giving it a more intense whiteness and shine. (White gold on its own is a softer color, more like a pale ivory.) Around 90% of the white-gold jewelry that you see in shop display windows are gold with rhodium coating. In answer to my quest for a silver coating, this second chemist advised palladium, a white metal in the platinum group. Palladium also stands up well to mechanical friction as a covering, lasting indefinitely. Although its grayish silver color is not so bright as rhodium, the crafted detail in the silver will remain constant, and – as I discovered to my delight – it will prevent oxidation. I used palladium plating in my “Pomegranate” pendant, which was more compatible with the gray-silver coloring. Even after many years, its color does not change. And if I really need the snowy-silver color of rhodium, I can use a double-layered coating of rhodium over palladium, for an extremely durable and resistant coating. It’s also possible to apply a double palladium layer with rhodium, or a platinum-rhodium blend. As you can see, it’s very important to choose the best coating for protecting each jewelry product. Rings and bracelets are exposed to constant friction against the skin, so they are especially in need of protection from wear and tear on the precious metal. Second in importance after preservation is the appearance. If you are going to use certain jewelry items every day, and you want to buy (or order) a protective coating for them, I would advise a material the same color as the basic metal. A silver jewelry piece calls for a white coating: palladium or platinum, or perhaps a double layer of palladium-rhodium, or platinum-rhodium. Gilding (gold plating) is better for use on brass or copper. Diffusion there works more slowly and gradually, so the color change is not immediately noticeable.

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