Depression and Mercury Glass
Q: I purchased a set of mercury-glass salt and pepper shakers at a thrift shop. They have a small amount of old salt encrusted on the top of the shaker, and I would like to know how to clean it without harming the finish.
-- Jacqueline Stallone, Ocala, Florida
Mercury glass, which is often called silvered glass, has neither mercury nor silver in it. It is, in fact, clear glass that has been mold-blown into double-walled shapes and coated on the inside by means of a silvering formula inserted through a hole in the bottom. The hole is then sealed or plugged. Some silvering formulas are composed of silver nitrate, others from alcohol or ammonia, and others from oil of cassia or tartaric acid.
To remove the encrusted salt from mercury glass, a warm, damp rag should be sufficient. You can use ammonia since you don't have to worry about harming the finish (it's the glass you're cleaning, not the finish within it), but don't use a scouring pad or the glass could be scratched. Don't put mercury glass in the dishwasher or submerge it in water; if water gets between the layers of glass, it will harm the silvered finish. Exposure to air and humidity can destroy the silvering through oxidation, so if your mercury glass has lost its seal in the bottom, plug the hole with a small piece of cork or wax.