S. F. P. - An excellent preparation for oxidizing silver is a solution of platina in hot aqua regina (or one part nitric acid to two parts muriatic). Let the fluid evaporate; wash the powder in water and dry it. Then dissolve the powder in water or alcohol and keep the solution in a bottle for use. The silver object having been well cleaned, apply the solution with a soft brush and let it dry. If a very dark oxide be desired, warm the silver until it gets as black as is required. Wash off what is superfluous and clean as with ordinary silver.