Here is a method of darkening or blackening brass tacks and pins. Add to a solution of copper sulphate (Milestone) a strong solution of washing soda; allow this to settle, pour off the liquid, and add a quantity of water equal to the liquid poured off; then allow to settle again. Then pour off as completely as possible, take the green sediment with four times its value in water, heat to 140° F., and add ammonia gradually until the articles immersed in it assume the desired colour.