Ferrous sulfate is also known as Copperas or Green Vitriol. It is light green in color and when dissolved in water, it is used to stain woods. Dissolve 2 ounces in a pint of water and use this as a stock solution. Dilute the stock solution to use as a mordant. It works best on woods that are high in tannin and depending on the amount of tannin present will stain them a grey to steel blue color. It can also be used on woods that are low in tannin by prestaining the wood with a tannic acid solution. It is also used as a mordant for varying the colors of natural dyes when staining wood. Use this chemical with caution as it is moderately poisonous.