I did some research into the old technique of staining wood with a solution made by putting iron nails in vinegar. I read some explanations from people with a better knowledge of chemistry than me who said the reaction was actually the result of iron ions in the solution reacting with tannins in the wood. They went on to suggest iron sulphate dissolved in water would achieve the same reaction with less fuss. So I decided to test it.

This is iron sulphate as sold to me by a garden centre for killing moss in my lawn. There are apparently two types of iron sulphate:

Iron (II) Sulphate (chemical formula FeSO4), which is green when in powdered form, and,

Iron (III) Sulphate (chemical formula Fe2(SO4)3), which is yellow when in powdered form.

Because this is clearly yellow I'm presuming it to be Iron (III) Sulphate.

Because I'm interested in repeatability more than in the detailed chemistry and because I don't have a set of scales that measure accurately to the gram I made up my solutions measuring the water in millilitres with a syringe and the powder in 1/8 teaspoonfuls with a measuring spoon. Dissolving the powder in water gave me a cloudy solution with small quantities of undissolved solid particles, which may have include various impurities. I filtered the solutions through coffee filter paper before applying them to wood samples using an old perfume spray bottle.