That's the classic recipe for preparing iron acetate. When applied to white oak, which is high in tannins, the iron solution causes the tannic acid to turn black. Red oak doesn't have as much tannin so the color reaction is much less.Make an iron oxide solution by putting a piece of steel wool (OOOO is good size to use) in a jar of white vinegar. You could get by with a small piece of OOOO steel wool in a cup of vinegar. Vinegar dissolves the steel wool completely in 6-7 days but 1 or two days is all that's needed for identification purposes. Iron oxide will turn white oak black. Red oak will hardly change color at all.