Iron stain, an unsightly blue-black or gray discoloration, can occur on nearly all woods. Oak, redwood, cypress, and cedar are particularly prone to iron stain because they contain large amounts of tannin-like extractives. To find out more about iron stain, and what can be done about it, click on over to this newly revised FinishLine.
The discoloration from iron is caused by a chemical reaction between extractives in the wood and iron in steel products, such as nails, screws, and other fasteners. Contaminating wood is easy. For example, merely striking wood with a hammer can cause iron stain on some wood. (Covering the head of the hammer when nailing redwood and western redcedar siding is a good idea.)