I am the director of product development for a company allied to the finishing industry. May I suggest some alternatives to your answer to a reader’s request for a “non-painting” alternative to painting wood fencing, which appeared in your column in the November 2001 issue.
Ammonium hydroxide (ordinary, full strength, household ammonia) reacts with some woods, redwood for one, to form a very persistent black color. This is the result of a reaction of the ammonium ion with naturally occurring compounds inside the wood’s cells. The black material formed appears to be chemically stable, insoluble in water, not sensitive to UV degradation and resides inside the first few layers of the wood’s cells. It only fades as the wood itself weathers away, opening individual cells to washing out of their contents by rainfall. To the best of my knowledge, it offers no protection to the effects of weathering or the various forms of biological degradation.
This is only one of many in situ conversion schemes developed over the millennia to color natural materials such as wood, leather, and fibers (cotton, flax, wool, etc).
I do not know whether the use of pressure treated woods react as do the untreated ones. It might be possible to convert some of the copper in commercial rot resisting pressure treating compounds, after their infusion into the wood, to black copper sulfide. Both the +1 and +2 copper sulfides have low solubility products, meaning that they should be quite stable. I would probably try a surface application of an aqueous solution of one of the compounds, which maintain a reasonable level of hydrogen sulfide in solution. A candidate, thioacetamide in water, hydrolyzes to produce hydrogen sulfide in solution without making a very smelly excess. It also yields an ammonium ion that may also produce the original coloration reaction that I mentioned.
The USDA’s Forest Products Research Laboratory in Madison, WI, probably would be able to give authoritative information, rather than my speculation, on the original question asked. D.S.
Thanks for the information. It adds to the “store of general knowledge.” Hopefully it will be recalled during one of my archival searches, so that it can be retrieved in the future. Remember this is a clinic. Our intent is to solve problems. If I don’t have the answer, I know that one of my loyal readers does. I really appreciate you and others who are willing to share your expertise with us.