Ammonia fuming

Ammonia fuming is a wood finishing process that darkens wood and brings out the grain pattern. It consists of exposing the wood to fumes from a strong aqueous solution of ammonium hydroxide which reacts with the tannins in the wood. The process works best on white oak because of the high tannin content of this wood. Fumed oak is also called smoked oak. Other species may also be fumed but usually will not darken as much as white oak. The introduction of the process is usually associated with the American furniture maker Gustav Stickley at the beginning of the twentieth century, but fuming was certainly known in Europe some time before this.

The wood to be fumed is placed in a sealed chamber with all the surfaces to be fumed exposed to freely circulating air. A large shallow container of ammonium hydroxide solution is placed on the floor of the chamber and the chamber is sealed. If the chamber is large or the fuming is to be done for a long time then more than one container may be provided or the ammonia may be replenished during the process. The fuming time depends on the amount of darkening required, the size of the chamber, and the strength of the ammonia used. It is usual to oil the wood after fuming to fully bring out the effect.

Fuming has an advantage over staining in that it does not obscure the grain, it just darkens it. Unlike staining, there is no possibility of blotches or runs. Fuming is also colourfast. Fuming has the disadvantage that it is not a very precise process. Different batches of wood will react to fuming differently. For this reason wood that is to be fumed for a particular project is often taken from the same tree. Even so, boards from the same tree, and even different regions of the same board, can have a noticeably different colour. Where a consistent colour is important, staining or dyeing may be better options.

Fuming has some inconvenient safety issues. The solution of ammonium hydroxide used is much stronger (26% to 30%) than in household ammonia and is corrosive. The fuming must be done in an enclosed sealed chamber. Ammonia splashes can burn skin and the fumes can cause burns to eyes and lungs. Operators need to wear gas masks, gloves and eye protection.

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Wikipedia