Plasticizing wood with ammonia‐control of color change

Abstract

AbstractRecent studies of plasticizing wood with anhydrous ammonia have demonstrated that pronounced changes in the color of wood can occur. These changes are thought to be the result of various oxidation and condensation reactions that are enhanced by ammonia. It was postulated that pretreatment of the wood with sulfur dioxide would effectively block these reactions and thus lead to little, or no, color change in subsequent ammonia treat‐ment. A series of tests were run on maple, birch, red oak, and douglas fir specimens that confirm this hypothesis. Reflectance spectra over the range of visible wave lengths show that both liquid ammonia at −35°C and gaseous ammonia at +25°C darken the wood with more darkening occurring at the higher temperature. Pretreatment with sulfur dioxide followed by gaseous ammonia treatment to a highly plastic state results in only slight changes in color while the reverse procedure (NH3 first followed by SO2) results in considerable color change.