4 BULLETIN- 1037, U. S. DEPARTMENT OF AGRICULTURE. 



DEFINITION OF SAP-STAIN. 



The term " sap-stain " refers to the blue, green, brown, or red dis- 

 coloration which often may be observed in the sapwood of timber 

 derived from several kinds of broad-leaved and coniferous trees. 

 It must not be confused, however, with the superficial discolorations 

 produced mechanically, i. e., by collections of dirt and coal dust, by 

 deposits from the drip in leaky cars and sheds, or from the condensed 

 moisture in kilns. Such deposits may occur upon heartwood as well 

 as sapwood. Neither should it be confused with the common blue- 

 purple stain apparent when rusty saws are used on certain green 

 wood, such as oak. This stain results from the reaction between the 

 tannic acids of the wood and the iron compounds from the saw. 

 Finally, it must not be confounded with the variously colored super- 

 ficial growths of molds or the more or less deep seateci sap-rot, witli 

 its brown to bleached appearance and its tendency to produce a 

 punky consistency of the sapwood itself. 



There are two quite generally recognized classes of sap-stain: (1) 

 The chemical stain, said to be produced by chemical reactions brought 

 about through the agency of certain oxidizing enzyms present in the 

 wood itself; and (2) fungous stains known to be caused by several 

 species of fungi. 



CHEMICAL STAINS. 



Chemical stains due to enzyms cause discolorations in both the sap- 

 wood and the heartwood of sugar pine and hard maple (Tiemann, 

 51, p. 185; also Pratt, -5^ p. 305-307). Such stains develop during 

 air drying, particularly under warm and humid conditions, or in 

 the kiln, and give more or less permanent discolorations to the wood, 

 to wit, a brown stain in sugar pine (fig. 1) and a cherry color in 

 hard maple. These defects cause degrading {34) and often result in 

 financial losses. According to Bailej^ (5), when freshly cut sapwood 

 of alder, birch, cherry, or red gum is exposed to the air during ex- 

 tremely warm and humid weather, chemical reactions often take place 

 and within a few hours produce colored substances in the wood. Bailey 

 (5) states that the microscopic examination of sections of such wood 

 indicates that the colored substance develops particularly within the 

 pith rays and the parenchyma cells. He states that certain soluble 

 enzj^ms which assist in the oxidation of organic compounds and are 

 of prime importance in the nutrition and growth of living organ- 

 isms are widely distributed in plants and animals and may also pro- 

 duce post-mortem discolorations of certain organic compounds (see 

 also Clark <?, ^9). Yoshida {59) discovered in 1883 that an oxidizing 

 ferment is responsible for the oxidation of the latex in certain species 

 of Rhus and the formation thereby of black varnish, or lacquer. 

 Investigations have also shown that discolorations in fruit juices, 



