CLASSIFICATION AND CHARACTER OF WOOD 83 



latter, and consequently the wood shrinks more in drying. 

 The pores in the heartwood are more or less filled with gum, 

 tannin, and other substances, yet are never without moist- 

 ure until thoroughly seasoned. When the moisture is evap- 

 orated the wood necessarily shrinks, the fibres become more 

 compact, it becomes stronger, not necessarily tougher, 

 but, on the other hand, more brittle, harder, and more 

 serviceable. Evaporation of the moisture proceeds most 

 rapidly from the ends of the pores or veins when exposed 

 although in but few woods are they continuous ; hence a 

 board or stick will dry more quickly at the ends, and, 

 shrinking faster there than elsewhere, cracks are liable to 

 occur. Consequently, to prevent unequal shrinking, and 

 the resultant checking, evaporation should go on evenly 

 over the entire surface. To avoid checking through the un- 

 equal evaporation of the moisture they contain, logs of 

 valuable species, such as Black Walnut, Ash, Cherry, etc., 

 are frequently painted at the ends as soon as cut. 



The moisture in the fibres and pores of the wood is not 

 compelled to pass out through the ends of the wood alone, 

 as it can and does escape elsewhere. Unfortunately the 

 escape of moisture is not uniform in any species of wood, 

 but may be greater in some parts than in others ; and this 

 gives rise to irregular shrinking, which results in warping 

 and twisting of the stick or board, which is a very serious 

 defect even if checking is avoided. It is greater in some 

 species of wood than in others. This irregular drying, and 

 consequent irregular shrinking and checking, are largely 

 brought about but not entirely so by the fibres not 

 running parallel with each other. As has been shown, the 

 fibres of some species of tree are much distorted and run in 

 various directions, twisting and interlacing, and when lumber 

 is sawed from them the open ends of the pores are presented 

 and evaporation takes place more rapidly there. Shrinkage 

 endwise will occur in cross-grained wood practically in the 

 ratio of the departure of the fibres from parallelism with 

 the axis of the stick or board. 



