64 



TECHNICAL PROPERTIES OF WOOD. 



Frost-crack and cup-.sl 



elms and beech are most subject to frost-crack, but silver-fir, 

 spruce, larch, ash, sycamore, maple and birch do not escape. 

 Whenever a stem has suffered from frost-crack, its utility as timber 



is very questionable ; even 

 when a lonf,' crack has 

 been completely occlu- 

 ded, the stem cannot, in 

 the case of oak, be used 

 in the round and rarely 

 for staves. If, however, a 

 frost-rib has formed and 

 decay commenced, only 

 certain parts of it can then 

 be used as timber. In this 

 and many other cases of 

 defect, the extent of the 

 damage done is a matter 

 for appreciation, after the 

 tree has been felled, 

 (c) Cup-shake. — Cup-shake occurs when some of the annual 

 zones of the timber have separated partly or entirely from one 

 another ; sometimes such clefts may meet at their ends, and 

 surround a loose central axis of wood and are termed ring-shake, 

 but usually they do not extend so far (fig. 10). They are due 

 to difterent causes : sometimes to shrinkage of the central part 

 of the tree, probably owing to its drying-up ; in many cases 

 to the action of fungi, as Hartig has shown in the case of Scotch 

 pine, spruce, silver-fir and larch, where ring-shake may be due 

 to Trametcs Pini,K\\([ proceeds right up the stem. Ring-shake 

 may be caused after frost when a thaw suddenly sets in and 

 expands the sapwood, separating it from the central woody zones 

 [and also by a forest fire. — Tr.]. 



Cup-shake frequently occurs at the junction of two woody 

 zones of unequal dimensions, especially in the case of silver-fir 

 and spruce which have grown slowly as underwood for a number 

 of years and have then suddenly been exposed to full light. 

 The action of the wind also causes cracks of all kinds. 



Duhamel states that willow-pollards show as many cup- shakos 

 as the number of times they have been pollarded. As a rule 



