Paper for discussion - Incising of spruce to improve preservative penetration
W B Banks
Large quantities of whitewood or Norway spruce (Picea abies) are regularly imported into the United Kingdom from continental Europe. In addition, increasing amounts of Sitka spruce (Picea sitchensis) are likely to become available over the next decade from the Forestry Commission. In many ways spruce is a good structural timber, eg it is relatively cheap and easily available, it has adequate strength properties and is easily worked. However, as a long life structural material it suffers the disadvantage of low resistance to fungal and insect attack of both its sapwood and heartwood. In addition, the material is resistant to impregnation with preservative fluid using normal pressure methods. If preservative penetration were improved, the species could be utilised in several building and fencing situations from which it is at present excluded because of its susceptibility to rapid biodegrade. Considerable effort has been spent in trying to improve the preservative impregnation of spruce wood and some degree of success has been achieved in the treatment of round timbers. Here the problem is to improve treatability of the sapwood band so that this is fully penetrated, providing a protective sleeve around untreated heartwood. Successful treatments of this type may be attained by water storage on 'ponding' prior to seasoning followed by pressure impregnation or by sap displacement treatment of freshly-felled logs (eg the Boucherie process). In 'square' sawn timber however, exposed surfaces contain a significant proportion of heartwood, which does not respond to either of the processes mentioned above. Penetration and absorption of impermeable species can be improved by incising. Generally, however, in the United Kingdom, incising has been employed only on large section engineering timbers (eg railway sleepers and marine piling) using a tooth size which damages the timber surface to an unacceptable extent see BS 913:1954 and 4072:1966) for application to the smaller section sizes encountered in building or light fencing material. The present work is concerned with the development of a level of incision sufficient to allow protection against biodegrade, without weakening the timber or damaging its surface to an unacceptable extent. The purpose of this account is to show the degree of improvement in penetration and absorption brought about by such incisions.