Saw-mills, their arrangement and management, a companion volume to 'Woodworking machinery'. by Manfred Powis Bale

Author:Manfred Powis Bale
Language: eng
Format: epub, pdf
Published: 1883-03-25T05:00:00+00:00

for di-ymg are fitted with exhaust or ventilating pipes; the hot ail- enters at the bottom of the room, and circulates round and through the wood, and is pressed out through the circulating pipes, which are alsoj fixed at the bottom of the room on the opposite side. This method of drj'ing does not warp or discolour the wood, and is a preservative against dry rot.

Some valuable experiments on the action of steam oa

varlons kinds of wood were made by M. Violette, a Frenchman, some years back. The woods treated were oak, ash, ehn, walnut, and fir, specimens of which he submitted to the action of a current of steam at a temperature of 100'' Centigrade, gradually raising it to the different points of 125% 160% 175% 200% 225% 260% without the addition of any water, so that the vapour was no longer saturated, but was rendered capable of extracting the moisture contained in the wood. Portions of the various woods were weighed and exposed to these temperatures for two hours in closed chambers, and again weighed when cool, in order to find the amount of loss of weight by desiccation.

Tliis loss was found to increase in a constant ratio according to the temperature ; but great variations were experienced with different woods. At the temperature of 176" elm and oak lost one-third of their weight, and at 250* one-half; ash and walnut lost one-fifth at 176% and two-fifths at 260*; and fir, one-sixth and one-third at the same temperatures. Until the heat reached 176* thoy each preserved their primitive colours, but fi:om that point to 200* a slight change took place. Above 200* the colour gi'adually deepened, and at 260* oak became black. Tliis change of tint indicates the formation of tar in the wood, which seems to be necessary for its due preservation.

The partieulai' result of these trials to which we would direct the attention of the worker in wood is the great increase in strength which this treatment causes; this has been acom*ately determined at the different degrees of temporatm*e, showing the remai'kable fact that timber may be thus improved in tensile strength to an immense extent. Elm obtains its maximum point of sti'ength at a tempeniture between 160"* and 176*, whilst that for the

other woods varies from 125° to 150°. Ash receives an accession of two-thirds its original strength; oak^ five-ninths ; walnut, nearly one-half; fir, two-fifths; and elm, more than one-third. The order of classification here given is according to that of the temperatures. It appears that the process condenses the fibres, and gives to the wood the properties of solidity and firmness, equalling an amount of outdoor seasoning of a number of years.

A process for preserving timber, known as carbolizing, has latterly come somewhat extensively into use; imder this system the antiseptic properties of carbolic and other tar acids are, by means of superheated steam, carried through the pores of the wood, and are so combined with its fibres as to preserve them. The process is continued as long as any fermentable sap or water is extracted.

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