284 Increasing the Durability of Timber, [july, 



because they are poisonous to animals, or on account of their 

 being easily washed out by rain, or because they corrode 

 metal, they have all been supplanted more or less completely 

 by creosote, except in countries where this substance is much 

 dearer than in Britain. 



On a large scale creosote is usually applied under pressure, 

 and this process entails the provision of a somewhat costly 

 plant. But on account of the thoroughness and rapidity 

 with which the impregnation is carried out, the outlay on 

 a pressure plant is justified where a large amount of timber 

 has to be dealt with, and such plants are now to be found 

 on many estates. In outline the process may thus be de- 

 scribed : — The timber to be treated is well-seasoned so as to 

 get rid of surplus moisture, and the posts (barked), rails, 

 gates, troughs, or whatever the articles may be, are finished 

 in every respect before being placed in the creosoting cham- 

 ber. It is very important that there should be no cutting 

 of the wood after the treatment, as this would mean inter- 

 ruption of the layer where most of the creosote is deposited, 

 and water would enter, more or less freely, through this 

 point of weakness. The articles are then loaded on to small 

 waggons or bogies that run on rails, and are pushed into 

 the creosoting chamber, which is essentially of the character 

 of a large horizontal steam boiler. The door is then closed 

 and firmly bolted, and an air-pump is set to work to reduce 

 the pressure in the chamber. The air is thus partially re- 

 moved from the wood, and when the vacuum gauge indicates 

 a pressure of about half an atmosphere, the creosote, which 

 should be previously heated, is allowed to fill the chamber. 

 The air pump is now stopped, and a pressure of 80 to 85 lb. 

 per sq. in. is applied, the creosote being thus forced into the 

 wood. In two or three hours, or less, the wood will have 

 taken up as much creosote as is deemed desirable ; when the 

 surplus liquid is pumped from the chamber into a suitable 

 receptacle, the door is opened, and the bogey, with its load, 

 run out. 



The amount of creosote or other fluid that wood will absorb 

 varies greatly with species and other causes. Heartwood 

 takes up much less than sapwood, damp wood takes up less 

 than dry, slow-grown pine takes up less than fast-grown, 



