192 Processes for Wood-preservation. 



by pumping, and at other times by the condensation of stoam, as in 

 low-pressure steam-engines. 



760. It may be remarked of all methods of injection, that woods 

 with an open, porous grain, receive better than those that are solid, 

 and that the sap wood is more easily injected than the heart wood. 

 The coniferous species, especially the sap wood, if not already full 

 of resin, generally afford good results, while the oalis and other 

 solid woods can scarcely be injected in their heart wood. It is fur- 

 ther found that in wood that has begun to decay the tissues appear 

 to be disorganized, so that solutions will not penetrate, even under 

 great pressure. Without attempting to classify the various processes 

 employed for increasing the durability of timber, we will briefly 

 describe some of the more common methods. 



761. Impregnation with Oils. This is an ancient process. The 

 essential oils were used in wooden structures in Rome. Perhaps 

 the best is linseed oil, or other vegetable fixed oils, in preference to 

 animal oils, which cause the wood to become brittle. In India, 

 cocoa-nut oil, beaten up with shell-lime, is used as a varnish to plank. 

 Our common paints derive their well-known preservative effect 

 largely from the oils that they contain, but they should never be 

 applied except to wood thoroughly seasoned. Trenails are sometimes 

 boiled in oil before using. 



762. Crude Petroleum is found to be an excellent preserving agent, 

 and has been employed in ship and house-building with great suc- 

 cess. 



763. Common Salt is sometimes put into ships, while building, and 

 has the effect of rendering the wood durable. It is liable to be dis- 

 solved away, and to cause a corrosion of iron fastenings. It also 

 renders vessels damp -and unwholesome, and if in excess, it swells 

 the tissues by the crystals that form in drying. The method of 

 salting is required by lake underwriters in new first-class vessels, 

 and it is recommended by the American Lloyds.' 



'" The mode of salting is to fix stops of boards between the timbers of 

 the frames about the height of the load-line, and when the ceiling and plank- 

 ing are worked and the plank-shear readj' to go into place, the spaces 

 between the timbers are filled with salt. Near the end of the vessel the salt 

 is sometimes put between the frames, quite down to the dead wood. A 

 vessel of 500 tons will take 100 barrels of salt applied in the usual manner." 

 ( W. W. Bates, Ag. Eep. 1866.) 



