HOW WOOD IS PRESERVED 151 



cations are hard to obtain in a single substance. At the 

 present time zinc chloride gives good results where tim- 

 bers are not subjected to continuous moisture, but in 

 spite of its higher cost creosote is being more widely 

 used on account of its lasting qualities.. 



Small round timbers or shingles may be made quite 

 durable by an application of hot creosote or coal tar ap- 

 plied with a brush. This preservative kills any bacteria 

 or fungi present in the outer layers and the antiseptic 

 coating also renders the timber impermeable to water 

 and air. Such a treatment is very beneficial but its 

 effect is not lasting. Deeper penetration may be ob- 

 tained by submerging small timbers to be preserved in 

 a tank half full of hot preservative. This kind of 

 treatment is spoken of as "open tank 5 ' preservation and 

 is often done on a small scale by farmers. By heating 

 creosote in a tank or open barrel to a high temperature 

 and then putting the fence posts to be preserved in 

 the boiling liquid and allowing them to remain until 

 it is cool, a large amount of the preservative is absorbed 

 and the material so treated proves very durable. Where 

 deeper penetration is needed or the preservation of 

 ties or timber is done on a large scale, as in the case 

 of railroads or regular treating plants, the preservative 

 is generally applied in a cylinder where pressure can 

 be applied to force it deeply into the wood. When 

 timbers are to be placed in a moist situation or where 

 for any reason conditions will be favorable to rapid 

 decay, complete impregnation should be obtained. This 

 is possible only under high pressure and is a very expen- 

 sive operation owing to the difficulty of the process 

 and the amount of preservative consumed. Complete 

 impregnation is rarely used, however, and as a rule if 

 the outside of timber is soaked with creosote to the 



