HOW TO PROLONG THE LIFE OF FENCE POSTS 471 



treatment, " as it is called, should only be applied to thoroughly 

 seasoned posts. Where creosote or a similar preservative is 

 used, it should be applied hot and at least two coats given. One 

 of the defects of a brush, or surface treatment, is that it is dif- 

 ficult to get the preservative into all the cracks and checks. This 

 can be overcome by dipping the posts in the hot preservative. 

 At best, surface coatings are not durable. They are easily 

 broken or worn off, exposing the wood to fungi and admitting 

 moisture to the interior of the post. 



Hoiv to preserve fence posts hy the use of creosote. — The most 

 effective method of preserving fence posts is to thoroughly 

 impregnate the outer layer of the wood with some preservative 

 substance that will poison the wood and deprive the fungus plant 

 of its food. Many substances have been used for this purpose, 

 but the cheapest and most effective is creosote, or dead oil of 

 coal tar, formed as a by-product in the manufacture of coal gas. 

 It is not only poisonous to the fungus plant, but, being an oil, it 

 also tends to exclude moisture from the wood. Most of the high- 

 priced, patented preservatives have this substance as their base. 



The treatment is best carried out by the so-called "open tank 

 method." Thoroughly seasoned posts are heated for several 

 hours in hot creosote, and then allowed to cool down in cold 

 creosote. When the posts are heated in hot creosote, the high 

 temperature causes the air and water in the wood to expand, so 

 that a portion of this air and Avater is forced out. When the 

 posts are then placed in cold creosote, the air and water left in 

 the wood contract, forming a partial vacuum; and the creosote 

 is forced into the wood by atmospheric pressure, to take the place 

 of the air and water that have been forced out. This forms a 

 shell of creosoted wood from one-eighth of an inch to two inches 

 in thickness around the post, that effectively excludes moisture 

 and prevents the entrance of fungi. 



Hoiv to apply the treatment. — A\niere a large number of posts 

 are to be treated, it can be done more quickly and economically 

 by using two tanks. The posts are heated for several hours in 

 boiling hot creosote in the heating tank, and then immediately 

 transferred to the second tank to cool for about an equal length 

 of time, in cold creosote. But for treating a few posts for farm 

 use, the single tank method is advised. In this case, the posts 

 are allowed to cool down in the tank in which they were heated. 

 As good results can be obtained by the use of one tank as 

 with two. 



The simplest form of treating tank would consist of an iron 



