DURABILITY OF WOOD. 239 



be much more soluble than the dead oil, and on this account 

 more liable to wash out of the wood when in contact with 

 the soil. 



Zinc Tannin Process. In this process the chloride of 

 zinc is protected from being washed out of the ties by 

 coagulated albumen. The process is as follows: The ties 

 are impregnated with chloride of zinc mixed with a small 

 percentage of dissolved glue. They are then subjected 

 to heavy pressure, after which the solution is drawn off 

 and a tannin solution added at a pressure of 100 pounds. 

 This material combines with the glue, and forms a leath- 

 ery, waterproof substance which permanently closes the 

 pores or outer cells of the wood, excluding moisture and 

 retaining the zinc. 



Burnettizing. In this process the timber is impregnated 

 with chloride of zinc, the operation being similar to that 

 of creosoting. It has a wonderful preservative effect 

 upon the timber, the only objection to it being that the 

 solution is liable to be washed out of the ties. This is 

 overcome in the modern treatment of the ties in Ger- 

 many by using a certain per cent, of dead oil with it, 

 as previously noted in describing the method of impreg- 

 nating railroad ties. 



Fire-proof Wood. It has been known for many 

 years -that wood could be made fire proof by filling it 

 with certain chemicals in much the same way that rail- 

 way ties are impregnated. The most common chemical 

 used for this purpose was phosphate of ammonia, and it 

 is perhaps the best material for this purpose that has 

 ever been used, but it is so expensive that the use of it 

 is quite impracticable. The next best material that 

 has been used for this purpose is sulphate of ammonia, 

 but like phosphate of ammonia this somewhat injures 

 the flexibility of the fibre and corrodes metal, and in addi- 

 tion deadens the color and causes the wood to be more 



