THE VALUE OF AMMONIUM POLYSULFIDE AS A 



WOOD PRESERVATIVE 



DONALD H. CLARK, '16 



In the history of wood preservation, a large number of compounds 

 have been tested as to their efficiency as agents to prevent decay. Of 

 those that have been tried, a few have proven to b3 of commercial value, 

 others more or less of a failuie. A large proportion of the compounds 

 so tried have been inorganic salts of various kinds. Most of these have 

 proved to be of little or no value as preservatives, the chief reason for 

 their failure being their solubility in water, and the consequent leaching 

 of the salt from the timber after impregnation. Even though a compound 

 has a high toxicity and is effective in preventing the germination and 

 growth of fungi in the wood tissues, it is of little commercial value if it 

 cannot be used for the preservation of timbers which are to be placed 

 under outdoor conditions where leaching may occur. A good example of 

 such a compound is copper sulfate, which has been proved to possess a 

 high toxic value, but which leaches from the wood if the latter is placed 

 in a moist situation. 



Recently a method of preservation has been suggested whereby the 

 wood cells are filled with a substance almost insoluble in water, namely, 

 sulfur. Although the details of the process have not been worked out, 

 owing to the fact that the commercial value of the method is not yet 

 proven, the essentials of the method of impregnation may be briefly given. 

 The wood products to be treated are placed in a retort somewhat similar 

 to those used in impregnation with creosote. The retort is then filled with 

 ammonium polysulfide and pressure applied. When the desired pene- 

 tration has been secured the pressure is released, the excess of ammonium 

 polysulfide is drawn off and the retort heated with steam coils. The 

 effect of the heating is a regeneration of ammonia gas. This reaction is 

 common to all the salts of ammonium when heated. In the case of 

 ammonium polysulfide, ammonia gas and a small amount of hydrogen 

 sulfide are given off, the residue being precipitated sulfur. When heat 

 is applied to the retort containing the impregnated wood, the ammonium 

 polysulfide passes through the chemical changes outlined above. The 

 ammonia gas is collected for further use and the precipitated sulfur is left 

 in the cells of the wood. 



Before perfecting the mechanical details of the process or figuring 

 the costs of operation and materials, it is necessary to prove the value 

 of the sulfur as a preservative by testing its toxicity. The value of lime- 

 sulfur as a fungicide is universally recognized. In this case, however, 

 there is a reaction which is lacking in the case of the insoluble, precipi- 

 tated sulfur. When slacked lime is boiled with sulfur in the preparation 

 of lime-sulfur compound, a calcium salt of sulfur is formed which 

 hydrolyzes in the presence of water, giving both sulfurous and sulfuric 

 acids. Both of these are effective fungicides. In the case of ammonium 

 polysulfide, however, the insolubility of the precipitated sulfur in the wood 

 cells prevents the formation of such acids. 



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