149 



The Penetration of AVood by Zinc Chloride. 



By FjUWAed (t. Mahin. 



Duriiiir the last fifteen years the supply of all kinds uf sawed lumber 

 iias rapidly diminished. The conse(pient rise in prices has stimulated the 

 use of methods for prolonging tlie life of structural timbers that are ex- 

 l)»,sed to excessive weather conditions. Treatment of such timbers to 

 prevent decay has been practically limited, in a commercial sense, to 

 impregnation of the wood with creosote oils or with antiseptic salt solutions, 

 the salt most extensively used being zinc chloride. Against the use of 

 creosote oils two objections have been urged, these being the somewhat 

 excessive cost of the preserving material and the tendency of the lightei- 

 tractions of the oils to evaporate under the influence of wind and sun. 



While the cost of the zinc chloride would not prohibit its use, it has 

 been noticed for a long time that exposure for a term of years seemed to 

 cause a loss of the efficiency of the preservative. This was long thougbt 

 to be due to tlie fact that from moist wood the zinc chloride was gradually 

 lost through a process of outward diffusion similar to that causing "efflores 

 cence" of salts on brick walls. Tliis theory was apparently confirmed by 

 the fact that analysis of old treated wood showed the presence of little zinc 

 chloride in the interior of the piece. In order to remove this objection o 

 process^ was devised abroad and in lOOS was patented in the United 

 States, for treating wood with zinc chloride, with the addition of alumin- 

 ium sulphate. It was thought at the time and was originally claimed l>y 

 the patentees that this process resulted in a fixation of the zinc salt withii; 

 the wood fibers in such a manner as to practically prevent the outward 

 difl'usion and consequent loss of the preservative. It was their theory 

 that some sort of compound was produced between the cellulose of the 

 n-ood cells and the aluminium sulphate, this compound resulting in a 

 retaining action upon the zinc chloride. The action was likened to the im- 

 perfectly understood action of aluminium salts when used as mordants in 

 dyeing. Experiments with washing treated sawdust did not confirm this 

 theory but later unpublished investigations showed that solutions of zinc 

 chloride containing aluminium sulphate, under the same conditions of 

 treatment, i>enetrated wood ties farther than did solutions of zinc chloride 

 alone, the zinc salt having the same concentration in the two solutions. 



1 The Bruniiig-Marraetisclike process. U. S. patent No. 898,246. 



