149 



The Penetration of Wood by Zinc Chloride. 



By Edward G. Mahin. 



During the last fifteen years tlie supply of all kinds of sawed lumber 

 has rapidly diminished. The consequent rise in prices has stimulated tlie 

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

 l)osed 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 sohitions, 

 the salt most extensively used being zinc cliloride. 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 lighter 

 inu-tions 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 

 laiise a loss of the efticiency of the preservative. This was long thought 

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

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

 cence" of salts on brick walls. This 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 a 

 process' was devised abroad and in 1908 was patenteil 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 by 

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

 the wood fibers in such a ni;inner as to practically prevent the outward 

 diffusion and consequent loss of the preservative. It was their theory 

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

 wood cells and the aluminium sulphate, this compound resulting in a 

 retaining action upon the zinc ohloride. The action w'as 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 theory but later unpublished investigations showed that solutions of zinc 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, peneti'ated wood ties farther than did solutions of zinc chloride 

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



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



