238 
The N.Z. Journal of Science and Technology. [July 
liquid into the pores of the timber. The pieces were weighed before and 
after treatment, and, the strength of the solution being known, the weight 
of dry salt introduced could be computed. The blocks were now heated 
in an air-oven until all excess moisture was removed. This was essential 
to the accuracy of the ignition tests, which would be affected by excess of 
moisture. 
The tests were then carried out as before, but with this modification : 
that the burning was continued for twenty minutes after ignition, the loss 
in weight so caused being the determining factor as to the efficiency of 
the various chemicals. This will appear obvious when one considers that 
impregnation will not prevent the ignition of the inflammable gases given 
off, but will hinder or prevent the continued burning of the wood itself. 
In elaborating these tests it was found that the strength of the solutions 
could be cut down to 5-10-per-cent, solution only, the results becoming 
thereby very little altered, while the cost as a commercial process thus 
becomes not only reasonable, but quite low. 
Salts employed. 
1. Chemicals that sublime at high temperatures, the vapours of which 
are non-inflammable : ( a ) ammonium sulphate ; (b) ammonium chloride. 
2. Chemicals that lose their water of crystallization, or decompose with 
the evolution of non-inflammable gases, on being subjected to high tempera¬ 
tures : (a) ammonium phosphate; (b) sodium bicarbonate; (c) oxalic 
acid ; (cl) aluminium sulphate ; ( e ) borax ; (/) ammonium alum. 
3. Chemicals that fuse, or that melt in their own water of crystallization 
when heated : (a) borax ; (b) ammonium alum ; (c) ammonium phosphate. 
The strength of solution of the most efficacious salts or mixtures of 
salts— i.e., to prevent free combustion—with rimu (red-pine) are given in 
the following table :— 
Ammonium phosphate 
Ammonium sulphate 
Ammonium chloride 
Sodium borate 
Equal parts sodium borate and am¬ 
monium sulphate 
5-6-per-cent, solution. 
8-10-per-cent, solution. 
8-10-per-cent. solution. 
8-10-per-cent. solution. 
8-10-per-cent, solution. 
A mixture of ammonium phosphate and sulphate gave good results, 
requiring even less concentrated solutions than those given, the total per¬ 
centage strength being 6. Although this mixture gave the best results, 
the tendency for phosphates to encourage the growth of moulds and fungi 
must not be overlooked, as well as the cost of ammonium phosphate. Taking 
into account the relative corrosive actions of the salts used, since this affects 
the cost of treatment owing to depreciation of plant, we might choose 
the following for use in ordinary steel impregnating-cylinders : ammonium 
sulphate ; ammonium phosphate ; ammonium chloride ; sodium borate ; 
mixtures of these. 
As regards cost of treatment, based on cost of material, sodium borate 
and ammonium borate have the advantage. 
Since wood-preservation by means of pressure treatment with salts soluble 
in water— e.g., zinc chloride—requires apparatus that could without modi¬ 
fication be used for fireproofing, it would seem to offer advantages in treat¬ 
ment of structural timber, such as is used for factories, storehouses, &c. 
Double treatment, first with borate and then with zinc chloride, leads to 
