The majority of water-borne salts in current use contain hexavalent chromium, usually as dichromate or chromium trioxide. In about 1913 it was suggested that chromates, phosphates or borates could be added to the sodium fluoride and dinitrophenol mixtures then in use in order to inhibit corrosion. Dichromates were found to be most suitable and the resulting mixture known as Thriolith and later Triolith, was the first of the group of wood preservative products that are now usually described as Wolman salts. Although the dichromate was originally introduced in order to reduce the corrosive properties of the dinitrophenol it was soon appreciated that it considerably improved fixation and the resistance of the treatment to leaching, and dichromate contents have been progressively increased since then in attempts to further improve fixation. Chromate and dichromate were similarly found to assist fixation of copper salts, and copper-chromium-arsenic salts now account for most of the chromium used in wood preservation. The permanence, distribution and influence on materials of chromium containing water-borne preservatives has previously been reviewed in a paper prepared for this Working Group III (Belford, 1970).

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