GOLD TONING (Pr., Virage d l'or ; Ger., Tonen (Schanen) mit Goldsalz) The purpose of the gold toning bath is to convert the somewhat unpleasant yellowish red colour of the fixed silver image into a more pleasing brown, purple or bluish purple. This has been erroneously described as " gilding," whereas the action is purely chemical, the gold taking the place of the silver of the image, and the silver being converted into chloride in place of the gold. If instead of a plain solu tion of gold chloride the auric chloride were used, the reaction could be expressed as follows :— + 3 Ag = 3 Agel + Au From this it will be seen that one atom of gold replaces three atoms of silver, and the fine details in the high lights would disappear and the image lose considerably in vigour. If, on the other hand, the gold is reduced to the aurous chloride Audi, the reaction would be represented by AuCl Ag = Agel + Au and one atom of gold would replace one atom of silver. It has been stated that an intermediate aurous chloride, is formed, but it is such an unstable salt that its existence is doubtful. To convert any toning bath into the proper toning condition—that is, when the gold is reduced to the aurous state—it is " ripened " either by allowing it to stand or by the use of hot water to dissolve the salts, which should make the bath either neutral or distinctly alkaline. In the case of the sulphocyanide bath,
a double salt of sulphocyanide of ammonium or potassium and gold is formed; this may also be in the auric or aurous state, and a similar chemical action takes place. Of recent years a more reasonable method of using the gold bath has been generally adopted, this allowing a definite quantity of gold to a definite area of print, instead of using one bath for a number of prints in succession and then adding fresh gold. It is important to wash prints well before toning, so as to free them from the excess of silver nitrate, which would decompose the gold chloride and prevent its deposition on the print. When a toning bath has been used, all the gold is not exhausted ; but some which has passed into a stable aurate will not deposit. Obviously this may be collected for the sake of the residues, or the old bath may be used instead of water to make a new bath, the former plan being preferable. (See also "Residues.")