SEED LAC. See "Lac." Seed-lac is said to be more soluble in alcohol than shellac, and therefore to make clearer varnish.
A A 2 SEL D'Olt. All. 0, S2 02 + 3 [Na. 0, S2 02] + 4 H O.
This salt is a double hyposulphite of gold and soda, containing 4 atoms of water of crystallization. It is made by adding 1 part of perchloiide of gold to 3 parts of hyposulphite of soda, each dissolved in 500 parts of water. The liquid is colourless, and contains several salts besides sel d'or. The solution of gold must be added to that of soda, and not the solution of soda to that of gold. In the reaction which takes place, 8 atoms of hyposulphite of soda, 1 atom of chloride of gold, and 4 atoms of water, form 2 atoms of a sulphur salt of soda, 1 atom of sel d'or, and 3 of chloride of sodium. Thus :— 8 [Na. 0, S2 02] + Au. C1.3 + 4H0= 2 [Na. 0, S4 06] + 2 Sel d'or + 3 Na. Cl.
The sel d'or is precipitated from the solution made as described above by the addition of alcohol. It crystallizes in fine neeclles, which are very soluble in water.
Sel d'or sometimes contains a considerable proportion of common salt and hyposulphite of soda.
Neither hyposulphite of gold, nor hyposulphite of silver can be isolated. They only exist in double salts.
Sel d'or is used for toning positive prints. It acts very energe tically if used before the print has been fixed in hypo. The rationale of its action appears to be, that the gold of the sel d'or is substituted for the silver of the print, so that oxide of silver, plus sel d'or, becomes oxide of gold, plus the soluble double hyposulphite of soda and silver. In this method of toning no sulphide of either metal is formed, and it cannot be considered in the light of a sulphur toiling process. When chloride of gold is added to hyposnlphite of soda in excess, the sulphur salt Na. 0, S4 06, which is produced would, if not removed, constitute a sulphur-toning bath. Hence the advantage of using sel-d'or instead bf the toning and fixing bath commonly employed, and described in the article " Albumen ized-paper Printing Process."