6S New Pigments, 



of 212" Fahr. they lo?e their water and become Iroivnl In this 

 wnhyrlrous state they form a body of such good colour as to pro- 

 mise to be useful as a pigment. 



But pa'iiters have been more at a loss for a durable purple thar» 

 for browns, and tiiis desideratum seems at length to be supplied. 

 His Excellency the Count le Maistre of St. Petersburgh, in a 

 letter to Dr. Creighton (published by Dr. Thomson) describes a 

 process for obtaiiiing this unchangeable colour. The Count first 

 notices the tendencyof the muriate of gold to form purple coloured 

 compounds — with the oxide of tin ; with gelatine and starch, when 

 mixed and boiled with them ; and with the earths, if precipitated 

 with them and heated : and then gives the following process for 

 preparing the new colour : — One part of dry muriate of alumina^ 

 one of sulphate of magnesia, four parts of muriate of barytes, and 

 five of carbonate of soda, are each pulverised separately. They 

 are then mixed in a glass mortar, and a little water is added, 

 merely enough to moisten the mixture. Then a diluted solution 

 of gold is added, by little and little, pounding the matter all the 

 time in the mortar, till the whole has acquired a light sulphur- 

 yellow tint, and the consistence of cream. The pounding is con- 

 tinued a long time, to produce the decomposition of the salts with 

 as little water as possible. When no more effervescence is per- 

 ceptible, and when the salts cease to crack under the pestle, a 

 sufficient (juantity of water is to be added for their complete so- 

 lution. This tedious process is essential toimite the oxide of gold 

 >vilh the earths ; and the whole success of the operation (which 

 is pretty capricious) depends on it. The precipitate is to be left 

 24 hours in the mortar, stirring it from time to time with a glass 

 rod. It is then to be poured into a saucer, or a similar vessel, and 

 left till the powder subsides. Tiie supernatant liquid is then drawn 

 off with a syphon, and the deposit is dried in the shade, without 

 wa«Iiing it. The precipitate when dry is yellowish white. The 

 muffle in which it is to he baked, ought to be red hot. The powder 

 is put (of the thickness of one or two lines) upon a silver or porce- 

 lain plate ; and it must l)e withdrawn from the fire the instant that 

 it acquires its purple colour. If too long exposed to heat it ac- 

 quires a violet tinge. This is occasioned bv the salts it still con- 

 tains; ior, after it has been washed, it may be kept red hot without 

 io^hi^ any of its colour, which indeed acquires greater lustre. 

 These trials v^-eri' uiide on a small scale, and, the Count oliserves, 

 nay be improvcu. The colour appears to want intensity, but the 

 mixture of oils or gums renders it sufficiently deep. For oil-paint- 

 ing it must be carefully rub! cd with a mixture of drying oil and 

 varnish. The painting is to be begun by a thin transparent coat: 

 a second is sufficient to give it all the lustre of which it is suscep- 

 tible. The under coats ought to be prepared with raw terra de 

 Sienna. kkw 



