Account of certain Colours dug 7ip at Pompeia. 413 



whitish grains. Analysis has discovered the same prin- 

 <5iples in it as in the preceding; we mav consider it as a 

 composition of the same nature in which lime and aluminc 

 are in stronger proportions. 



No. 7- has a fine rose tint; it is soft to the touch; is 

 reduced between the fingers to an impalpable powder, and 

 leaves on the skin an agreeable rose-colour. 



When exposed to heat, this colour becomes at first black, 

 and ends in white. It gives out no perceptible smell of am- 

 monia. 



The muriatic acid dissolves it with a slight efFervescence: 

 ammonia produces in the solution a flaky precipitate, which 

 potash redissolvc'S entirely. 



The infusion of gall-nuts and hydrosulphuret of am- 

 monia did not denote the presence of any metal. 



We may therefore regard this red colour as a true lac, in 

 which the colouring principle is founded upoualumine. In 

 properties, shade, and in the nature of the colouring prin- 

 ciple, it has an almost perfect analogy with the madder lac 

 which I have mentioned in my treatise on dyeing cotton. 

 The preservation of this lac for nineteen centuries without 

 any perceptible alteration, is a phaenomenon which must 

 astonish chemists. 



Such is the nature of the colours presented to me by the 

 empress: they seem to have been destined for painting: 

 nevertheless, if we examine the varnish of the Roman 

 earthen-ware, of which we find such immense quantities 

 wherever the Roman armies have been, we shall be con- 

 vinced that most of the earths in question have been em- 

 ployed in forming the varnish with which these pieces of 

 earthtn-ware are covered. 



In fact, the greater part of these earthen-wares is covered 

 with a red coating which has nothing vitreous in it, and 

 which may have been given, either with yellow ochre, or 

 with the reddish-brown kind, reduced by bruising to a fine 

 paste, incorporated with a mucilaginous, gummy, or oily 

 body, and applied with a pencil. M. d'Arcet, who has 

 written a very ingenious work on the Roman potteries, is 

 in possession of a vase, the composition of which is of a 

 dirty red colour, and the surface has been covered with the 

 coating above mentioned. We observe in it the place 

 where the potter has left off covering it; and we perceive 

 cn (he bottom, which is not coated, red streaks, which 

 the workman had laid on, in order to clean his pencil, or 

 to try the colour. 



It is by no means rare to find other vases, the body of 



which. 



