COLOURS. 413 



even in enamel ; but the great price prevents the 

 general use of it. 



Ultramarine ashes. This is the residuum after 

 washing the lapis lazuli, in which a portion of the 

 ultramarine still remains. It is very subject to be 

 adulterated. It is not so bright as ultramarine, 

 being, like that colour, with a tint of red and white 

 in it. When genuine it stands well. 



Prussian blue. This colour is iron combined 

 with the prussic acid. It is made in the following 

 manner. 



Two parts of purified potash are most intimately 

 blended with three parts of dried and finely pul- 

 verized bullock's blood. The mass is first calcined 

 in a covered crucible, and on a moderate fire, until 

 no more smoke or flame appear ; and it is after 

 this brought to a complete, yet moderate, ignition. 

 Or equal parts of potash and finely-powdered coals, 

 prepared from bones, horns, claws, &c. are min- 

 gled, and heated in a covered crucible to a moderate 

 redness. This done, either of these two calcined 

 masses is, after cooling, lixiviated with boiling 

 water, and the lixivium filtered. Nothing remains 

 now but to make a solution of one part of green 

 vitriol and two parts of alum, and to add to it 

 while yet hot the above lixivium, little by little how- 

 ever, and to separate the greenish-blue precipitate, 

 which then forms by means of a filtre. If after- 

 wards a slight quantity of diluted muriatic acid be 

 affused upon this precipitate, it assumes a beautiful 

 dark-blue colour. The operation is terminated 

 by edulcorating and drying the pigment thus 

 prepared. Prussian-blue is an extremely beau- 

 tiful colour when properly prepared, and stands 

 well. Common Prussian-blue is apt to contain 

 some iron, which causes it to turn greenish or olive. 



