

ENAMELS 271 



white ; as a little azure blue is added to linens, to brighten or counteract the dulness 

 of their yellow tint. 



A white enamel may bo conveniently prepared also with a calcine composed of 

 two parts of tin and one of lead calcined together ; of this combined oxide, one part 

 is melted with two parts of fine crystal and a very little manganese, all previously 

 ground together. "When the fusion is complete, the vitreous matter is to be poured 

 into clear water, and the frit is then dried, and melted anew. The pouring into 

 water and fusion are sometimes repeated four times, in order to secure a very uniform 

 combination. The crucible must be carefully screened from smoke and flame. The 

 smallest portions of oxide of iron or copper admitted into this enamel will destroy its 

 value. 



Some practitioners recommend the use of washed diaphoretic antimony (antimoniate 

 of potash, from metallic antimony, and nitre deflagrated together) for white enamel ; 

 but this product cannot be added to any preparation of lead or other metallic oxides ; 

 for it would tend rather to tarnish the colour than to clear it up ; and it can be used 

 therefore only with ordinary glass, or with saline fluxes. For three parts of white 

 glass (without lead) one part of washed diaphoretic antimony is to be taken ; the sub- 

 stances are well ground together, and fused in the common way. 



Blue Enamel. This fine colour is almost always obtained from the oxide of cobalt 

 or some of its combinations, and it produces it with such intensity that only a very 

 little can be used, lest the shade should pass into black. The cobalt blue is so rich 

 and lively that it predominates in some measure over every other colour, and masks 

 many so that they can hardly be perceived; it is also most easily obtained. To bring 

 it out, however, in all its beauty, the other colours must be removed as much as pos- 

 sible, and the cobalt itself should be tolerably pure. This metal is associated in the 

 best known ores with a considerable number of foreign substances, as iron, arsenic, 

 copper, nickel, and sulphur, and it is difficult to separate them completely ; but for 

 enamel blues, the oxide of cobalt does not require to be perfectly free from all foreign 

 metals ; the iron, nickel, and copper being most prejudicial, should be carefully 

 eliminated. This object may be most easily attained by dissolving the ore in nitric 

 acid, evaporating this solution to a syrupy consistence, to expel the excess of acid, and 

 separate a portion of arsenic. It is now diluted with water, and solution of carbonate 

 of soda is dropped slowly into it with brisk agitation, till the precipitate, which is at 

 first of a whitish grey, begins to turn of a rose-red. Whenever this colour appears, 

 the whole must be thrown on a filter, and the liquid which passes through must be 

 treated with more of the carbonate of soda, in order to obtain the arsenate of cobalt, 

 which is nearly pure. Since arsenic acid and its derivatives are not capable of com- 

 municating colour themselves, and as they moreover are volatile, they cannot impair 

 the beauty of the blue, and hence this preparation affords it in great perfection. 



Metallic fluxes are not the most suitable for this colour ; because they always com- 

 municate a tint of greater or less force, which never fails to injure the purity of the 

 blue. Nitre is a useful addition, as it keeps the oxide at the maximum of oxidation, 

 in which state it produces the richest colour. 



Yellow Enamel. There are many processes for making this colour in enamel ; but 

 it is somewhat difficult to fix, and it is rarely obtained of an uniform and fine tint. It 

 may be produced directly with some preparations of silver, as the phosphate or sul- 

 phate ; but this method does not always succeed, for too strong a heat or powerful 

 fluxes readily destroy it, and nitre is particularly prejudicial. This uncertainty of 

 success with the salts of silver causes them to be seldom employed ; and oxide's of 

 lead and antimony are therefore preferred, which afford a fine yellow when combined 

 with some oxides that are refractory enough to prevent their complete vitrification. 

 One part of white oxide of antimony may be taken with from one to three parts of 

 white lead, one of alum, and one of sal-ammoniac. Each of these substances is to be 

 pulverised, and then all are to be exactly mixed, and exposed to a heat adequate to 

 decompose the sal-ammoniac. This operation is judged to be finished when the yellow 

 colour is well brought out. There is produced here a combination quite analogous to 

 that known under the name of Naples yellow. 



Other shades of yellow may be procured either with the oxide of lead alone, or by 

 adding to it a little red oxide of iron ; the tints varying with the proportion of the 

 latter. 



Clouet says, in his ' Memoir on Enamels,' that a fine yellow is obtained with pure 

 oxide of silver, and that it is merely necessary to spread a thin coat of it on the spot 

 to be coloured. The piece is then exposed to a moderate heat, and withdrawn as soon 

 as this has reached the proper point. The thin film of metallic silver revived on the 

 surface being removed, the place under it will be found tinged of a fine yellow, of 

 hnrdly any thickness. As the pellicle of silver has to be removed which covers the 

 col HIT, it is requisite to avoid fixing this film with fluxes : and it ought therefore to 



