422 ENAMELLING. 



quired. If the heat is too low, the enamel does not spread and 

 vitrify as it ought ; if too high, it may be enough to melt the metal 

 itself, whose fusing point is but a small step above that of the ena. 

 mel, or else (what is an equal mortification to the artist) the d< li. 

 cate figures, laid on with so much care and judgement, meltdown 

 in a moment, and the piece exhibits only a confused assemblage of 

 lines, and fragments of designs. 



The exact composition of the opaque white enamel, is a matter 

 of considerable importance, and is procured by the enamellers 

 from persons whose business it is to prepare it. A good enamel 

 of this kind, fit to be applied to porcelain and metals, should be of 

 a very clear fine white, so nearly opaque, as only to be translucent 

 at the edges ; and at a moderate red heat it should run into that 

 kind of paste, or imperfect fusion which allows it to extend itself 

 freely and uniformly, and to acquire a glossy even surface, without, 

 however, fully melling into a thin glass. The opaque white of this 

 enamel is given by the oxyd of tin, which possesses, even in a 

 small proportion, the property of rendering vitrescent mixtures 

 white and opaque, or in still less proportion, milky; and when 

 otherwise coloured, opalescent. The oxyd of tin is always mixed 

 with three or four times its quantity of oxyd of lead ; and it ap. 

 pears necessary that the metals should be previously mixed by 

 melting, and the alloy then calcined. The following are the direc. 

 tions given by Clouet for the composition of this enamel. Mix 

 100 parts of pure lead with from 20 to 25 of the best tin, and 

 bring them to a low red heat in an open vessel. The mixture then 

 burns nearly as rapidly as charcoal, and oxidates very fast. Skim 

 off the crusts of oxyd, successively formed, till the whole is tho- 

 roughly calcined. It is better then to mix all the skimmings, and 



a"ain heat as before, till no flame arises from them, and the whole 



o 



is of an uniform grey colour. Take 100 parts of this oxyd, 100 

 of sand, and 25 or 30 of common salt, and im-lt the whole in a 

 moderate heat. This gives a greyish mass, often porous and appa. 

 rently imperfect, but which, however, runs to a good enamel when 

 afterwards heated. This is the enamel used for porcelain, but for 

 metals and finer works the sand is previously calcined in a very 

 strong heat with a fourth of it* weight, or, if a more fusible com. 

 pound is wanted, as much of the oxyd of tin and lead as of salt is 

 taken, and the whole melted to a white porous mass. This is then 

 employed instead of the rough sand as in the above-mentioned pro. 



