420 EN A MULLING. 



are gold and copper ; and w!th the latter the opaque enamels are 

 only used. Where the enamel is transparent and coloured, the 

 metal chosen should he of that kind, as not only to have its sur- 

 face unalterable when fully red hot, but also to be in no degree 

 chemically altered by the close contact of melted glass, contain, 

 ing an abundance of some kind of metallic oxyd. This is the chief 

 reason why coloured enamelling on silver is impracticable, though 

 the brilliance of its surface is not impaired by mere heat, for if (for 

 example) an enamel made yellow with oxyd of lead, or antimony 

 is laid on the surface of bright silver, and kept melted on it for a 

 certain time, the silver and the enamel act on each other so power- 

 fully, that the colour soon changes from a yellow to an orange, 

 and lastly to a dirty olive. Copper is equally altered by the co- 

 loured anamels, so that gold is the only metal which can bear the 

 long contact of the coloured glasses at a full red heat, without 

 being altered by them. 



The simplest kind of enamel is that fine white opaque glass, 

 which is applied to the dial plate of watches. The process of lay. 

 ing it on (which may serve as a general example of the art) is the 

 following. 



A piece of thin copper sheet, hammered of the requisite con- 

 vexity, is first accurately cut out, a hole drilled in the middle for 

 the axis of the hands, and both the surfaces made perfectly bright 

 with a scratch brush. 



A small rim is then made round the circumference, with a thin 

 brass band rising a little above the level, and a similar rim round 

 the margin of the central hole. The use of these is to confine the 

 enamel when in fusion, and keep the edges of the plate quite neat 

 and even. The substance of the enamel is a fine white opaque 

 glass, the material of which will be presently mentioned. This is 

 bought in lump by the enamellers, and is first broken down with a 

 hammer, then ground to a sufficiently fine powder, with some 

 water, in an agate mortar ; the superfluous water being then 

 poured off, the pulverised enamel remains of about the consistence 

 of wetted sand, and is spread very evenly over the surface of the 

 copper-plate by many dexterous manipulations. On most enamel- 

 lings, and especially on this, it is necessary also to counter ena- 

 mel the under concave surface of the copper plate", to prevent its 

 being drawn out of its true shape, by the unequal shrinking of the 

 metal and enamel on cooling. For this kind of work, the counter 



