1901.] on Enamels. 519 



whom this is chiefly due is Petitot, a jeweller of Geneva, who mi- 

 grated to Paris, where he executed a great number of most beautiful 

 miniatures. 



I now pass to the mode of making the enamel. I do not propose 

 to describe the various recipes that exist. I will only say that the 

 simplest method is the best. For whereas in order that a glaze may 

 not craze upon china or earthenware, it must be most carefully com- 

 pounded to suit the substance upon which it is placed ; any glass that 

 is easily fusible may be melted on to, and will adhere to copper. 



For cheapness in glazing earthenware, the raw ingredients of 

 glass are mixed together, sometimes with a little fritting or half 

 melting, and are put on in this raw state. It would be better, of 

 course, to use a well-made glass. For enamelling on metals wbich 

 cannot be exposed to very great heat, it is necessary to compound the 

 glass beforehand. 



As in art work the expense of the material is no object, tbe 

 enameller will begin with good flint glass. This, as you know, is 

 what is used for the lenses of optical instruments and for cut table- 

 decanters, in which the high refractive power due to the lead gives 

 lustre and the colours of the rainbow. Sham diamonds are also made 

 of very heavily leaded glass. 



We therefore mix powdered flint glass with oxides of various 

 metals and melt it. Two or three hours' heat is enough to incor- 

 porate the mass. It is then poured out into cakes upon a surface of 

 greased iron, and marked while hot with the maker's monogram. 



There is one colour that is so interesting that I must specially 

 call your attention to it. If to a glass, about -j-qV o" °^ i* s weight °f 

 chloride of gold is added, together with some reel-lead and an oxide 

 of uranium, tin, iron, or some electro-negative metal, then, when melted 

 and poured, it remains white. 



But if it is then re-heated, it suddenly becomes crimson. The 

 reason of this curious change is not known, but the subject was 

 investigated by Faraday. 



We now come to the method of putting the enamel upon the 

 metal. To do this, it must be pounded finely and washed. For if 

 the grains are too fine, bubbles of air are entangled in the mass 

 and the transparency is spoiled. Therefore, by elutriation, the finer 

 particles are removed, and only those left which are about T ^ of an 

 inch in diameter, like sand. 



The plate bein^ cleaned, the enamel is put on wet, smoothed and 

 patted down, dried over a hot plate, and then put into a muffle- 

 furnace at a temperature of about 1200° Fahr. — a nice clear red. 

 There are a number of details which have to be observed as to the 

 thorough drying of the plate and the method of manipulation, with 

 which I will not weary you. I have dealt with them at length in a 

 little work which I have published. 



If the copper plate is thin, then, if the enamel is only put on one 

 side, the unequal contraction of the glass and copper causes it to fly 



