270 ENAMELS 



oxygen, that it may bo calcined with tho greatest c-aso in a flat cast-iron pot, and at a 

 temperature not above a cherry red, provided tho dose of tin is not too great. Tho 

 oxide is drawn off to tho sides of tho molted metal, according as it is generated, now 

 pieces of the alloy being thrown in from time to time, till enough of the powder bo 

 obtained. Great care ought to bo taken that no metallic particles bo loi't in tho oxide, 

 and that the calcining heat be as low as is barely sufficient ; for a strong firo frits t)u> 

 powder, and obstructs its subsequent comminution. The powder when cold is ground 

 in a proper mill, levigated with water, and elutriated. In this state of fineness :md 

 purity, it is called calcine or flux, and it is mixed with siliceous sand and some alka- 

 line matter or sea-salt. The most ordinary proportions are, 4 of sand, 1 of sea-sail, 

 and 4 of calcine. Chaptal states, that he has obtained a very fine product from 100 

 parts of calcine, made by calcining equal parts of lead and tin, 100 parts of ground 

 flint^ and 200 parts of pure sub-carbonate of potash. In either case, the mixture is 

 put into a crucible, or laid simply on a stratum of sand, quicklime spontaneously 

 slaked, or wood-ashes placed under a pottery or porcelain kiln. This mass undergoes 

 a semivitrification, or even a complete fusion on its surface. It is this kind of frit 

 which serves as a radical to almost every enamel ; and by varying the proportions of 

 the ingredient, more fusible, more opaque, or whiter enamels are obtained. The first 

 of these qualities depends on tho quantity of sand or flux, and tho other two on that 

 of the tin. 



The sea-salt employed as a flux may be replaced either by salt of tartar, by pure 

 potash, or by soda ; but each of these fluxes gives peculiar qualities to tho enamel. 



Most authors who have written on the preparation of enamels, insist a great deal 

 on the necessity of selecting carefully tho particular sand that should enter into tho 

 composition of the frit, and they even affirm that the purest is not tho most suit- 

 able. Clouet states, in the 34th volume of the Annales do Chimie,' that the sand 

 ought to contain at least 1 part of talc for 3 of siliceous matter, otherwise the enamel 

 obtained is never very glassy, and that some wrinkled spots from imperfect fusion 

 are seen on its surface ; and yet we find it prescribed in some old treatises, to make 

 use of ground flints, fritted by means of salt of tartar or some other flux. It would 

 thence appear that tho presence of talc is of no use towards the fusibility of the 

 silica, and that its absence may be supplied by increasing the dose of the flux. In all 

 cases, however, we ought to beware of metallic oxides in tho sand, particularly those 

 of iron and manganese, which most frequently occur, and always injure the whiteness 

 of the frit. 



The frits for enamels that are to be applied to metallic surfaces require greater fusi- 

 bility, and should therefore contain more flux ; and the sand used for these should be 

 calcined beforehand with one-fourth its weight of sea-salt; sometimes, indeed, metallic 

 fluxes are added, as minium or litharge. For some metallic colours, the oxides of lead 

 are very injurious, and in this case recourse must be had to other fluxes. Clouet states 

 that he has derived advantage from the following mixtures, as bases for purples, blues, 

 and some other delicate colours : 



Three parts of siliceous sand, one of chalk, and three of calcined borax ; or, throe of 

 glass (of broken crystal goblets), one of calcined borax, one-fourth of a part of nitre, 

 and one part of well-washed diaphoretic antimony. These compositions afford a very 

 white enamel, which accords perfectly well with blue. 



It is obvious that tho composition of this primary matter may be greatly varied : 

 but we should never lose sight of the essential quality of a good enamel ; which is, 

 to acquire, at a moderate heat, sufficient fluidity to take a shining surface, without 

 running too thin. It is not complete fusion which is wanted, but a pasty state, of 

 such a degree as may give it, after cooling, tho aspect of having suffered complete 

 liquefaction. 



Dead-white Enamel. This requires greater nicety in the choice of its materials 

 than any other enamel, as it must be free from every species of tint, and be perfectly 

 white; hence tho frit employed in this case should bo itself composed of perfectly 

 pure ingredients. But a frit should not bo rejected hastily because it may be some- 

 what discoloured, since this may depend on two causes; either on somo metallic 

 oxides, or on fuliginous particles proceeding from vegetable or animal substances. 

 Now the latter impurities may be easily removed by means of a small quantity of 

 peroxide of manganese, which has tho property of readily parting with a portion of 

 its oxygen, and of thus facilitating the combustion, that is to say, tho oxidation of 

 tho colouring carbonaceous matter. Manganese, indeed, possesses a colouring power 

 itself on glass, but only in its highest state of oxidiscmont, and when reduced to tho 

 lower state, as is done by combustible matters, it no longer communicates colour to tho 

 enamel combinations. Hence tho proportion of manganese should never l>o in ex 

 for tho surplus would cause colour. Sometimes, indeed, it becomes necessary to ^ivn 

 a little manganese-colour (i.e. a pink tint) in order to obtain a more ngrwaMc- s! 



