ENAMELS.

441

the necessity of selecting carefully the particular sand that should enter into the composition of the frit, and they even affirm that the purest is not the most suitable. Clouetstates, in the 34th volume of th ^Annales de Chimie , that the sand ought to contain atleast 1 part of talc for 3 of silicious matter, otherwise the enamel obtained is never veryglassy, and that some wrinkled spots from imperfect fusion are seeen on its surface; andyet we find prescribed in some old treatises, to make use of ground flints, fritted by means°1 salt of tartar or some other flux. It would thence appear that the presence of talc is°1 no use towards the fusibility of the silica, and that its absence may be supplied by in-creasing the dose of the flux. In all cases, however, we ought to beware of metallicox ydes in the sand, particularly those of iron and manganese, which most frequently occur,a »dalways injure the whiteness of the frit.

dhe ancients carried the art of enamelling to a very high perfection, and we occasion-find beautiful specimens of their work, of which we know neither the composition,or the manner of applying it. Then, as at present, each artist made a mystery of theea ns that succeeded best with him, and thus a multitude of curious processes have beenUr ied with their authors. Another cause contributes powerfully to this sort of declen-j o ° n ln the arts. Among the vast number of recipes which have been published for theIn Dla *'* on °f enamels, there are several in which substances are mentioned that can no„ = er be procured, whether owing to a change of denomination, or because the substan-1 ? c annot now be found in commerce, or because they are not of the same nature as of_ ' Hence, in many cases, we find it impossible to obtain satisfactory results. What® have now said renders it desirable that the operations should be resumed anew, or; P°h new bases, and availing ourselves of all the known chemical facts, we should employ

1,1 the

production of enamels, raw materials of the purest kind.

jY be Venetians are still in possession of the iest enamel processes, and they supply thehch and other nations with the best kinds of enamel, of every colored shade,hav llaine ^ s are distinguished into transparent and opaque; in the former all the elementstfhd ex E )ei 'ienced an equal degree of liquefaction, and are thus run into crystal glass,thei St ' a tbe otbers > some of their elements have resisted the action of heat more, so thateff Particles retain sufficient aggregation to prevent the transmission of light. This'vhb Is Produced, particularly by the oxyde of tin, as we shall perceive in treating ofe »amel.

ity e frits for enamels that are to be applied to metallic surfaces require greater fusibil-bef 0 s bould therefore contain more flux; and the sand used for these should be calcineda 'He 1 ] "d w ’th one fourth its weight of sea-salt j sometimes, indeed, metallic fluxes areihrio ’ aS or litharge. For some metallic colors, the oxydes of lead are very in-

derif f’ :iru ^ * n Ibis case recourse must be had toother fluxes. Clouet states that he hadoth e , , advantage from the following mixtures, as bases for purples, blues, and somedelicate colors :—

glass parls op silicious sand, one of chalk, and three of calcined borax; or, three ofpa°! broken crystal goblets), one of calcined borax, one fourth of a part of nitre, andena m op . We li washed diaphoretic antimony. These compositions afford a very whiteIt [ S J T^ich accords perfectly well with blue.

' Ve shon] i 10US composition of this primary matter may be greatly varied ; but

a t a mod^ never * ose sight of the essential quality of a good enamel; which is, to acquire,ft is erare heat, sufficient fluidity, to take a shining surface, without running too thin.Sive i t S? m Plete fusion which is wanted ; but a pasty state, of such a degree as may, e . r Co °ling, the aspect of having suffered complete liquefaction.at »y °ther ^ 1 ™ ~^ name ^—This requires greater nicety in the choice of its materials thanhen Ce ^ enamel, as it must be free from every species of tint, and be perfectly white ;

. jj 1 em Ployed in this case should be itself composed of perfectly pure ingre-Slt >ce thi s U a ft should not be rejected hastily because it may be somewhat discolored,P ar ficle s Dr may ^. e P en 4 on two causes; either on some metallic oxydes, or on fuliginousth eas fty ren? 66 i ^ *' rom Ve s eta ^ e or nnimal substances. Now the latter impurities mayae Property °f^ means of a small quantity of peroxyde of manganese, which has0ln hustion th 5®**^ P art ‘ n g with a portion of its oxygen, and of thus facilitating theese indeed* n & 1S t0 say ’ destruction °f the coloring carbonaceous matter. Manga -.^ydizemgut; P !? ss . ess ^ s a Coring power itself on glass, but only in its highest state ofa ° 'ouger co 0 W * n re d uce d to the lower state, as is done by incombustible matters,a 0ganese shoulT UniCales co ^ or t0 the enamel combinations. Hence the proportion ofp/ 1168 ) indeed it h never excee d what is just; for the surplus would cause color. Some-con' e a " r eeable ne ? essar y to give a little manganese color, in order to obtain a

Pteraet the Hni a( e °‘. wll ft e 5 as a little azure blue is added to linens, to brighten orp* ' Vt >^ ena£“ 5 of K their * «nt.

Wi.if of ' tin and one Him i conveniently prepared also with a calcine composed of twol ' v o parts of finp ! ' il: ® a ' CIne d together; of this combined oxyde,one part is meltedystal and a very little manganese, all previously ground together.