442
ENAMELS.
When the fusion is complete, the vitreous matter is to be poured into clear water, and thefrit is then dried, and melted anew. The pouring: into water and fusion are sometimesrepeated four times, in order to secure a very uniform combination. The crucible must becarefully screened from smoke and flame. The smalleft portions of oxyde of iron or cop-per admitted into this enamel will destroy its value.
Some practitioners recommend the use of washed diaphoretic antimony (antinaoniateof potash, from metallic antimony and nitre deflagrated together) for white enamel; butthis product cannot be added to any preparation of lead or other metallic oxydes ; for itwould tend rather to tarnish the color than to clear it up; and it can be used thereforeonly with ordinary glass, or with saline fluxes. For three parts of white glass (withoutlead) one part of washed diaphoretic antimony is to be taken ; the substances are wellground together, and fused in the common way.
Blue enamel .—This fine color is almost always obtained from the oxyde of cobalt orsome of its combinations, and it produces it with such intensity that only a very little canbe used, lest the shade should pass into black. The cobalt blue is so rich and livelythat it predominates in some measure over every other color, and masks many so thatthey can hardly be perceived; it is also most easily obtained. To bring it out, however,in all its beauty, the other colors must be removed as much as possible, and the cobaltitself should be tolerably pure. This metal is associated in the best known ores with aconsiderable number of foreign substances, as iron, arsenic, copper, nickel, and sulphur,and it is difficult to separate them completely; but for enamel blues, the.oxyde of cobaltdoes not require to be perfectly free from all foreign metals; the iron, nickel, and copper,being most prejudicial, should be carefully eliminated. This object may be most easilyattained by dissolving the ore in nitric acid, evaporating the solution to a sirupy consis-tence, to expel the excess of acid, and separate a portion of arsenic. It is now dilutedwith water, and solution of carbonate of soda is dropped slowly into it with brisk agit a 'tion, till the precipitate, which is at first of a whitish gray, begins to turn of a rose-red*Whenever this color appears, the whole must be thrown on a filter, and the liquid whichpasses through must be treated with more of the carbonate of soda, in order to obtainthe arseniate of cobalt, which is nearly pure. Since arsenic acid and its derivatives areHot capable of communicating color themselves, and as they moreover are volatile, theycannot impair the beauty of the blue, and hence this preparation affords it in great P er 'fection.
Metallic fluxes are not the most suitable for this color; because they always cominU'nicate a tint of greater or less force, which never fails to injure the purity of the blu •Nitre is a useful addition, as it keeps the oxyde at the maximum of oxydation, in wh* cstate it produces the richest color.
Yellow Enamel .—There are many processes for making this color in enamel; but iis somewhat difficult to fix, and it is rarely obtained of a uniform and fine tint,may be produced directly with some preparations of silver, as the phosphate or sulpha 1 ■»but this method does not always succeed, for too strong a heat or powerful fluxes ream >destroy it, and nitre is particularly prejudicial. This uncertainty of success withsalts of silver causes them to be seldom employed; and oxydes of lead and antimonytherefore preferred, which afford a fine yellow when combined with some oxydes tharefractory enough to prevent their complete vitrification. One part of white oxj ^
antimony may be taken with from one to three parts of white lead, one of a ^. nl ''. e t£ >one of sal-ammoniac. Each of these substances is to be pulverized, and then allbe exactly mixed, and exposed to a heat adequate to decompose the sal-ammoniac. . goperation is judged to be finished when the yellow color is well brought out. /i>r a plesproduced here a combination quite analogous to that known under the name oiyellow. or by
Other shades of yellow may be procured either with the oxyde of lead alone,^adding to it a little red oxyde of iron; the tints varying with the proportionlatter. 0 xyde
Clouet says, in his memoir on enamels, that a fine yellow is obtained with P^ e ^ e co l-of silver, and that it is merely necessary to spread a thin coat of it on the spo . ] ia sored. The piece is then exposed to a moderate heat, and withdrawn as soon as ^ e \ n greached the proper point. The thin film of metallic silver revived on the sur a j. neS s.removed, the place under it will be found tinged of a fine yellow, of hardiy any aV oi4As the pellicle of silver has to be removed which covers the color, it is requisifixing this film with fluxes; and it ought therefore to be applied after the t«si ^ of aresl. The yellows require in generaf little flux, and they answer better winmetallic nature. ^ • tnreof
Green Enamel .—It is known that a green color may be produced by a mix c btaia"and blue ; but recourse is seldom had to this practice for enamels, as theycan ^ rde 0 fed almost always directly with the oxyde of copper; or still better withchrome, which has the advantage of resisting a strong heat.