272 ENAMELS 



be applied after the fusion of the rest. The yellows require in general but little 

 alkaline flux, as they answer better with one of a metallic nature. 



Green Enamel. It is known that a green colour may bo produced by a mixture of 

 yellow and blue ; but recourse is seldom had to this practice for enamels, as they can 

 be obtained almost always directly with the oxide of copper; or, still better, with the 

 oxide of chrome, which has the advantage of resisting a strong heat. 



There are two oxides of copper, the suboxido of an orange colour, which com- 

 municates its colour to enamels, but it is difficult to fix ; the protoxide is blue in the 

 state of hydrate, but blackish -brown when dry, and it colours green all the vitreous 

 combinations into which it enters. This oxide requires, at most one or two proportions 

 of flux, either saline or metallic, to enter into complete fusion ; but a much smaller 

 dose is commonly taken, and a little oxide of iron is introduced. To 4 pounds of frit, 

 for instance, 2 ounces of oxide of copper and 48 grains of red oxide of iron are used ; 

 and the ordinary measures are pursued for making very homogeneous enamel. 



The green produced by oxide of chrome is much more solid ; it is not affected by a 

 powerful fire, but it is not always of a fine shade. It generally inclines too much to 

 the dead-leaf yellow, which depends on the degree of oxygenation of the chrome. 



Red Enamel. Wo have just stated, that suboxide of copper afforded a fine colour 

 when it could be fixed, a result difficult to obtain on account of the fugitive nature of 

 this oxide ; slight variations of temperature enabling it to absorb more oxygen. The 

 proper point of fusion must be seized for taking it from the fire whenever the desired 

 colour is brought out. Indeed, when a high temperature has produced peroxidation, 

 this may be corrected by adding some combustible matter, as charcoal, tallow, tartar, 

 &c. The copper then returns to its lower stage of oxidation, and the red colour which 

 had vanished, reappears. It is possible, in this way, and by pushing the heat a little, 

 to accomplish the complete reduction of a part of the oxide ; and the particles of 

 metallic copper thereby disseminated in a reddish ground, give this enamel the aspect 

 of the stone called aventurine. The surest and easiest method of procuring suboxide 

 of copper is to boil a solution of equal parts of sugar, and sulphate or rather acetate 

 of copper, in four parts of water. The sugar takes possession of a portion of the 

 oxygen of the cupric oxide, and reduces it to cuprous oxide ; when it may be precipitated 

 in the form of a granular powder of a brilliant red colour. After about two hours' 

 moderate ebullition, the liquid is set aside to settle, decanted off the precipitate, which 

 is washed and dried. 



The suboxide properly employed by itself, furnishes a red which vies with the 

 finest carmine, and by its means every tint may be obtained from red to orange, by 

 adding a greater or smaller quantity of peroxide of iron. 



The preparations of gold, and particularly the oxide and purple of Cassius, are like- 

 wise employed with advantage to colour enamel red, and this composition resists a 

 powerful fire tolerably well. For some time back, solutions of gold, silver, and plati- 

 num have been used with success instead of their oxides ; and in this way, a moro 

 intimate mixture may be procured, and, consequently, more homogeneous tints. 



Black Enamel. Black enamels are made with peroxide of manganese or protoxide 

 of iron ; to which more depth of colour is given with a little cobalt. Clay alone, 

 melted with about a third of its weight of protoxide of iron, gives, according to Clouct, 

 a fine black enamel. 



Violet Enamel. The peroxide of manganese in small quantity by itself furnishes, 

 with saline or alkaline fluxes, an enamel of a very fine violet hue ; and variations of 

 shade are easily had, by modifying the proportions of the elements of the coloured 

 frit. The great point is to maintain the manganese in a state of peroxidation, and, 

 consequently, to beware of placing the enamel in contact with any substance attract ivo 

 of oxygen. 



Such are the principal coloured enamels hitherto obtained by means of metallic 

 oxides ; but it is to be wished that new trials bo made with such oxides as have not 

 yet been employed. From such researches some interesting results would unquestion- 

 ably be derived. 



Of painting on Enamel. Enamelling is only done on gold and copper ; for silver 

 swells up, and causes blisters and holes in the coat of enamel. All enamel paintings 

 are, in fact, done on either copper or gold. 



If on gold, the goldsmith prepares the plate that is to bo painted upon. The gold 

 should be 22 carats fine : if purer, it would not bo sxifficiently stiff; if coarser, it would 

 be subject to melt ; and its alloy should be half white and half red, that, is, half silver 

 and half copper ; whereby the enamel with which it is covered will bo less disposed 

 to turn green, than if the alloy wore entirely copper. 



The workman must reserve for the edge of tin- pl.it r a small fillet, which IK- calls the 

 border. This ledge serves t-> retain the enamel, and hinders it from falling off when 

 applied and pressed on with a spatula. When the plate is not to be counter-enamelled. 



