As these would not do for iron, he tried the following : 1. minium, nine parts ; flint glass, six ; pure potash, two; nitre, two ; borax, one ; were ground to gether, put into a covered crucible, which they only half filled, and fused into glass. This poured out on a piece of marble, quenched in water, powdered and made into a thin paste, was laid on both sides of an iron vessel. After hav ing been dried and heated gradually, the vessel was put under a male, well heated in an assaying furnace, and in half a mi nute the enamel melted. The vessel being then withdrawn, was found ena melled of a beautiful black colour, which appeared to be owing to a thin layer of oxided iron seen through the transpa rent glaze. 2. The same, with one hun dreth part of oxide of cobalt prepared as above, covered the vessel more per fectly with a blue enamel. 3. The same, ground with potters' white lead, which consists of four parts of lead and one of tin, produced a very smooth grey enamel more firm and hard than the preceding. A small quantity of red oxide of iron gave it a fine dark red colour. 4. Flint glass, twelve parts ; minium, eighteen ; potash, four; nitre, four ; borax, two ; oxide of tin, three ; oxide of cobalt, one eighth of a part; gave a smooth pearl coloured enamel, not brittle or subject to crack, and capable of enduring sud den changes of heat and cold, as well as the action of oils, alkalies, and weak acids : but it cannot resist the stronger vegetable acids, and still less the mine ral.
These enamels were applied only on hammered iron, cast iron being too thick to be heated with sufficient quickness. But they have been applied to the thin cast vessels in England. It seems unne cessary to add, none of them will bear hard blows ; and this is perhaps the rea son why they have not been more used with us.
The application of enamel colours to glass or earthenware constitutes a pecu liar branch of the art. M. Brougniart, of the porcelain manufactory at S4vres, has given a good account of them. (Ni cholson's Journal, Vol. III. 4to.) These bodies may be divided into three very distinct classes, from the nature of the substances that compose them, the effects produced on them by the colours, and the changes they undergo. These are, 1. enamel ; soft porcelain, and all the glazes, enamels, or glasses, which contain lead in any considerablequantity. 2. Hard porcelain, or such as is glazed with feldspar. 3. Glass, in which there is no lead, such as the common window glass. The principles of composition of these colours, and the general phenome na they present on these three grounds or supporters, are regularly treated of.
Colours in enamel painting have been longest known. Enamel is a glass ren dered opaque by oxide of tin, and very fusible by the oxide of lead. It is this
last, which, in particular, gives it proper ties very different from those of the other excipients of metallic colours. Hence all the glasses and glazes which contain lead have the properties of enamel, and what we may assert of the one will apply to the other with very little difference.
Such are the white and transparent glazes of Dutch or Delf ware ; and the glaze of the porcelain called soft ware.
This porcelain, the first made in France, particularly at Sevres, and in deed for a long time almost exclusively at that manufactory, has for its base vi. treous frit, nearly opaque, capable of be ing acted upon by marle, and its glaze is very transparent glass, containing much lead.
The colours made use of are the same as those for enameling, consequently the changes these colours undergo in enamel must take place in this species of porce lain : the causes of the change being the same in both.
The colours for enamel and soft por celain require less flux than the others, because the glass on which they are placed softens sufficiently to be pene trated by them.
This solvent may be either the mix ture of glass of lead and pure silex, call ed rocaille, or this same glass mixed with that of borax.
Montamy says, that glass of lead ought not to be used in the flux or enamel ; he employs borax alone. He then dilutes or makes up his colours in a volatile oil.
On the contrary, th e painters of the ma nufactory at Sevres use only colours with out borax, because they dilute them with gum, and borax does not dilute them well this way. M. Brougniart is convinced that both methods are equally good, and that Montamy is not justified in exclud ing the fluxes of lead, as they are em ployed without inconvenience every day, and even render the management of co lours more easy.
It is remarked, that in the baking of these colours the glaze is softened so much as to be easily penetrated by them; and this is one great cause of the change they undergo. They become diluted by the mixture with the glaze, and the first fire changes a painting, apparently finish ed, into a very slight sketch.
The oxide of lead contained in the glaze is a more powerful cause of the great changes these colours undergo. Its destructive action is principally exercis ed on the reds of iron, and is very re markable.
It has already been shewn that the two principal causes of the change, which co lours on enamel and tender porcelain un dergo, do not relate to the composition of these colours, but entirely to the na ture of the glass on which theyare placed. The assertion that the colours of porce lain are subject to considerable change, relates to the colours of soft porcelain, a species of ware now almost totally aban doned.