DELFT ware, a kind of pottery of baked earth, covered with an enamel or white glazing, which gives it the appear ance and neatness of porcelain. Some kinds of this enamelled pottery differ much from others, either in their sustain ing sudden heat without breaking, or in the beauty and regularity of their forms, of their enamel, and of the painting with which they are ornamented. In general, the fine and beautiful enamelled potte ries, which approach the nearest to por celain in external appearance, are at the same time, those which least resist a brisk fire. Again, those which sustain a sudden heat are coarse, and resemble common pottery. The Casis of this pot tery is clay, which is to be mixed with such a quantity of sand, that the earth shall preserve enough of its ductility to be worked, moulded, and turned easily ; and yet that its fatness shall be sufficient ly taken from it, that it may not crack or shrink too much in drying or in baking. Vessels formed of this earth must be dried very gently, to avoid cracking. They are then to be placed in a furnace to receive a slight baking, which is only meant to give them a certain consistence or hardness. And, lastly, they are to be covered with an enamel, or glazing, which is done by putting upon the ves sels thus prepared, the enamel which has been ground very fine, and diluted with water.
As vessels on which the enamel is ap plied are but slightly baked, they readily imbibe the water in which the enamel is suspended, and a layer of the enamel ad heres to their Surface ; these vessels may then be painted with colours composed of metallic calces, mixed and ground with sensible glass. When they are be come perfectly dry, they are to be placed in the furnace, included in cases of baked earth, called seggars, and exposed to a heat capable of fusing uniformly the ena mel which covers them. This heat gi ven to fuse the enamel, being much stronger than that which was applied at first to give some consistence to the ware, is also the heat necessary to com plete the baking of it. The furnace and colours used for painting this ware, are the same as those employed for porcelain.
The glazing, which is nothing but ,white enamel, ought to be so opaque as not to show the ware under it. There are many receipts for making these enamels ; but all of them are composed of sand or flints, vitrifying salts, oxide of lead, and oxide of tin ; and the sand must be perfectly vitrified, so as to form a glass consider ably fusible. Somewhat less than an equal
part of alkaline salt, or twice its weight of oxide of lead, is requisite to effect such vitrifications of sand. The oxide of tin is not intended to be vitrified, but to give a white opaque colour to the mass ; and one part of it is to be added to three or four parts of all the other ingredients ta ken together. From these generil prin ciples various enamels may be made to suit the different kinds of earths. To make the enamel, lead and tin are oxyd ed together with a strong fire ; and the sand is also to be made into a frit with the salt or ashes. The whole is then to be well mixed and ground together. The matter is then to be placed under the furnace, where it is melted and vitrified during the baking of the ware. It is next to be ground in a mill, and applied as above directed.
Concerning the earth of which the ware is made, pure clay is not a proper material when used alone. Different kinds of earth, mixed together, are found to succeed better ; pieces of ware made of clay alone are found to require too much time to dry, and they crack and lose their form, unless they are made ex ceedingly thick ; all addition of marle di minishes the contraction of the clay, ren ders it less compact, and allows the water to escape, without altering the form of the ware in drying. It affords also a bet ter ground for the enamel, which appears more glossy and white than when laid on clay alone. The kinds of clay which are chiefly used in the composition of delft ware, are the blue and green. A mixture of blue clay and marle would not be sufficiently solid, and would be apt to scale, unless it were exposed to a fire more intense than what is commonly used for the burning of delft ware. To give a greater solidity, some red clay is added ; which on account of its ferru ginous matter possesses the requisite binding quality. The proportions of these ingredients vary in different works, according to the different qualities of the earths employed. Three parts of blue clay, two parts of red clay, and five parts of marle, form the composition used in several manufactories. See ENAMEL.