Page:Popular Science Monthly Volume 40.djvu/313

Rh which was held by the water in suspension, upon the surface of the piece. This piece of porous biscuit covered with glaze is now cleaned of glaze upon its foot, or that part upon which it rests, to prevent its sticking or burning fast to the clay "sagger" or firing case; otherwise the glaze on the bearing parts would, at the time of flowing, form a cement, fastening the piece and the sagger together. The pieces are placed separately in the saggers. The heat in firing hard porcelain is carried to such a high degree that the ware touches the point of pliability, almost the melting-point. At this point of heat the body is vitrified; at the same time the glaze, from its slightly softer composition, is melted into the body of the ware, producing a hard, vitreous, and homogeneous material properly known as true, hard porcelain. This is the process used at Sèvres, Meissen, Berlin, and elsewhere.

The earthenware method is just the reverse of this. The body is composed of much the same materials as a porcelain body, but differently compounded, and it is baked in biscuit at the first firing at a greater heat than is required for porcelain biscuit, and receives during that first burning the greatest heat to which it is subjected in the entire process of manufacture. The glaze is composed partly of the same materials as compose the body, with the addition of oxide of lead and boracic acid, which latter, being soft, fluxes in the fire, enabling the glaze to flow at a low heat. It is fired the second time in the gloss-kiln at a lower temperature than it has previously been fired in biscuit. This results in flowing the soft glaze over the surface of the ware, making substantially a lead-glass film or coating upon the surface of different compounds and materials, not homogeneous, not a part of the