Características de la base y su relación con el esmalte[]
Not only the chemical composition of the glaze exerts an influence on the properties of the same, but also the physical condition of the body and glaze. This fact is founded in the nature of the case. The reception of the glaze by the body generally takes place in such a way that the vessel to be glazed, after it has been more or less hard burnt, is dipped into the glaze which has been made up into a fluid mass. The layer of the glaze should form an even and homogeneous coating on the ware, without cracks, bare or dry spots, or holes. This requires that, on the one hand, the body must be homogeneous, for all inequalities will be marked by thick or thin spots in the glaze, and on the other hand, that the glaze shall possess the same property, in order that it shall not, on burning, form a cracked or wrinkled covering on the surface to be glazed.
In order to obtain a faultless glaze, it is above all things necessary that the body shall be free from air-holes and shall be homogeneous. Irregular bodies, which after burning contain softer and harder parts, absorb unequal quantities of the glaze slip, on the different portions. Porous bodies allow the air bubbles, which form when the glaze is applied, to escape unequally and produce a "pin-holed" glaze. Also, the degree of the biscuit burn has a great deal to do with the uniformity of the glaze layer. As a rule, bodies, which absorb water well, will also glaze well, though there is a limit to the operation of this statement. If the bodies are too soft burnt, or if they show a powdery, dusty surface, or if this dust is not thoroughly removed, they will take the glaze but poorly, and the glaze surface will show cracks on drying, or at the beginning of the burn; it rolls together and forms drops in one place and in another place it shows spots bare of glaze.
In porcelain another trouble comes as a consequence of too soft-burning in the biscuiting process which is called "pockmarking." The air escapes from the body with violence when it is dipped in the glaze slip, and if the body is too soft burnt, it may happen that the body is softened, or its internal cohesion overcome. Through this arise hollows, cracking and splitting of the body which, although not always noticeable, gives rise on burning to small prominences, owing to the fact that the air which is enclosed by the melting of the glaze expands in the heat.
If the glaze is to be carried by a raw clay (as is the case in the glazing of ordinary pottery, Bunzlau stoneware, etc.), the body must be in such a condition as best to endure the effect of the water on its surface. This is that condition known as "leather-hard," or "water-hard." If one attempted to put the glaze on the dry ware, the latter would, in consequence of the rapid absorption of water, be subjected to the danger of softening or swelling on the surface. In this case the deposition of the glaze layer does not occur from the absorption of the water by the body, but by the drying of a very thickly fluid slip on the surface.
If colors are used under the glaze, they often occasion defects, because they interfere with the adhesion of the glaze layer. Those colors especially act in this way, which are difficultly attacked by the glaze, and are infusible by themselves such as chrome oxide, cobalt-alumina oxide, pink, and others. Such colors form a separating layer between the glaze and the body, and hinder by their interposition, the necessary adhesion between the two. They occasion either a scaling off of the glaze, or a collecting together, or heaping up in single places (curdling). In such cases the mixing of the color with some glaze is the proper remedy in order to cause a fritting of the same.
It has already been said that the ingredients of the glaze must be applied in a finely ground condition to the surface of the ware. This is not done only to occasion an intimate mixture of the ingredients of the glaze, but also to permit an adhesion between them and the surface of the ware, for it is evident that the ingredients can only have the necessary adhesion after the water has been absorbed or evaporated from the surface of the ware, if they are sufficiently fine to secure a kind of interlacing or matting.
But one can go too far in this respect and introduce new troubles. The more a glaze is ground, the more water it takes up, and the more it becomes a sort of plastic substance. But the more plastic it is, the more it tends, on drying or on vitrifying, to crack, and there arise, through this, cracks or a scaling off of the glaze, or the glaze gathers together in spots and allows wrinkles or thick places to form. This defect occurs in direct proportion to the thickness of the layer of the glaze.