Although pottery glazes are true glass, glazes are meant to be applied as a thin coating on the surface of the pottery. To hold their position on the surface of the pottery; they must be quite stiff when melted. The type of glass used in fusing is fairly runny when molten and it is applied mainly in the form of cut sheets. Glazes are made by blending the un-melted, raw materials and spreading these blended materials on the surface of pottery, where the glaze is melted into place. Glass, on the other hand, is melted first in a pot of molten liquid material and then fashioned into objects and sheets. A glaze on the surface of pottery is in slight compression. This means that there is stress on the surface of ordinary pottery. If you were to make the glaze thicker the stress would be too great, and cause crackling of the glaze and in extreme situations cracking of the pottery.