And here I may say that certain important technical difficulties, that must always have hampered the use of true transparent enamels on glass, have scarcely received the attention that they deserve. I mean the relations of the enamels, as regards the softening-point and rate of contraction on cooling, to the ground on which they rest. The question here is very similar to that which presents itself in the case of porcelain. Our present problem is, however, somewhat simpler, for with the latter material we have not only to consider the relation of the enamels to the glaze on which they lie (this takes, indeed, the place of our glass ground), but in addition the relation of the glaze itself to the porcelain body beneath must not be neglected. The first condition for the successful application of an enamel is that it should be more fusible than the glass to which it is applied; not only that, but at the temperature at which the enamel fuses, the glass must still maintain its rigidity, otherwise the vessel on coming from the enameller’s stove will not preserve its original symmetry. It has been already suggested that the partial collapse so often observed in the large Cairene lamps may probably be explained in this way. On the other hand, if the surface of the glass is not to some degree softened, there will be no intimate connection between it and the enamel, and the latter will be likely to scale off before long. This tendency will be increased if there is much difference in the rate or amount of contraction between the two materials. Difficulties of this kind long hindered the employment of certain fluxes and colours—that of cobalt, for instance, combined with a transparent flux. Such obstacles may, however, be surmounted in a measure, and the process simplified by employing (in place of a transparent lead flux) an opaque white, stanniferous enamel merely stained, in cases only externally, by a little colouring material. This apparently was the plan universally adopted by the Venetians in the fifteenth century, and it is here that their experience of the use of a similar enamel on copper may have served them. One cannot but marvel at the technical dexterity so early acquired, and, alas! so soon lost by the Saracens, in the application of enamels to glass. The means by which they avoided the use of a lead flux in the case of their famous translucent blue, is above all worthy of admiration (see above, Chapter X.).