32. Diapers scratched out.
Now that the reader may be presumed to have a perfectly clear idea of the process of the early glazier, and to realise the distinctly mosaic character of old glass, it is time mention should be made of two important intermediate methods of glass staining which presently began to affect the character of stained glass windows.
Allusion has been made ([page 2]) to the Roman practice of making glass in strata of two colours, which they carved cameo-fashion in imitation of onyx and the like; at least, one tour de force of this kind is familiar to every one in the famous Portland vase, in which the outer layer of white glass is in great part ground away, leaving the design in cameo upon dark blue. The mediæval glass-blower seems from the first to have been acquainted with this method of coating a sheet of glass with glass of a different colour. As the Roman coated his dull blue with opaque white glass, so he coated translucent white with rich pot-metal colour. It was not a very difficult operation. He had only to dip his lump of molten white into a pot of coloured glass, and, according to the quantity of coloured material adhering to it, so his bubble of glass (and consequently the sheet into which it was opened out) was spread with a thinner or thicker skin of colour. The Gothic craftsman took advantage of this facility, in so far as he had any occasion for its use. The occasion arose owing to the density of the red glass he employed, which was such that, if he had made it of the thickness of the rest of his glass, it would have been practically opaque. To have made it very much thinner would have been to make it more fragile; and in any case, it was easier to make a good job of the glazing when the glass was all pretty much of a thickness. A layer of red upon white offered a simple and practical way out of the difficulty.
33. Diaper scratched out.
What is called “ruby” glass, therefore, is not red all through, but only throughout one half or a third of its thickness. The colour is only, so to speak, the jam upon the bread; but the red and the white glass are amalgamated at such a temperature as to be all but indivisible, to all intents and purposes as thoroughly one as ordinary pot-metal glass.
For a long while glass painters used this ruby glass and a blue glass made in the same way precisely as though it had been self-coloured. But in shaping a piece of ruby glass, especially with their inadequate appliances, they would be bound sometimes to chip off at the edges little flakes of red, revealing as many little flaws of white. This would be sure to suggest, sooner or later, the deliberate grinding away of the ruby stratum in places where a spot of white was needed smaller than could conveniently be leaded in. As to the precise date at which some ingenious artist may first have used this device, it may be left to archæology to speculate. It must have been a very laborious process; and the early mediæval ideal of design was not one that offered any great temptation to resort to it during the thirteenth or even the fourteenth century. It was not, in fact, until the painting of windows was carried to a point at which there was some difficulty in so scheming the lines of the lead that they should not in any way mar its delicacy, that the practice of “flashing” glass, as it is termed, became common. That is why no mention of it has been made till now. It will be seen that it is a perfectly practical and workmanlike process, rendering possible effects not otherwise to be got in glass, but lending itself rather to minuteness of execution and elaboration of detail than to splendour of colour or breadth of effect.
