The obstacles which, on the revival of the art, have interposed to check its further extension, and therefore to diminish also the general demand for its productions, are much rather to be attributed to those in whose hands it rests, than to anything properly belonging to itself; they originate in fact less in the art than with the artists.
One of the principal causes of the earlier decay of glass-painting was that its rules being based so entirely upon empirical principles, those who practised it were accustomed to consider the knowledge they had acquired in the thorny path of tedious and long continued experiment as their most valuable personal property, forming at once the means of their subsistence, and the foundation of their future artistical fame. They therefore not only kept the information they had gained profoundly secret during their lives, but even carried it with them to their graves, in preference to leaving it behind them to be made use of by their scholars.
Glass-painting or staining may be defined to mean the art of painting on transparent glass, (either colorless or already colored in the process of its manufacture), with vitrescible metallic colors, which are afterwards burnt into the surface of the glass on which they are laid, leaving it more or less transparent.
All colors used in glass-painting are oxides of metals, or other metallic combinations. They may be divided into two principal classes:
1. Those whose coloring base, or the oxide, is laid upon the glass simply in its original combination with an earthy vehicle.
2. Those whose coloring base, or the oxide, must be made to adhere by the help of a glassy body, namely, the flux.
The colors which require a flux may be divided again into,
1. Those in which the oxide unchanged, but only mixed with the flux, is attached to the glass.
2. Those in which the oxide requires to be vitrified, by previous fusion with the flux, before it is laid on the glass.
The last may be called fused colors, all others mixed colors.