Lynne Carson Rickards and Sally Joyce Rush on colouring glass over the years.

There are three basic ways to colour glass: first, by adding metal oxides to the molten material and producing homogeneous colour throughout the body of the glass; secondly, bycoveringa smailgather of coloured glass with a larger gather of clear glass and blowing it into a cylinder to produce flashed glass, having a thin layer of coloured glass on one side; and thirdly, byapplyingstain, paint orenamel to the surface of the glass and firing it at a high enough temperature for it to fuse with the glass.

The art of colouring glass in its molten statewasfirstdevelopedinancienttimes, and probably derived from the glazing of ceramics. For many centuries, glass colouring was carried out by trial and error with little understanding of the chemistry involved, and effective recipes were passed on through generations of glassmakers with considerable secrecy. Medieval artisans achieved a very high degree of skill in the colouring of glass, and the industry thrived across Europe until the mid-16th century.
Religiousandpoliticalupheavalduring the Reformation brought widespread destruction of decorative church windows in Europe, as Protestants pulled down offending monuments to Roman Catholicism. The demand for coloured
glass was thus greatly reduced, and artisans turned to secular commissions such as civic and domestic windows decorated with armorial designs. In the latterhalfofthe l6thcentury, glassmaking suffered a serious decline, as religious conflict and economic hardship combined to close down all but a few glassmaking centres. Lack of demand led to a virtual cessation of coloured glassmaking, and any decorative glass was coloured using stain or enamel. The 17th century saw a gradual loss of expertise, and, by the time interest in the art of coloured glass was revived in the late 18th century, glaziers were forced to reinvent the techniques of medieval craftsmen.
Makingpot-colouredglassisacomplex process, since the same metal oxide can produce arange of colours depending on the quantity added, the composition of theglass itself andthefurnace conditions. The state of the furnace is of particular importance, since the amount of oxygen being consumed by the fuel will affect the reaction between the added metal oxide and the glass batch. A reducing environment is produced when the fuel is newly added and is oxygen-hungry. As the fuel burns oxygen is drawn out of the furnace atmosphere and out of the glass batch as well, altering the number of oxygen ions in the molten glass. The reduced metal oxide does not melt, but instead is distributed evenly in tiny particles, creating a suspension (colloid)
which reflects light and produces the appearance of colour. An oxidising environment, on the other hand, is produced when the fuel has been more fully burned, and requires less oxygen to maintain itself. In this case the metal oxide added to the glass batch fuses with the silica to produce a coloured silicate, and the higher levels of oxygen ions in the glass produce a different range of colours.

The invention offlashed glass came about because certain colours of glass, particularly ruby, were so intense that even the thinnest of panes absorbed most light and appeared black; applying athin layer of the colour to a clear body allowed the glass to transmit enough light for the colour to be visible. The process of flashing glass is described in Theophilus’ SchedulaDiversarumArtium(datingfrom the 12th century), and was used to great advantage in decorative windows, since the coloured glass could be abraded in chosen areas to produce a two or three- colour design on a single pane of glass (red and white, blue and white, or either ofthesewiththe addition ofyellow stain, which will be discussed later).

The First Enamels
The earliest known examples of painted
window glass, found at San Vitale in
Ravenna, are fragments of crown glass