In the first quarter of the twelfth century, a German monk, who adopted the pen name Theophilus, wrote a description of the techniques of making stained glass. Glass was made by melting sand, potash and lime together in clay pots.
It was coloured by the addition of metallic oxides - gold for red, copper for green, cobalt for blue and so on. Pot-metal glass, especially red glass, was often too dark to transmit much light.
The designer would indicate the principal outlines of his drawing, the shape and colour of the individual pieces of glass to be used, and the position of the lead strips (calmes) that would eventually hold all the pieces of glass together.
The panes of coloured glass were cut to shape with a 'grozing iron' and laid on top of the drawing.
Through the glass, details of the drawing - faces, hands, drapery, etc.
- could be seen and these details were traced with an iron oxide pigment on the surface of the glass.
As paper was scarce and parchment very expensive, the full scale outline of the design for a stained glass window was drawn out on a whitened table top.His work relies more on strong design than the extensive use of paint.This circular panel illustrates the use of slab glass, with uneven texture and stunning colours.Although coloured glass continued to be made in the 17th and 18th centuries, the craft declined and skills were lost.Only in the 19th century was there a serious attempt to rediscover the techniques of the medieval glazier.