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. The basic methods have hardly changed. Glass was

made by melting sand, potash and lime together in clay pots. The glass was colored by the addition of metallic

oxides - copper for red, iron for green, cobalt for blue and so on. This is called pot-metal glass. Pot-metal glass,

especially red glass, was often too dark to transmit much light. To overcome this, 'flashed' glass was made by

dipping a lump of white glass on the blowpipe into a pot of red glass and then blowing, This provided sheets of

glass with a thin surface layer of co lour. Later, parts of this layer could be removed by grinding with an abrasive

wheel; this produced two colors, red and white, on the same piece of glass. Because 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. The designer would indicate the principal outlines of his drawing, the shape and color of the

individual pieces of glass to be used, and the position of lead strips (calmes) that would eventually hold all the

pieces of glass together. The panes of colored 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. After painting, the pieces were fired in

a small furnace for sufficient time to fuse the paint to the surface of the glass, and then re-laid on the table and

assembled by the glazier, using strips of lead H-shaped in section, which allowed the glass to be slotted into the

grooves on each side. The lead provided a strong but flexible bond. The intersections of all the lead strips were

then soldered, and an oily cement was rubbed into all the joints in order to make them watertight. The panels

were then held in place in the window openings by a grid of iron bars set into the masonry. From the early

fourteenth century a further range of colors varying from a pale lemon to a deep orange could be achieved on

one piece of glass through the discovery of 'silver stain', a silver compound painted on the back of the glass and

then fired in a kiln. By the mid sixteenth century many different colored enamels were being used. As a result,

windows began to be painted like easel pictures on clear glass of regular rectangular shape, with lead calmes

no longer an integral part of the design. These methods prevailed from the seventeenth to early nineteenth centuries.

However, the earlier techniques were revived in Victorian times.