Glass-making techniques in the 12th century were much more primitive than they are today and the glass they produced was much rougher. To produce stained glass, the molten glass was colored by mixing it with chemicals when it was still in the furnace so that it retained its translucency. When it was illuminated, it glowed like a jewel. Manganese was used to obtain a red color and cobalt blue.
While the plate of glass was still warm, it was cut into the pieces needed to create the picture in the window, and then allowed to cool and harden. The designer would then arrange the pieces of glass into the required picture and add details - for example the faces of the figures or the folds in the clothes - with black paint.
Once the cut glass was completed, each piece was enclosed in soft lead. They were then assembled by welding the pieces of lead together to form a picture, which resulted in a design in the form of outlines around figures. The assembled pieces were then fitted into a window frame. Because of the weight of the lead and glass, the panels were hung from iron frames by hooks.
