Page 8: Medieval Red Glass

What is so unique about the red glass of the middle ages? Glass blowers were using copper as a coloring agent. 

Glass that is colored with copper is a very difficult glass with which to work. If it is heated the wrong way it will turn black. Gathering out of a solid tank of copper red results in an opaque glass. 

IMAGE: Corning Museum of Glass

Only on the edge, where the glass is very thin, does it reveal its transparency. 

German Medieval Copper Red

Glass blowers in the middle ages were called upon to produce vessels for utilitarian use as well as sheet glass for windows. Obviously, only transparent glass was suitable for the purposes of stained glass. In order to be able to create a sheet thin enough to read as transparent red, glass blowers developed the technique of flashing or casing the red over a gather of clear glass. The resulting sheet would have a layer of red stretched thin enough over the clear base layer to reveal its transparency. 

IMAGE: Corning Museum of Glass

To color the glass copper was added to the crucible in the form of copper scales which were stirred into the molten glass. Copper does not dissolve in glass but forms a colloidal suspension. Viewed under a microscope this presents as many small streaks of red within the glass.