Older windows made of stained glass have a great deal of texture. They are lumpy, and many of them have small air holes. This may appear to be a case of glass being made improperly. When stained glass was first made, the glass mixtures were heated in wood-fired kilns. There was very little control over the temperature, and many times it was insufficient to melt all the glass ingredients. While there are minerals, such as alumina, that reduce the heat necessary to melt glass, these were not known to artists of the time. Completely melting glass in these ancient kilns was a hit or miss procedure that produced glass with character rather than perfect plates of glass.