I’m often asked technical questions about the process of making glass so here I’ll try to answer some of those questions over time.
Starting with the myth about glass slowly creeping and flowing like a liquid from top to bottom so it gets thicker at the bottom
This myth came about when repairs were done to a section of a medieval window which had buckled and needed re-leading. The design of the lead work was in the shape of diamonds called ‘quarries’ and these quarries of glass were thicker at the bottom than at the top. It was thought the glass had flowed like a liquid under the influence of gravity. This misconception was fortified in the 20th century when scientific studies revealed the molecular structure more like that of a liquid than a solid. However, the science is clear that there is no movement of glass over time under the normal range of temperatures and there is no flow. Glass actually starts to bend at around 600’C. It turns out that the reason the medieval glass was thicker at the bottom was because it was leaded that way around. In other sections of the window it was later found that the glass quarries were thicker at the top but by then the myth was established.