Early window glass was made by mixing sand, soda, and lime and then heating this mixture at high temperatures.
The Basic Process
The fundamental process of early glassmaking involved these key steps:
-
Mixing the Ingredients: The primary ingredients were silica (sand), soda (sodium carbonate), and lime (calcium oxide). These were carefully measured and mixed together. The specific proportions affected the final properties of the glass, such as its melting point and clarity.
-
Heating the Mixture: The mixture was then placed in a furnace and heated to extremely high temperatures, typically around 1500 degrees Celsius (2732 degrees Fahrenheit). This caused the ingredients to melt and fuse together.
-
Forming the Glass: Once molten, the glass was shaped into the desired form. Several techniques were used for window glass:
-
Crown Glass: This was a popular method where a bubble of glass was blown, then spun rapidly to create a flat, circular pane. This method left a characteristic "bullseye" in the center.
-
Broad Sheet Glass: This involved blowing a cylindrical bubble of glass, cutting it open, and flattening it into a sheet. This method resulted in a wavy, distorted surface.
-
Cylinder Glass: Similar to broad sheet, a large cylinder of glass was blown, cut, and then flattened.
-
-
Cooling and Annealing: The shaped glass was then slowly cooled in a process called annealing to reduce internal stresses and prevent cracking.
-
Cutting and Finishing: Finally, the glass was cut to the required size and shape for the window and its edges were finished.
Differences in Techniques and Appearance
The method used to form the glass had a significant impact on its appearance. Crown glass often featured a central "bullseye" where the glass was attached to the punty rod. Broad sheet glass and cylinder glass tended to have wavy surfaces due to the flattening process. These imperfections are often seen in antique windows.
