When you think of a window, you think of a flat piece of glass. But window glass didn’t always start out flat. It once began as a massive bubble on the end of a glassblower’s pipe.
To flatten the bubble, the glassblower could spin it rapidly into a huge disk called a crown. After it was cool, the uneven, distorted disk was cut into small panes.
If, instead, the glassblower swung the bubble over a pit, he could stretch and elongate it into a cylinder. When the glass cylinder was cool, it was cut lengthwise, then reheated and flattened. Larger panes could be made this way, but the glass was still distorted—and the work was just as exhausting.
Some centuries-old window glass is thicker at the bottom than at the top. Is that because the glass flowed down over time?Older windowpanes have an uneven thickness because they were cut from uneven, handmade glass. The large glass disks made by the crown method, for example, were thicker in the middle than at the edge. Glaziers usually installed the thicker, heavier part of the pane at the bottom for stability.