Old Glass Defies Cutting?

Q: Why can't you cut old glass with a scoring tool? From my materials education class, glass can be regarded as a super-cooled liquid that is in a continual state of flow. This would explain why old windowpanes appear to distort an image, but why can't it be cut?
Dave, Aberdeen, England.

A: Actually, you can cut old glass with a scoring tool, but it's tricky, because old windowpanes are uneven. Your material education class, however, taught misinformation. Glass is not in a continual state of flow. That theory has been disproved (see www.cmog.org) says Robert H. Brill, research scientist of the Corning Museum of Glass.

Instead, it's the glass-making process that distorted the glass. Before the mid 19th century, glassblowers gathered molten glass on the end of a blowpipe, and blew it into a balloon shape. They transferred the red-hot balloon to a long, solid rod. Then, while heating the balloon, a glassworker twirled the rod rapidly until the balloon flattened, and spun out into a disk. See figure. They cut the disk rim into panes. The glass was thicker toward the outer rim edge where the spinning rod flung it.

In the early 20th Century, glassmakers used a process that also resulted in uneven glass. They poured molten glass on large cooling tables. The glass was thicker in the center where it was poured. Then they cut the big sheets into smaller windowpanes, thicker on one end than the other.

Windowpane installers usually placed the thicker side down for stability. Occasionally, though, we have found the thinner side down.

That's why old glass is difficult to cut: it's uneven due to a manufacturing process.

"Additional issues that can make it more difficult to cut and break glass are how well the glass has been annealed and excessive damage on the surface," safety glass consultant Siegfried Herliczek emails.

I checked the Internet for techniques to successfully score-cut old glass, and have included them in Further Reading. Here's the one that appealed to me: "When it comes to old glass, I usually have a local glass shop do the cutting. They have a very sharp and expensive cutting tool that makes an unusually deep score mark," says 'TranquilityBase' in a discussion forum.

Further Reading:

Why glass can shatter unexpectedly, WonderQuest

Old glass, Killifish discussion list

Cutting old glass, Don Wilkins

Window panes from spun hand-blown glass, London Crown Glass Company

(Answered May 14, 2007)