What happens to excess glass in handmade glassmaking?

September 27, 2019 11:56 PM

I've been watching handmade glassblowing and making videos (they're very soothing) and there's a lot of excess glass that gets broken off during the creation. Is this like clay and metal workshops where the leftover pieces can be set aside for reuse as lower-quality materials in other projects, or are they discarded outright? Bonus for historical workshops methods!
posted by dorothyisunderwood to Grab Bag (6 answers total)

I watched a glass blowing demonstration at NovaScotian Crystal a few years ago and it is my understanding that the extra glass (and the things that accidentally get smashed) are just melted back down to be used again, but this might be particular to the kinds of things they produce (mouth blown crystal) all basically having the same properties (uh, the same amount of ...lead content I think?). The tour was very interesting. I had no idea how long it takes to be a glass blowing apprentice. It also looked sweaty. I regret not buying a Christmas ball in the storefront afterward. In conclusion: check YouTube for traditional mouth blown glass methods of the east coast!
posted by janepanic at 12:19 AM on September 28, 2019 [1 favorite]

Best answer: Crushed glass, often sourced from broken or otherwise rejected glass from previous production runs, is called "cullet." Cullet is a standard ingredient in most new glass, sometimes making up one third or more of the recipe for glass. The use of cullet when making glass not only saves energy, money, and time, but can also contribute to predictability and consistency in the process of making new glass. The use of cullet goes back millennia.
posted by RichardP at 12:37 AM on September 28, 2019 [13 favorites]

I went to a glassmaking workshop in Japan. The guy teaching the workshop said they reuse the glass.
posted by starlybri at 6:03 AM on September 28, 2019

Best answer: Most art glass type places discard it because they are usually not batching their own glass—they’re buying pellets or cullet of their own to start with. Also, clear and colors get mixed in the crack-off barrels and you can’t make clear glass by starting with colored glass.

A larger factory setup is better positioned to keep color and clear separated and probably batches their own glass because they can afford people to do that. So, I guess “it depends.”

The school near me that has a program buys pellets, but when they started thirty-five years ago they were using free cullet from a ketchup bottle factory. Turns out glass formulated for mechanical blowing is very different from glass for offhand blowing—it would stiffen up over a very narrow range of temperatures. I am glad it preceded my participation in the program. Those people really did walk both ways uphill through the snow!
posted by Gilgamesh's Chauffeur at 1:12 PM on September 28, 2019 [2 favorites]

The glass blowing class I took had a barrel for clear and a barrel for color. The clear was mixed in with the cullet they bought. The color was thrown away. I'm not sure if it was to prevent muddy color in the clear glass out of the furnace or if it was because different colors and clear or milky glass has different melting points. It makes a difference if you're glass blowing, not so sure out or the furnace.
posted by stray thoughts at 8:27 PM on September 28, 2019 [1 favorite]

Response by poster: thank you all for the magic word cullet! I have been able to read a bunch of interesting articles and locate a medieval techniques book in the library to read further. And apparently there is now a US-shortage of cullet and it was a major trade item in antiquity and is in itself a technical resource to master because of how to melt and handle. Glass art is amazing.
posted by dorothyisunderwood at 3:45 PM on September 29, 2019 [1 favorite]