After the glass is blown, it’s placed in an annealing oven where it slowly comes down in temperature to prevent it from cracking. The annealer usually cools down the next day, unless you’re making really thick stuff like giant telescope lenses, which are so thick they have to stay in there for a few years.
Fire cracking is when you scratch a line in the glass with a diamond or carbide bit, and then rotate the piece while heating it up around the line with a very small but intense flame. The difference in temperature between the cold glass and the hot flame is enough to cause the piece to crack, more or less following the control line that’s been scored into it. This was my first time doing it on a turntable, usually I’ve used a banding wheel and turned it by hand.
At some point the top part of the glass will crack. It can help to take the heat away for a few seconds to let it go from hot to cool and start the crack. After that’s done the top of the glass can be polished flat on a lapping wheel or on a machine with belts. Just like sanding wood, you start coarse and work up to a finer grit, getting a decent polish on cork or felt pads with cerium oxide powder.
The outside edge of the glass can be rolled over while polishing to make it less sharp, but the inside edge can’t be done the same way. There are machines to polish the insides of things, but I don’t have access to one, so I used diamond hand pads to smooth that edge.
