Cracking Off

As if glassblowing wasn't fast enough—it takes under three minutes to make a Roman bottle—cracking-off made the process even faster. This technique was well known by A.D. 20 or so, and cut the manufacturing time of simple tumblers in half.

Category:

Howto & Style

Tags:

• Corning Museum of Glass
• glassmaking processes
• corning
• glass
• museum
• newyork
• ny
• glassmaking
• makeglass
• cracking off
• 54.1.84
• Free Blowing

[willyreed]

thank you so much for this wonderful instruction.

i have tried it on wine bottles that i want to make into drinking glasses and i have 2 questions; i'd be grateful for any reply.

1. the bottle does not cut evenly - that is flat, or perpendicular to the bottle's long axis. how can i fix this?

2. what is the best way to make a nice smooth lip from the cracked rim? easy way?

thank you very much!

[corningmuseumofglass]

There are many things that can cause the rim to not break evenly, including turning too fast, turning too slow the flame being to broad, or bushy, and more. Be sure you flame comes to a point, and that you don't hold it too close to the glass. You should see the blue point while heating.

[ozzyscruggs1]

You don't think the glass carries internal stresses from being unannealed?

[corningmuseumofglass]

@ozzyscruggs1 the thicker the glass, the more stress remains from the cracking off process. The glass is annealed originally after it is blown, and that lack of stress is one thing that allows the glass to crack off so perfectly. The stress in general pales in comparison to the stress of being firepolished (where the same flame heats he glass until it softens and surface tension pulls it round and smooth). If you were not cold working the rim, i would re-anneal it for safety.

[corningmuseumofglass]

CONTINUED- Turn fast enough that the flame never pauses on one area for more that a tenth of a second. Try a little slower and a little faster if you're having trouble. For the lip, use wet dry sandpaper to remove the sharp edge, but finishing it very smooth will require some more serious cold working, with a machine or much hand working.

[azeromedia]

Thank you sooo much for this great video! I am gathering all the tools, and just wanted to know...do I need a "diamond" cutter as they are quite expensive. I have a normal bog standard, glass wheel cutter. All the bottles ready to go!

[corningmuseumofglass]

all you need is a definative and visable scratch. The glass cutter may work, a carbide lathe tool head would work. Try it. If the cutter doesn't work just google diamond scriber, and you can find many option for under 20 dollars.

[KyleJamesRichards]

Sounded like he said jacking off

[juventus86]

@willyreed also, this works best with a very fine flame. If you are using a standard plumbers propane torch the flame is too large. A small jewelry torch works very well

[ozzyscruggs1]

Wonder if I couldn't drill holes in glass panes with this? Use a lathe faceplate or similar fixture to turn the glass on a horizontal axis while pointing a torch flame at it.

Very cool demo, definitely gets your attention when the glass cracks dead on in a circle.

[harmonicgrunt]

the best i have seen.

[missplumeau]

Thank you for posting this very understandable video! I have been looking for ways to do this for a long time. I tried the "string" method that made a very ugly cut. Thank you!!!