Sciencemadness Discussion Board

Pyrex heat resistance after cutting

Quince - 4-10-2006 at 05:38

I gut a large pyrex jar, removing the bottom and a ring off the other side, to use as a transparent chamber for the coffee roaster I built. I cut it with a small diamond wheel on a hand-held rotary tool. How much is the resistance to cracking from heat affected from the rough cuts? I don't see any way I can really polish the edges (I don't have a kiln or any flame nearly large enough to fit the whole thing in).

Quince - 4-10-2006 at 06:02

Oh shit I broke it... nvm

BeerChloride - 4-10-2006 at 14:53

Short thread..:o

But.. If you make another one.., My guess is that a really clean cut will probably not affect the heat resistance, but such an edge would be easily prone to very tiny chips on the edge, which could potentially (probably negligibly) slightly reduce the heat stress resistance and could be a source of a crack. A rougher cut may be similar.

Pyrex glass CAN be polished with just a propane torch. You have to try to get the edges as hot as possible with the torch. Without proper annealing, you will not be able to remove all stresses from the glass, but with a simple shape like you have you can "hand anneal". After firepolishing the edge, slowly work the torch heat smoothly further from the edge in as uniform manner as possible. You would be trying to "feather" the stress into the glass surrounding the edge. Using a homebrew polarimeter to see the residuall stresses, I've seen that hand annealing can be quite effective. If done well, I wouldn't be worried about it's heat resistance.

As an alternative, you CAN sand glass. Sanding by hand takes forever - use a disk. To smooth an edge, I would shape the edge using around 220. You will have to use succesively finer grits down to smoothness. Go to 400, 1500, then whatever you have to polish, but 1500 might be good enough. I bought some 4,000 and 15,000 grit sheets a while back. I find lots of uses for it.


leu - 5-10-2006 at 00:21

You can anneal borosilicate glass in a self cleaning electric oven by setting the oven to clean itself. this is also the best way to carbonize stubborn tars :P

The_deadly_dustbin - 5-10-2006 at 04:21

The rough edges will not effect the heat resistance of the glass piece substantially, as this is a function of:
a.) Thermal expansion coefficient and
b.) Thickness of the glass

As the TEC (or CTE) for the boro3.3 is very small it will not build up substantial stresses during heating and cooling. The rough edges will however effect the mechanical properties of the glass as mechanical properties depend on the crack size and the crack geometry. So flame polishing with a propane torch is definitely advised. Hand annealinng afterwards is also recommended (and as BeerChloride pointed out highly effective).
The technique of cutting is also very important. DO NOT CUT several times. This will only lead to a rough fracture surface. Scoring the surface ONCE is also very effective and leads to smoother results as cutting (or scoring) several times. A cotton thread soaked in a flammable liquid and put on the surface of the glass (if complicated geometries of the cut are desired) is the best way to go. Light the thread and quench the glass to be cut in cold water. Works pretty good.
But removing the bottom of a glass container with breaking it requires a lot of skill.

Quince - 6-10-2006 at 22:09

Is a self-cleaning gas oven OK as well, or is the temperature insufficient?

Crack Detection

leu - 7-10-2006 at 17:16

Is a self-cleaning gas oven OK as well, or is the temperature insufficient?

It's common knowledge that combustion produces water vapor which is unlikely to help in annealing and would probably cause some other unhappy event than what's planned :P A polariscope can be used to detect invisible cracks in glass, as explained at:

If a polariscope/polarimeter is impossible to obtain or construct another method of detecting invisible cracks in glassware is by means of electricity which doesn't require as much experience:

Often, when using glassware, a crack which has been overlooked, or which is invisible, will cause the apparatus to break when heated. To avoid such an accident, the glassware may be examined in the following manner. A wire is run along the inner wall of the glassware and another, parallel to the first, along the outside. The wires are attached to an induction coil. If, when the current is turned on, sparks appear along the whole length of the wires, accompanied by a bluish glow, there are no cracks. However, if there is a crack, its position maybe located by a thick spark jumping between the two wires. By revolving the apparatus and repeating the operation; the piece may be thoroughly examined.


[Edited on 8-10-2006 by leu]

Quince - 7-10-2006 at 21:16

That is an interesting trick. I may use it to test the glassware should I end up distilling H2SO4.

Unfortunately, I only have access to a gas stove, not an electric one. :(