PDA

View Full Version : Stress Induced by Coldworking?


Zach Jorgenson
01-25-2012, 10:48 PM
I've recently heard some boro workers talking about reannealing their work multiple times after lathe cutting. Coldwork induced stress?

Graham Stone's book states that cutting and especially drilling can weaken the glass. He talks somewhat about temporary stress by dropping a piece or heating an annealed piece too quickly, but that it isn't permanent stress, meaning it can be annealed out.

What of this reannealing? Is it necessary after lathe cutting? Could this have more to do with negligible compatibility or is it a common practice. I've never heard of anyone else annealing after cold working a piece.

Jon Myers
01-26-2012, 12:39 AM
I've seen moldblown pieces reannealed after they were "popped" and polished (flame polished)

Rick Sherbert
01-26-2012, 06:09 AM
Annealing after flamepolishing makes sense, after grinding and cold polishing doesn't (at least to me). I'm sure Pete will chime in here if I'm off base.

Heating a piece that is below strain point too quickly is called thermal shock and yes technically there is strain induced (witnessed by the piece blowing up) but it is not strain due to improper annealing in the first place.

"negligible compatibility " as you call it, can't be annealed out. Ever.

Do some searches on annealing and compatibility here. I think it's been discusses a lot.

It also makes sense to me that cutting and drilling can weaken glass, you're putting a damn hole in it!:)

Pete VanderLaan
01-26-2012, 06:28 AM
Annealing is the relaxation of strain from the softening point to the strain point. Once glass is annealed, which normally occurs at the low point around 700F, anything you are doing to it from that point on is just trying to avoid thermal shock.

Given the rough math, soft glass expands about one ten thousandth for every three degrees C it is heated. It's a little less and varies from glass to glass. Boro expands very little comparatively, about 1/3 of the expansion of a soft glass which is why you can zap it with a torch and it doesn't blow up. It isn't expanding much.

But that match is still there. When you polish you are heating the glass. When you cut, you are heating the glass and when you drill you are heating the glass, so it expands. Too much and the stress builds and lets go as presented it's thermal shock and has nothing to do with annealing.

When I make my cut forms I can see where the annealing comes in to play in grinding. I have to put them away really hot into a 1000F annealer. If I don't, I can't grind them without their cracking once I get to knife edge grinding. They still have too much strain in them and that kind of strain will express itself even though you could say the piece is sufficiently annealed for normal purposes. Once it has had a brief soak at 1000F, I can drop it to 935F. My problem usually comes on the polishing end where heat is getting generated at those knife edges. It's really easy to overheat and crack them but at that point it isn't annealing. Compatibility really comes in to play on that stuff as well and should be considered in the type of problem Zach is talking about. The glasses that make big crystals are moving targets when they get cooled and heated a lot.

Jon Myers
01-26-2012, 11:20 AM
When you're polishing, the heat is a product of the work being done. I've been told that the glass polishes better when it's hot (from the felt not the annealer) this never made sense to me, I always assumed it was a product of friction, just a byproduct so to speak. Is the heat helpful? (is it doing work or is it just there)

Zach Jorgenson
01-26-2012, 12:02 PM
Thanks for the perspectives. I was just curious is anyone else had done this, needed to, or heard of it. Reannealing didn't make sense to me, the need for it. I'll have to do my own testing.

Jon, I've read here that heat helps in cerium polishing and I use it myself when I can, also to keep my fingers from freezing.

Pete VanderLaan
01-26-2012, 01:21 PM
I too have heard that cerium has that effect but I have no way of proving it. It is true that you can get a greater amount of cerium in to solution when the water is hot. It's also true that cerium does best staying in solution in an acidic solution. So, piss in your cerium and let me know how it works out.

Kenny Pieper
01-26-2012, 04:15 PM
I was told by someone who worked in the glass polishing industry that on a microscopic level the cerium works best when it is melting the glass on the surface and that is why it works so much better when it is a bit thick and warm.

Michael Ahlefeldt-Laurvig
01-26-2012, 04:53 PM
Is it a matter of terminology? I know that boro workers put cylinders of boro in a lathe and work on the glass with burners as the lathe is turning....
That would make you want to reanneal it when youre done

Pete VanderLaan
01-26-2012, 05:10 PM
You certainly make sense Michael. I don't even know what "lathe Cutting" would be.

Ray Laubs
01-26-2012, 05:29 PM
Anyone have a piece to look at under a polariscope?
I have a paperweight here that started out as a large angle sulphide marble. It got chippped and from there I turned it into a paperweight using a cab machine, wet diamond, down to about 1500 grit. I checked it under the scope and theres no apparent stress from the grinding/polishing.

Pete VanderLaan
01-26-2012, 05:46 PM
Stress from grinding should ( should in my mind ) only be present while the actual temperature differences exist. Once they have stabilized, the stress should be gone. You aren't unannealing the piece by grinding it unless it gets so hot as to pass back into the range above the strain point. It's unlikely it would survive that.. This is a thermal shock issue.

Ray Laubs
01-26-2012, 05:58 PM
Stress from grinding should ( should in my mind ) only be present while the actual temperature differences exist. Once they have stabilized, the stress should be gone. You aren't unannealing the piece by grinding it unless it gets so hot as to pass back into the range above the strain point. It's unlikely it would survive that.. This is a thermal shock issue.

oic, thanks.

Zach Jorgenson
01-26-2012, 07:48 PM
Sorry, Merker's terminology stuck with me. Diamond wheel engraving lathe is what I was talking about. Not the hot blowing lathe.