Stress-O-Meter

Kitty · Post by **Kitty** » Tue Aug 05, 2003 1:40 am

i am thinking maybe i need to have the means to look at glass to detect stresses. i saw some stuff advertised here:

http://www.artglass1.com/fgsuppliesw.htm

wouldnt 2 of the 3" pieces be sufficient, instead of the $40 rig?

i've not used this type of thing, so any info would be welcome. many thanks. kitty.

Tony Smith · Post by **Tony Smith** » Tue Aug 05, 2003 7:15 am

Kitty,

I think you'll find that the BEConnection catalog has a better price on the polarizing film. I can't put my hands on my catalog at the moment, but I seem to recall that they sold single sheets (much larger than 3x3) for $11. You could just cut the sheet in half.

Tony

Bert Weiss · Post by **Bert Weiss** » Tue Aug 05, 2003 8:38 am

Tony Smith wrote:Kitty,

I think you'll find that the BEConnection catalog has a better price on the polarizing film. I can't put my hands on my catalog at the moment, but I seem to recall that they sold single sheets (much larger than 3x3) for $11. You could just cut the sheet in half.

Tony

I bought a large sheet from Edmund Scientific a while back, and I seem to recall that the last time this discussion came up they still sold the polarizing sheet. Like Tony said, you cut it in half.

Larry Lunsford · Post by **Larry Lunsford** » Tue Aug 05, 2003 9:36 am

I found a good price on polarized film here:
http://www.polarization.com/shop/catalog/index.html

In practice, I have not found the stress-o-meter to be useful. For starters, your piece must be translucent. Also, I have not been able to tell the difference between stress and "normal" where two different pieces meet.

The only time I could actually see stress was with a test piece of two layers of 4" x 4" window glass that I deliberately stressed by opening the kiln at 1000 dF. It didn't break, but I can actually see a pattern of shadows when viewed with the polarizing film.

Has anyone found polarizing film to be useful for real projects?

Larry

Mike Byers · Post by **Mike Byers** » Tue Aug 05, 2003 1:32 pm

I've found it pretty useful, but it does work only on translucent or transparent glass, and I've found a good light source (such as a light table) is needed. You can see the stress risers fine this way; they appear as white areas. An interesting aside here... the Pantera sports car was constructed so that the body carried the load as well as the frame. If you wore polarizing sunglasses while driving a Pantera, you could see the stress in the windshield when you went through a corner at speed.

Bert Weiss · Post by **Bert Weiss** » Tue Aug 05, 2003 1:52 pm

Larry Lunsford wrote:I found a good price on polarized film here:
http://www.polarization.com/shop/catalog/index.html

In practice, I have not found the stress-o-meter to be useful. For starters, your piece must be translucent. Also, I have not been able to tell the difference between stress and "normal" where two different pieces meet.

The only time I could actually see stress was with a test piece of two layers of 4" x 4" window glass that I deliberately stressed by opening the kiln at 1000 dF. It didn't break, but I can actually see a pattern of shadows when viewed with the polarizing film.

Has anyone found polarizing film to be useful for real projects?

Larry

The idea is to test a glass by full flat fusing it on to a piece of tested clear. This will give you a usable reading. White stress is visible, when it gets to the blue stuff, it is more serious. Lundstrom, "Fusing Book 1" goes in to the process in detail,

Larry Lunsford · Post by **Larry Lunsford** » Tue Aug 05, 2003 2:47 pm

Bert: Your info was r.e. compatibility testing. Since I've only used BE, I haven't been worried about compatibility.

What I was concerned about (and I think Kitty is asking about) is testing for stress as a result of problems with anealing.

I've read Lundstrom's book, and he shows two pieces - one properly annealed and one not annealed as well.

I still don't know how to apply this to one of a kind pieces.

Any recommendations for checking stress in a finished piece?

Bert Weiss · Post by **Bert Weiss** » Tue Aug 05, 2003 8:30 pm

Larry Lunsford wrote:Bert: Your info was r.e. compatibility testing. Since I've only used BE, I haven't been worried about compatibility.

What I was concerned about (and I think Kitty is asking about) is testing for stress as a result of problems with anealing.

I've read Lundstrom's book, and he shows two pieces - one properly annealed and one not annealed as well.

I still don't know how to apply this to one of a kind pieces.

Any recommendations for checking stress in a finished piece?

It's the same test. Stress can only be detected with using a polarizer in clear glass, so you have to anneal a clear piece and look at the stresses. I have been told that the real view is through the edge.

There is much in the archives about this. I like the dishwasher test. If it survives the dishwasher, it is low on stresses including annealing. Not to mention sparkly clean.

Lauri Levanto · Post by **Lauri Levanto** » Thu Aug 07, 2003 2:53 am

To begin with,
I started looking thru to pairs of sunglasses.

A good polarizer is big enough, and widely framed.
If you get a lot of unpliarized light on the piece,
the results are not reliable.

My problems are with cast 3D forms of clear glass. As the light is refracted and reflected in the piece, the polarisation changes.

I do not dare to take 2" thick pieces into dishwasher test.
They are doomed to a thermal shock.

-lauri

Bert Weiss · Post by **Bert Weiss** » Thu Aug 07, 2003 11:01 am

lauri wrote:To begin with,
I started looking thru to pairs of sunglasses.

A good polarizer is big enough, and widely framed.
If you get a lot of unpliarized light on the piece,
the results are not reliable.

My problems are with cast 3D forms of clear glass. As the light is refracted and reflected in the piece, the polarisation changes.

I do not dare to take 2" thick pieces into dishwasher test.
They are doomed to a thermal shock.

-lauri

Lauri

I agree with you about 2" pieces in the dishwasher.

I have a friend who is a glass blower, who has a profesional polarizer. he uses it to look at colors encased in clear crystal. Stresses are easy to identify in his case.

Perhaps you could try one of these polarizers out at a glass factory? It could also be that your surface relief makes it too difficult to read.

There is always the wait 3 years and see if it breaks test.

rodney · Post by **rodney** » Thu Aug 07, 2003 12:12 pm

i have a pair of cheap polarized lens sunglasses, can i just pop out the lens and use them

thanks
rodney

Bert Weiss · Post by **Bert Weiss** » Thu Aug 07, 2003 1:49 pm

rodney wrote:i have a pair of cheap polarized lens sunglasses, can i just pop out the lens and use them

thanks
rodney

Yes. But you'll look funny wearing one sunglass lens.

To make the polarizer work all you need is a single light source and 2 polarizing filters. The glass you are looking at goes between the filters and when you turn the top filter it will get dark and reveal stress.