Bottle Glass Frit and Float Glass Stress Test

Last month we looked at fusing mixed bottle glass and the internal stress that it causes in the glass due to incompatibility. The post was very popular, and I received a number of requests both to explain how to stress test your own glass, and if I could test a bottle glass frit fused on float glass combination. Both are great ideas, so I combined them in what will hopefully be a helpful guide.

Testing for Incompatibility

Setting up the fusing test

Start with the glass that will be tested, we’re using four pieces of float glass and bottle frit in 3 sizes.

float and frit compatibility testing
Float glass squares for frit test
float and frit compatibility test
Bottle glass frit for float/frit compatibility test

Next I’ll put some of each frit size on a piece of float glass.

compatibility tests for frit and float
Frit and float compatibility test prefire

and fuse it using my standard fusing schedule.

fused float and frit
Fused float/frit compatibility test

Notice that one of my float pieces has no frit. This is my control piece. A control is critical because we are looking for stress in the glass, and stress can be caused by incompatibility AND/OR annealing issues. By having a control piece, I can check it for annealing problems and eliminate that as a possible cause of glass stress.

Checking for Glass Stress

For this we will need a light box and two pieces of polarizing film.

compatibility testing
Light box and polarizing film

You can buy a set of films from Delphi, but they are only 2″ square. I would strongly recommend buying a larger piece and cutting it in half. I bought mine from American Science and Surplus and have been very happy. If you search for it online, make sure you search for Polarizing Film, not polarized film, the search results are different. My light box is an old X-ray viewer.

Turn on the light box and put one piece of film on it, then the glass, then another film. Kind of a film and glass sandwich.

float/frit compatibility testing
Film with glass on top
compatibility testing for fused glass
Glass with top film to show stress

A quick word about using polarizing film, the film has grooves (very tiny) that channel the light, and go in one direction. The top film has to be turned 90 degrees so the grooves go the other direction. When you do this, the surrounding film will go dark. If your test is completely lit up, turn the top film and see what happens.

Now let’s talk about what we are seeing here. The top right square of glass is our control. You can see there is no yellow flare so it is safe to assume that the glass is properly annealed, and the stress we are seeing in our test is incompatibility.

And it is, quite incompatible. Although the glass isn’t cracking to the visible eye, there is a lot of stress in the glass that makes it unstable. I would not sell any piece of glass with this amount of stress in it.

Here are some closeup photos of the stress caused by incompatibility.

incompatibility in bottle frit
Fine bottle frit on float glass – incompatible

The fine frit causes an even flare of stress.

frit and float test
Medium frit on float test – incompatible

The frit pieces here are large enough to see the individual stress halos around each piece, they look like squares in the photo

 

frit float incompatibility
large frit on float test – incompatible

The large pieces of frit completely light up. Although all of these look different, they are all incompatible. Ideally, you would want all four of your squares to be dark in a stress test.

What this tells us? This bottle glass *only* this one is tested incompatible. I strongly suggest that you test every kind of bottle glass frit that you use, some may actually be compatible. The only way to know is to test!

Key things:

  • always have a control piece of glass
  • test everything
  • keep excellent notes

Let me know if you start compatibility stress testing mixed glass and what you learn.

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