Bottle Glass Frit and Float Glass Stress Test
Recently we looked at fusing mixed bottle glass and the internal stress that it causes in the glass due to incompatibility. That article is 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 window (float) glass from the same sheet and bottle frit in 3 sizes. It’s important to use glass from the same window to provide a control situation and eliminate a variable. We want to have the fewest number of variables possible, so when we test one, we know exactly what we are testing.

The next thing is the bottle glass frit that we’ll be testing. This is three sizes of crushed bottle frit, from the same bottle. Again, we are reducing the number of variables so we can test the one we want accurately.

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

and fuse it using my full fuse schedule.
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 Stress in the Glass
To check for stress inside the glass, we use some surprisingly low tech tools. For this test we will need a light box and two pieces of polarizing film.

My light box is an old X-ray viewer, as the medical industry switched over to digital, these were available everywhere for a while, I still try to pick them up when I can.
Turn on the light box and put one piece of film on it, then the glass, then another film. We’re making a kind of a film and glass sandwich.

Then add the second film on top of the glass and turn on the light box.
Quick Note about Polarized Film – Polarized film has micro grooves that run in one direction and direct the flow of light. We are going to turn the films so that the grooves run at right angles with each other. When there is nothing between them, this will effectively block all light and you’ll see a solid black square. If you are not seeing a solid black field around your glass, rotate the top sheet 1/4 turn.

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.

The field of little flares you see here are the incompatibility, the frit is almost powder, so you’ll see a broad streak of light, rather than individual flares.

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

The large pieces of frit completely lit 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.
Conclusions
What this test tells us: This bottle glass, *only* this one is tested incompatible with this particular sheet of window glass. I strongly suggest that you test every kind of bottle glass frit that you use, some may actually be compatible.
Our goal in this tutorial was to learn how to set up and read a stress test. Hopefully we’ve done that and you feel confident to pursue your own testing.
Key things:
- always have a control piece of glass
- test everything
- keep excellent notes and photos
Let me know if you start compatibility stress testing mixed glass and what you learn.