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 float glass from the same sheet 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 full fuse 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

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. We’re making a 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 and photos

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

Related Articles:

Mixed Bottle Glass Stress Test

Glass Studio Safety Hazards

Diagnosing Cracks in your Glass

Related Products:

Polarized Film Sheets