Saturday, March 30, 2013

Fused Glass Trial and Error

"Trial and error" and "test and learn" are used a lot in business and imply that one is optimizing or improving upon a process or product.  I've used these terms in product marketing and research, especially.  However, when they apply to fused glass, it's a different story.

Most people who see my work don't see behind the scenes.  When I venture into a new area, there's always a learning process.  Most of the time, it means failure.  Repeated failure until I get the firing schedule right.  And here, failure means cracked glass and lost time.

Part of the problem in the past has been my kiln, which I corrected by getting a new kiln.  Part of the problem is that the new work I'm doing is more complex, using multiple colors at higher temperatures with longer hold times (which can change the property of the glass).  And, part of the problem is that I'm firing multiple times and working on larger pieces.  My new work with pattern bars is particularly susceptible to breakage, typically along the seam where two bars are joined. Even though I have learned to fire very conservatively that doesn't always guarantee success.

Fortunately, glass can be repurposed so not all is lost.  I try to retain the design elements to use in new work.  Having a tile saw can be a real asset to cut up the glass and use it again.  Here's a picture of a piece that will be used to make a bowl.

Although glass failures are not ideal, there are still some lessons that I can pass on, so hopefully you won't have the same issues.

  • Check your kiln elements relative to the size of the piece.  I was having problems in my old kiln as I was pushing the size limit with 12" squares (kiln interior was 14.5") and elements that were circular, making it difficult to heat the corners (notice the crack line in earlier photo above).
  • Fix any sagging elements.  My "new" kiln was used and had one sagging element (hanging down about 2" from one end of the kiln to the other) which created uneven heating, especially on large pieces.
  • Keep good records of your firing schedules so that you can make adjustments on future firings.  When in doubt, slower is better.  You'll want to make sure to have a long enough annealing hold based on the size and thickness of your glass piece.  And, if you notice that the glass is cracking on the way up, you'll want to increase your annealing and/or ramp up slower to 480-600 degrees.   The more important the piece, the slower I go.
  • Don't coldwork the piece until the glass is cool.  This can add to its stress.
  • Test your kiln to see how the heat is being distributed. Bullseye Glass has a good tutorial on this here
  • Consider making a "test" piece first.  I typically don't do this but recently I've had a problem slumping a piece in my new kiln.  Before I make another attempt on a real project, I am going to make a test blank to make sure I have the right set-up and schedule.  After going through 3 firings and coldworking, I don't want to lose all that work on the last firing.
  • Consider varying thicknesses of glass and anneal for the thickest part, especially if you are tack fusing or have varying layers and are not full fusing.  This hasn't been an issue for me as I don't tend to create work that is tack fused but I thought I would pass it along.


  1. Thanks for the tips. I too have been trying to do 12" pieces in a 14" kiln with round top elements and I had never thought about this causing a problem for the corners. But this explains some of my issues. Thanks so much!


  2. Thanks for the tips..I just broke my first slumping project..I should have practiced first. I never would have thought of that.

    Laurie S

  3. Glad the tips help. It's so disappointing when projects don't turn out. I feel like I'm constantly "problem solving" as I continue to try new areas and deal with new kilns too.