Get the Stress Out

This presentation is derived from a talk given by Ted Sawyer, director of Research & Education at Bullseye Glass Co. The talk, entitled “Stress, Out!: Avoiding Painful Breaks and Strains” was given at the 2009 BECon conference in Portland, Oregon. It focuses on the history, theory, and process of getting the stress out of fused glass work and will hopefully remove some of the stress from studio practice. From the fundamental definition of annealing through the 900˚F revolution, ΔT < 5˚C, Young’s modulus, multi-point pyrometry and much, much, more!

Let’s Break it Down

Ok, so there’s a lot going on here, and it’s not very exciting. However, it is important. Let’s start with just how unorthodox this is in the tradition of the glass world. We’ve all been taught to believe a few fundamentals about annealing:

  1. Annealing can only happen in the annealing temperature range—between the annealing point and the strain point.
  2. No stress can be permanently added or removed from glass below the strain point.
  3. The closer to the strain point you anneal glass, the more difficult it is to anneal it.

So, let’s think about these concepts. First, let’s define these points.

  • Anneal (Glass) / v. to remove permanent stress/strain in a glass body which is set up by temperature differentials during its formation and subsequent cooling.
  • Annealing Point / n. the temperature above which glass begins plastic deformation… it softens.
  • Strain Point / n. the temperature below which no strain can be added or removed permanently from glass.
  • Annealing Range / n. the range of temperature between the aforementioned points where glass is practically annealed.

There’s one more concept we have to be familiar with before we can go on, and it has to do with the reason we can’t have a “one size fits all” annealing schedule. Basically, glass is a pretty decent insulator. Not great, but decent. That means that (at annealing temperatures) the surface of the glass is insulating the interior of the glass. So, that means it takes a while for heat to transfer through the surface and leave the interior of a glass object. Each millimeter of glass makes a difference, so a 10 mm thick object will lose heat slower than one 5 mm thick. In other words, the thicker the object the slower we have to cool it through the annealing range so that the surface and interior are never at wildly different temperatures. It’s the actual thickness of the material itself that dictates the speed we need to anneal something.

This is important. If it’s the thickness of the material that dictates the annealing speed, and that’s the only criteria, then it doesn’t actually matter where in the annealing range you start the annealing process. So long as you can equalize the surface and core—by using the anneal-soak—you can use the same degrees/hr to anneal something, whether you start at 960°F or 900°F. So, instead of soaking at 960°F (a common annealing temperature for both Bullseye and Spectrum studio glasses) you could equalize the temperature at a much lower temperature and save yourself time (and energy) and still have a well annealed piece of glass!

In other words, the old concept of annealing is dead wrong! Beginning your annealing closer to the strain point doesn’t make glass more difficult to anneal, it makes it easier. For one thing, it makes it a lot quicker. Just do the math. If your strain point is 900°F (Uroboros transparent glass) and you begin your annealing at 960°F, you’ve got to cool slowly for at least 60 degrees. Say your glass is 3 inches thick, then you’ve got to anneal at 3 degrees/hr for this initial “slow cool”. And, you’re probably going to want to make sure you actually cool at that speed all the way past the strain point, so you’re going to probably go this speed down to 850° or even 800°. Math:

  • 3″ thickness = 3°/hr
  • Anneal-soak at 960°
  • 3°/hr for 160° = 53.33 hrs.

That’s almost 4½ days. However, if you anneal-soaked at 910° the math is much kinder:

  • 3″ thickness = 3°/hr
  • Anneal-soak at 910°
  • 3°/hr for 110° = 36.67 hrs.

Basically, 3 days. That’s a day and a half saved! You could use that time cold-working, or casting another piece. If each student in a class of 10 saved that much time per project, that’s 2 weeks of annealing time saved! Not only is that a huge benefit to students in terms of fitting in more experience per semester, but it’s also a massive cost savings to the whoever is picking up the tab for the energy costs. And, of course, it’s environmentally responsible as well.


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