NEW Casting & Annealing Graph

NEW! CURING, FIRING & ANNEALING GRAPH (pdf) (click to view; right-click + “save link as” to download) has been added to Tech Downloads page. The new graph finally changes the annealing times & temperatures to more closely match that of Bullseye’s Annealing Thick Slabs chart.

Annealing Graph Thumbnail

The new chart also takes into account info about quartz inversion, which has been referenced in several glass casting publications—for example, Angela Thwaits: Mould Making for Glass and Bullseye’s Tech Notes: Basic Lost Wax CastingQuartz inversion is the phenomenon of the silica in standard plaster/silica mould mix suddenly expanding in the 1100ºF range. If we follow the standard practice of “spiking” the kiln from annealing temperature (or 1000ºF) to casting temperature—which was taught in many kiln-casting publications as a way to avoid devitrification—then we might push through the quartz inversion stage too quickly, causing the outside of the mould to expand before the interior. This uneven expansion can create cracks in the mould, leading to mould failure at casting temperature.

This new Firing Chart takes that into account and recommends a slower ramp from 1000ºF  to 1200ºF before spiking to casting temperature. At this writing I feel it’s still important to spike from 1200º to casting temp as fast as possible to avoid prolonged exposure in the devitrification range, especially when firing open-faced moulds where glass billets have been loaded directly into a mould.

Download more useful Tech Materials here!


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.

Spectrum Glass Update

The Great News

KSU Glass has been able to secure enough Spectrum Studio Nuggets AT THE OLD PRICE to keep our program running unchanged for the next year!

The Good News

Spectrum is selling off its formula and equipment to another manufacturer so that Spectrum Nuggets will continue to be produced and supply the glass studios around the country. In the meantime Spectrum itself has committed to Nugget production through Sept, and possibly longer.

The Bad News

This will probably mean an increase in price, which will eventually trickle down to student material fees. (Bullseye Glass has already raised their rates by 12% across the board.) However, since KSU Glass was prescient enough to act in time, that eventuality will be delayed as long as possible.

 Other Good News

KSU Glass has acquired thousands of pounds of System 96 cullet, both clear, crystal clear, and a variety of colors. This will enable us to continue to offer larger-scale glass casting to those Seniors and Graduate Students interested in economical, large-scale sculptural glass casting.

Bullseye Glass Struggles & Spectrum to Close Doors.

Bullseye Glass, one of the most popular manufacturers of studio art glass, has recently had trouble with their emissions. Here is an excerpt from their statement:

Some colors are currently suspended from production while we acquire, install, and test new equipment. These colors are made with cadmium (used for bright yellows, oranges, and reds) and chromium (used for greens). We plan to have an additional furnace for making cadmium glasses in early June, and we also plan to have the ability to produce glasses containing chromium in early August, at which time the required controls will be in place and functioning.

Read Full Letter Here.

This is happening at the same time that Spectrum Glass has been forced to announce that they are shutting their doors at the end of July 2016. Apparently the biggest factor has been economic.

Market factors have played the most significant role. Our facility was built to support product demand at the height of art glass movement, but our sales never fully recovered following the Great Recession. We have watched our sales dwindle dramatically to only 40 percent of production capacity, while overhead expenses have continued to increase. Our consistently reduced levels of sales simply cannot cover the fixed costs required to operate a facility of our size.

Full announcement here.

What this means for the glass art community in the long run is yet to be seen. Short term many studios will be caught off guard as their favorite source of affordable blowing glass (Spectrum Studio Nuggets) will be difficult (or impossible) to acquire. There are rumors that Olympic Color Rods is trying to step in and fill the gap, with inquiries into alternate vendors to keep the system going (System 96). It’s also rumored that Uroboros will be picking up colors that Spectrum was melting, but Uroboros is definitely more expensive than Spectrum, even though they’ve been working together on the System 96 line for years. Hopefully this doesn’t mark the end of an age. It would be a shame if Bullseye remained the only compatible glass system out there.