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Do You Have Rainbows in Your Halos?

June 24, 2009

Left ~ 960º...Right ~ 900º Anneal Soak Temps

Left ~ 960º...Right ~ 900º Anneal Soak Temps

If so, that’s not a good thing.  Halos are stress in kilnformed glass when viewed through polarizing film & if your halos have rainbows you have a lot of stress ~ meaning the glass is likely to break!  I just opened my kiln with the first load of cast glasswork (in commercial molds) using the new annealing temperature announced by Bullseye during BeCon, the glass conference I attended last week.  For my entire kilnforming career I’ve used 960º as the anneal soak, as taught by Lani McGregor at my very first glass fusing class at Bullseye many moons ago.  The new anneal soak temperature, 900º is closer to the strain point  which equals less risk and a more efficient anneal cool.  Ideally, you have multiple thermocouples in your kiln to check temps at critical points in relation to the glass, but I don’t want to get too technical here…

I couldn’t wait to check the results using a polarizing filter comparing the new & old temperatures.  Monday evening I loaded my kiln with a few Colour de Verre boxes filled with frit and powdered glass.  To have an exact replica of a box I’d already made for my test, I used Erbium pink course frit in the kidney shaped box.  Today, Wednesday, I pulled out the lid and placed it between sheets of polarizing film alongside my very first Erbium pink lid.  Please note the irregular edge of that first lid ~ it’s to show you how imperfect I am!  As careful as I’ve been with my film, I notice all the scratches on it, however I want you to focus on the glass atop the film.

So looking at the top photo, the lid on the left, my very first, and comparing it to the lid on the right from this morning you notice three times as many halos or whitish spots on the left!  I will be readjusting ALL my firing schedules!!  All the glasswork annealed properly at 960º is still good, durable, and safe from breakage, but as you can see, 900º is better.  For me as an artist, and for you as a patron of my work, a student learning from me, or an associate with whom I trade information 900º is the anneal soak temperature to use!  If interested, you can view Bullseye‘s new annealing chart for thick slabs here.

Blue Oval 960º : Aqua square 900º

Blue Oval 960º : Aqua square 900º

For a little color I also checked a couple boxes of differing colors, then wanted to compare the same shape box.  Interesting, yes?

Aqua 900º : Red 960º

Aqua 900º : Red 960º

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2 Responses to “Do You Have Rainbows in Your Halos?”


  1. Wow Linda. Really interesting stuff! Obviously I will be changing my annealing temp too! Wonder if they’ve posted it on their worksheets on the web yet. I know Cynthia has been checking.

    Most of my schedules call for a 960 at 1 hour. Is it now 900 at an hour? I’m sure I’ll need to read the worksheets for more details when they are published.

    Thanks for the photos of this. Really great.

  2. Toni Says:

    Hey Linda,

    I just commented on your most recent post about the pavers, but I’ve been thinking between then and now. Is the 900° anneal temp for all glass work or just the thick slabs? I’m talking about the normal two-three layer project. Just curious. I guess I could go their website and look, but since you were there… 😀


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