Testing chips waiting to go into the kiln

Testing chips waiting to go into the kiln

I sat down to write this article because I want to be transparent (apologies for the pun) about the way I work and about the products that I sell for use in fusing.   When I began working with glass, like most people I took courses with artists who had worked with glass for many years.  The thing that I remember the most (apart from the 6mm rule) was that old adage “you kids don’t know how good you’ve got it.”  My teachers were referring to the fact that manufacturers now produce glass that is tested compatible for fusing.  ‘Back in the day’, kiln glass workers had to perform their own tests for compatibility based on what they wanted to create, to make sure that their pieces wouldn’t suffer from cracking days, months or years after it was finished.  Luckily, this time-consuming aspect of working with the beautiful material that is glass is now not really necessary, because you can go online and buy a bunch of glass and as long as it’s all Bullseye, or System 96, you can pretty much be guaranteed that it’s compatible, if you work within the constraints of the tests run by the manufacturer (i.e., making normal bowls and fused suncatchers and things).

The unprompted cracking (i.e. you didn’t drop it…) of finished, properly annealed kiln glass pieces results from internal stresses that are caused by differences in – among other things – coefficient of expansion (CoE) AND viscosity.  These properties are controlled by the chemistry of the glass.  The important note, and one that kiln-glass manufacturers emphasise is that CoE is NOT a guaranteed measure of compatibility.  I’d recommend the Bullseye article “compatibility of glass”, as they explain this much better than I can.

So why does this matter?  Well, the problem with using lampworked glass elements in kilnforming is that during the torchworking phase, you are exposing the glass to conditions outside the constraints of the tests run by the glass manufacturerThis means that you can’t guarantee that the glass will behave in the same way; and it might not be compatible anymore – some unexpected chemical change might have happened due to what you did with it while torching.  In particular, you expose the glass to:

  • higher temperatures than you would during a full fuse, which are not consistent (the gather will go hot, then cold, quite a few times before it is finally pulled into a cane).
  • directional heat from a flame, which can set up temperature gradients within the gather.
  • Different atmospheric chemistry to that in a kiln.
  • Higher physical stresses to the ‘passive’ kilnforming process – you tug and pull the glass around, rather than it being controlled by it’s own weight in the kiln.

Sounds pretty bad, right?  This is why I always test every cane I make for compatibility using a slight modification of the polarizing film test (see the above Bullseye article).  This is to cover my own ass, as I don’t want someone thinking I’m responsible for cracking his or her masterpiece!

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The chip testing equipment

The chip testing equipment

Bullseye opal blue rod chips fused onto Bullseye TEKTA

Bullseye opal blue rod chips fused onto Bullseye TEKTA – note absence of haloes indicating compatibility.

[bs_col class=”col-sm-6″]My method, for a given murrini cane.

  1. Anneal the cane. I anneal for 50 minutes at 480°C, and cool at 98°C per hour to the strain point (432°C), then let the kiln cool naturally to room temperature (usually somewhere in the region of 200°C per hour, it’s only a tiddly kiln)
  2. Place two murrini chips in the middle of two 25mm squares of 3mm tekta.
  3. On one square, add 4 chads of 2mm clear to the corners
  4. Place one square face up, and one square face down.
  5. Fire to the following schedule:
    • Rate: 350°C/hr, Temp: 670°C, Hold: 20min[bs_row class=”row”]
    • Rate: AFAP, Temp: 810°C, Hold: 10min
    • Rate: AFAP, Temp: 480°C, Hold: 50min
    • Rate: 90°C/hr, Temp: 432°C, Hold: 0min
  6. Clean chips and place on a light box, on top of one sheet of polarising filter.
  7. Use my magical high-tech polarizing viewer (bog roll tube with polarising filter sellotaped over the end) to check for ‘halos’. Halos indicate stress in the glass.  This is not necessarily always caused by incompatibility – you will get halos around anything tack fused on the surface, and around the corners of a piece – but it is a good indicator, and is what they use at Bullseye so it’s good enough for me.
  8. If the halo is more than that seen around a round chip of opaque glass rod fused to the surface of 3mm tekta, I don’t sell the cane. What do I do with it?[/bs_col][/bs_row][bs_row][bs_col class=”col-sm-6″]
    104coe rod chips fused onto bullseye TEKTA

    104coe rod chips fused onto bullseye TEKTA – very incompatible!

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Some things I’ve learned while working with Bullseye murrini:

  • Opaque white is a pain in the ass. It’s shocky in the flame, and murrini which contain white always show more stress in the final chips.
  • It doesn’t really matter about the design of the cane, just the glass types used. This implies that the torch ‘work’ doesn’t affect the compatibility that much (if it did, canes that required longer working times, higher temperatures, or more physical ‘squidging’, like stars and flowers, would naturally show more stress than simpler canes).
  • Canes that contain opaque white can show devitrification on the finished surface where the white is exposed.
  • Bullseye opal canes are generally quite shocky and need to be preheated before use, and some really need to be annealed (Petal Pink, I’m looking at you!).
  • I hate opaque white.[/bs_col][/bs_row]

So, I hope that article helped anyone who was wondering about compatibility and flameworked murrini!  If you have any questions, please fire me an email or send me a message.  I’m quite interested in learning more about this fascinating material.