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Tips, Techniques, and Questions -- Technical questions or tips

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  #1  
Old 2010-11-15, 8:14pm
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Default Annealing

I know this question wasn't going to be framed the way I need it to be answered, so I'm sorry about not doing a search.

I took my 120 volt, small ceramic kiln with a kiln sitter and set it to a #19 cone. I did things a bit different from ceramics, and batched annealed some beads. (I think)

I started it on low 45min top wedged open (I can't remember if the top hole was un plugged)
went to med for another 45 minutes with the wedge opening the top, but did plug the top.
then I went to high until the cone cut it off, but unlike in ceramic, I had the kiln totally closed. I was surprised how quick this cone was to the ones I am used to working with.

I slowly started opening things up, first the top hole plug, then the second which was closer to the beads which were on a blanket. I waited for sometime, under 20 minutes, then opened the second plug, same thing again and I wedged the top, then I added another support to the lid to slowly open it higher, and proceeded like this a bit longer.

The beads did not crack or melt, I really can't see any "eye" type changes.

So here is the question... how do you know if a bead has be annealed? Is there a test? I read about a rimming type behavior, but that is not there. This may sound really stupid, but should I try to smash one with something hard?

These were a pile of my rejects--unfortunately I have a lot more of those to practice this with
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  #2  
Old 2010-11-15, 8:56pm
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I did a cone test on an old kiln, and I used cone 022 that should bend at 1085 degrees, which is above the temp that you need to anneal soft glass. I did it to ensure the kiln was reading accurately, not to anneal beads. Anyway, annealing is more about a steady hold at a temp above the strain point, then a slow controlled cooling until the glass is well below the strain point.
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  #3  
Old 2010-11-16, 6:33am
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Unfortunately what you are doing is not annealing. Glass must be cooled very slowly once it is at the appropriate annealing temperature. You can't let it drop 100 degrees, then hold it at that temp, then drop another 100 degrees, etc. You can't just turn off your kiln and let it cool naturally. Glass must be slowly, steadily cooled so that the inside of the glass is the same temperature as the outside. The thicker the glass the slower it needs to be cooled. If you allow the glass to be cooler on the outside than the inside, it sets up stresses in the glass and it will eventually crack.

Glass kilns are different from ceramic kilns in that they are not fired to cones. With a cone, once the kiln reaches the cone temperature, the kiln will shut off. Thermodynamics tells us that the inside of the kiln will cool fastest when there is the greatest disparity in the inside and outside temperatures, just the opposite of what you need for proper annealing. You will need a controller that allows you to slowly cool your glass in the high ranges of the temperature, perhaps 1 degree a minute, or 2. Cooling at a controlled rate of 100 degrees an hour is sufficient for most small glass pieces.
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Old 2010-11-16, 7:38am
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You can still use a ceramic kiln to anneal glass but you will need to get a controller like Pam described and hook it up to your kiln. I started with a ceramic floor kiln for annealing my beads and it works fine.

The second one listed on the page below is the one I used:

http://www.sundanceglass.com/controllers.htm

I drilled a hole in one of the plugs and inserted the thermocouple wire into the kiln so it shows about one inch of wire inside the kiln. See the bottom of the same page.
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  #5  
Old 2010-11-16, 2:15pm
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Argghhh,

Thanks guys--so what did I do to the beads

I did talk with the manufacture of my kilns, and the large one has a ring attachment, I guess so there is a door, but from the looks of the price, the most important part of the thermocouple, as mentioned by gink... can't have been included. Those are SO expensive!

So if I just plan on batch annealing since this is a hobby, is it better to find a place that can do it for me, or what is the least inexpensive kiln? Are ones that are used really trustworthy, if you do not know the seller?

I am a poor old starving artist...
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Old 2010-11-16, 2:42pm
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If you do not plan to ever give away or sell your beads then annealing is not at all necessary. If beads crack in the future it is a non issue. However, if selling, then annealing is a requirement for stability.

An inexpensive controller can be made by using a Fuji PXR-3 PID digital controller. The PXR-3 is $69 plus SSR for $15 and $14 for a thermocouple plus a few dollars for thermocouple wire. For a little over $100 you can have your present kiln converted to one that can anneal beads using recommended ramp and hold times as a DIY project.
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Old 2010-11-16, 5:50pm
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Thank you so much Alaska!

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