Making small cabochons with opalescents pickup kiln paper.

[JeffP]

Hi Everyone

I am using Bullseye 90 glass to make small (< 10mm) round cabochons. The 3mm glass is cut into 10mm squares and then full fused to a round cab, on BE kiln paper.

The transparent glass works fine, but opalescents all pickup the kiln paper or kiln wash and result in a scummy bottom. Cleaning this off is almost impossible with such small pieces and terribly time consuming.

Can anyone help with how I can make these with opalescent glass and have a clean bottom?

[Stephen Richard]

You may be firing too hot or too long.  You give no information on the firing schedule or conditions, so this has to be speculation.

Suggesions:

Fire lower and longer – peek in periodically to see what is happening.  As soon as the glass rounds, go to anneal cool.  On the next firing set the temperature 20C lower and fire for longer.  Peek in periodically to see how long it takes for the glass to ball up.  Record this and you will have a reference for the future.

As opals always are more prone to pick up residue from the separators, you could put a piece of clear glass under the opal.  Less work than cold working.

In general, I can get small pieces like this to ball up at rounded tack temperatures.  At these temperatures you should not be picking up separator residue.  This is why I think you are firing too hot.

Stephen Richard

blogs at: http://www.verrier-glass.blogspot.com/ and  http://www.glasstips.blogspot.com/

[JeffP]

Hi Stephen

They were fired at 790C (1450F) for 10 minutes. A quick look up seems to say rounded tack fuse is about 760C (1400F). Is that the temp you think may work?

I will try this and observe to see result, but what hold is normal for this temp, and what about the devit problem with opals?

[Stephen Richard]

I try to get all my fusing work completed with a 10 minute soak at top temperature.  Slumping and casting are different.

Well, the real test is what is the rounded tack fuse temperature for your kiln.  Not all kilns are the same, nor are the same effects achieved at the same temperature with different schedules.  You need to observe.  Looking up what someone else has done gives a guideline, not a definitive answer.

Heat work is the important concept.  A slow rise in temperature will achieve the desired effect at a lower temperature than a fast rise.

I’m not sure what you mean by “what about the devit problem with opals?”

If fired to 760C with a 10 min soak, I don’t think there should be much problem with devit, especially if you vent your kiln to 500C to allow burn out gasses from the thinfire out of the kiln.

Stephen Richard

blogs at: http://www.verrier-glass.blogspot.com/ and  http://www.glasstips.blogspot.com/

[JeffP]

Your comments have been very helpful. Venting is one thing I am not doing but will try.

I have a problem with opalescent Bullseye glass in that it tends to devitrify. From research I understood that rising and cooling faster through the temps around 730 – 760 minimizes devit. Hence I tend to rise quickly to temp after strain release.

Maybe I should try making these on a prepared shelf but keep the temp down and hold longer. My earlier attempt on a shelf and not paper resulted in a worse problem with picking up the shelf separator.

I will definitely try lower, slower and vent on the shelf.

 

[pgcrawley]

I have been using a lower temp(usually about 15 to 30 degrees F) with about a 2-10 minute more soak time and haven’t had any problems with devit.  I have never vented(because I always forget). As small as your glass is, you should be able to accomplish this with a lower temp quite easily.  Remember the color of your glass with affect your fusing as well.

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