Clear Capped Cabochons

[print-me]

In this firing I tried making cabochons with three layers of glass. This probably is a more normal method, capping with clear glass, so I expected this to work well and with few issues. As usual, I added a few other bits to see what happened.

This probably going to be my approach always; adding extra bits to fill in space and for testing. Making good use of the space in a kiln is surely much more economical and a good practice to adopt, but thinking of what to include does take a bit of thought.

Before Firing

The main cabochons were all done on a base of 3mm Bullseye COE 90 black, capped with 3mm clear. Sandwiched in the middle were a variety of small offcuts and some dichroic pieces. As we had only a vague idea what the finished pieces would look like we tried a few variations.

Both Ann and I layered up the main cabs.

We followed the recommendation about making the clear caps a little larger than the base, but simply added the middle bits in a random pattern.

In the ‘Before’ photo you can see that we did also have a few other items in there for a test.

Additionally, I added a few previously made cabochons with devitrification. I ground off the devitrification and partly shaped the cabs.

I also added some plain squares to make dots, and a couple of odd bits to see what would happen. I expected these to form nice round cabochons that I call ‘dots’.

After Firing

Well, it did go fairly well, but there were a few surprises.

The main cabs did nearly work perfectly. The only surprises were a couple that had fine lines along the sides. I think this was where the top cap edge had not fully fused. The dichroics looked great, and even the plain glass ones looked pretty good, but maybe not as colourful as many I have seen.

The cabs with devitrification didn’t work well. A couple were okay, but the others still had some devitrification. I really think it is the glass as it is nearly always the orange and pink that it happens to.

The dots simply all failed. They did not fuse into complete circles. This side of the shelf is near the bead door so that may have something to do with it, or maybe I didn’t fire high enough or long enough.

Firing Schedule

In this schedule I did try something new. That was to ramp at full speed up to the processing temperature. After seeing some videos and reading a few comments that with small pieces like cabochons you can just ramp up quickly, I thought why not try it.

I also decided to crash cool after processing was complete by opening the kiln door. Very carefully, with the correct gloves and glasses on I opened the door repeatedly until the temperature was down to around 560 C, being careful to not go down to below 516 C, the trigger temperature for the next segment. I then let it finish the cycle and cool naturally.

After some thought, I believe the processing target temperature was not high enough. The other thing I could have done is held the temp for longer.

Looking into the kiln and judging the result is obviously going to take practice. They looked fine to me, but obviously had not fused fully.

Segment	Rate (C/hr)	Target (C)	Hold (mns)
1. Ramp Up	FULL	780	5
2. Crash Cool	Open Door	560	0
3. Anneal	FULL	516	60
4. Cool	180	371	5