All kilns, as far as I know, have cool spots. Areas in the kiln that never reach the desired temperature. The result is that anything in those areas will not be processed to the same degree as the rest of the kiln.
After many firings I knew this to be the case in my kiln, but wasn’t sure just where those areas were.
A recent fire polish gave me the opportunity to look at this a little better.
This photo shows the pieces I wanted to fire polish. They are all designer fused glass cabochons.
These cabs were created earlier in another fusing as long slabs. Here I have cut them into 20mm wide cabs on a diamond saw. The sides were then ground to remove any deep saw marks.
If you look closely you can see I didn’t do anything else to these cabs and there are still small chips from the diamond saw. Yes, I know these can cause devitrification, but I have done this before without any problems. You can also see that the sides are all very square and have not been rounded in any way.
They were all thoroughly cleaned and the sides scrubbed before being fire polished on a prepared shelf, not shelf paper. I use Bullseye shelf separator and find it works well, so far.
Anyway, this post isn’t really about the cabs, more about the kiln.
In this photo you can see that the cabs have all polished quite well. But look closely.
You will notice that the cabochons in the middle of the shelf have well-rounded corners and sides, but with the cabs in the corners this is not as pronounced. They have still polished nicely, but the corners and edges are more square.
If you inspect the spread of cabochons you also see that the ones at the front of the kiln are also not as rounded. I assume this is because of the bead door in my kiln. While it is closed, heat does still escape.
Towards the front of the kiln on the left I have a viewing port. I expected this to have an impact but can’t see anything significant, any more than the cabs in the back corners.
In all cases the cabs are still polished, just not as contoured.
Another thing I noticed is that the black cabs have rounded more than other colours. I have read a little about how different colours respond differently to processing, and this is a great example of that.
I actually don’t mind having variation in the cabs. The crisper look of those that haven’t rounded as much is still cool and I expect that customers will vote with their purchases regards which they prefer.
If you are more concerned with consistency, and in some case I will be, you should run a test of your kiln and find out just where the cool spots are and load your shelf accordingly.