This morning I just learned…

…my kiln doesn’t heat evenly.

The hard way.

The project was simple enough.  I had this pot melt that was a bit on the small side that I didn’t quite know what to do with.

And I had this funky mold I wanted to try out.

So I got out my 12 inch ring mold, put the melt in the center, surrounded it with scrap blown glass, did a full fuse, smoothed out the rough edges with the grinder and put it in the kiln again for a contour fuse to round the edges. Now I’ve had a piece thermal shock during cool down from an annealing that was too short and open the kiln briefly when it was a 350 degrees. This is the first time something has blown up on me during the heat up phase. Now when I head up the glass I’ve been using the “rule of thumb” rates that I saw (if I remember correctly) on the Glass Campus web site. The basic rules were this:

• 6mm glass –> ramp up at 400 degrees per hour.
• 9mm glass –> ramp up at 300 degrees per hour.
• 12mm glass –> ramp up at 200 degrees per hour.

Something like that.  The blank that was in there was only 7mm thick but I used the ramp up for 9mm glass just to be safe.  Here’s what happened:

 

Ouch!

The only thing I can think of is that the the borders I’ve been using for my plates are made up of pieces of scrap blown glass.  Now even though the pieces are “fused” together their bonds may not be as strong as they are in the pot melt itself.  I’ve used the same ramping schedule on other plates but in those situations the melt made up the bulk of the plate mass and so was inherently stronger and more stable. There could have been a coating on a couple of pieces that introduced a weakness in the piece that mad it more sensitive to a kiln that didn’t heat evenly.  That’s my best guess.  I decided to go ahead and try to fuse the darn thing back together again.  This time I’m ramping up at a much more conservative 200 degrees per hour.

I did a little grinding to try to get the pieces to fit closer together and I added a few scraps along the break just to make sure the pieces thoroughly fuse together. I also put kiln posts up against the pieces to make sure doesn’t “move” during heating.  OK it’s probably a bit overkill but color me paranoid.  I’m going to need to grind the darn thing anyway because it will end up being a bit oblong from trying to mend the break.

 

I’ll keep you posted in the results..