I see said the blind man…

I my last post I spent a fair amount of time licking my wounds do to a massive air bubble that formed under a piece I was firing.   I was fairly confident I could fix things by doing another tack fuse.

Well it worked…sort of.

The bubble fused flat and there’s a hole where a check of glass broke off.  Not a big deal since that will be filled in with frit.

The point where I got hammered was the cool down after the anneal soak. After the piece had been out of the kiln for about a day a crack formed along the boundary of the pot melt.

I spent a couple of days scratching my head of over this one before I realized what the problem was.  I used a cool down schedule which I though was appropriate for a piece that had an overall thickness of ~9 millimeters.  the problem was my piece was not uniformly 9 millimeters.  Here’s the cool down portion of my schedule.

Ramp	Target	Hold (in minutes)
FULL	960	80
75	800	0
150	700	0
200	100	0

I think this would have been OK if the piece was uniformly 9 mm thick but this was too aggressive relative to what I was trying to do. 3mm glass is going to cool much faster than 9 mm and that’s what introduced stress along the boundary of the pot melt.  If anything I need treat as though I were cooling down something that was 12 – 15 mm thick due to the varying glass thicknesses.  Here is the schedule I plan on using next time.

Ramp	Target	Hold (in minutes)
FULL	960	100
50	800	30
75	700	30
100	100	0

I’m thinking (knock on wood) this should do the trick.

I’ll keep ya posted.

You probably don’t want to forget this step…

Normally after I put kiln wash on a shelf I put it in the oven and let it bake 200 degrees for an hour and then bump it up to 500 for 15 minutes or so to get all the moisture out.  On this occasion I forgot to put a timer on and it ended up baking at 200 degrees for a couple of hours.  I figured this was enough, let the shelf cool down again and then used it a few hours later to tack fuse this project.

The failure to bake it at 500 degrees turned out to be a mega fail.


OUCH!
I figured I could fix it by drilling a hole in the bubble and then taking the piece to slumping temperature.  I flipped it upside down and filled the depression with water.  I cut the hole using a 1/8 inch carbide tipped glass bit and a Dremel moto tool.


Of course one hole wasn’t enough for me.  I was being a bit paranoid and tried drilling a second one because I was worried that the hole would seal up as the glass settled and I’d still be left with a bubble.  The first hold drilled clean. The second one not so much.

But this doesn’t necessarily bother me either since I had planned on putting a layer of colored frit on top and then doing a contour fuse.  I’ll just fill the hole in with frit. Unfortunately the small cracks pictured here weren’t content on staying “small” and grew to split the piece in half.  I’m still pretty confident that I can salvage it and fuse it back together.  The lesson here is you want to make darn sure you’ve got all the moisture removed from your shelf before firing something like this.
Dang! This smarts though…