First pot melt had "issues"

Bruce Larion · Post by **Bruce Larion** » Sun Mar 21, 2004 7:12 pm

I tried my first pot melt yesterday with mixed results. Here are the details: Using a skutt 1014 kiln, used 4 colors of glass, lime green, black, orange and white. All Coe 90 and all BE. This was a 6" pot with a 7/8" hole dropping 3" into a 14" clay saucer that had been kiln washed then a 1/4 of talc leveled smooth.(got this from an archives search). The schedule was 700dph to 1650, hold 1hr then cooled using 1" gap in the lid to 1200, then heat back up to 1500( again something from the archives to supposedly relieve bubbles, then crash cooled to 1200 and naturally to 960 where I held it for 90 min. then 100 dph to 760 and finally cool to room temp.
When I opened it up this morning the pot melt had not cracked but the 14" saucer had a small crack in the side and my kiln shelf (20" mullite shelf space 1" oof the kiln floor)was cracked down the center. I had a few bubbles in the pot melt and 1 spot (top of picture) that appears like kiln wash on the surface. The back is very cloudy. My questions are Why did the shelf crack? What would cause the spot on the top surface? Can the back be cleaned up with something like lime away or do I need to have sandblasted? and finally I welcome critique on the piece itself-both good or bad. I am really trying to learn so I am open to critisism as mentioned in an earlier post.
Thanks for any feedback.
http://community.webshots.com/photo/127 ... 9076jSTkAT

rosanna gusler · Post by **rosanna gusler** » Sun Mar 21, 2004 8:19 pm

your kiln shelf probably cracked from heating too fast with too much mass on it. all clays like to go through quartz inversion rather slowly. even though mullite is very forgiving, you can stress it. clay pots and saucers are less forgiving. talc is best used under 1300f?. it outgasses around that temp. i forget exactly. rosanna

Barbara Muth · Post by **Barbara Muth** » Sun Mar 21, 2004 8:27 pm

Just in case you didn't realise this, you don't have to drop into a saucer. Just kilnwash your shelf, calculate how much glass can drop, go to 1/4 inch thick and not flow over the shelf, use no more than that, and do the pot drop. With a kiln that big, you should be able to get a nice sized pot drop without any worries about overflow. And you should expect the saucer and the pot you dropped the glass from to crack eventually. Their coefficient of expansions are different from that of glass.

Barbara

Bruce Larion · Post by **Bruce Larion** » Sun Mar 21, 2004 8:28 pm

Thanks Rosanna-I figured something to do with heat or cooling but was not sure what to change. It was a very heavy saucer and I sat it directly on the shelf.

Bruce Larion · Post by **Bruce Larion** » Sun Mar 21, 2004 8:33 pm

Thank Barabara-It certainly would be simplier to just use the shelf. I use high fire kiln wash but some of the things I read in the archives lead me to believe you need to drop onto something like a saucer or another piece of glass. It would appear I lost a shelf anyway so next one will try to use shelf only. I used 3#4oz of glass for this one.
Thanks again

Lauri Levanto · Post by **Lauri Levanto** » Mon Mar 22, 2004 2:42 am

Dadll
you had a very difficult thermal distribution.
Part of the shelf was exposed to direct IR radiation.
The middle was ahadowed by the plate that absorbs its share of heat. The talc which is conteins much air was an insulator.

On the way down the roles were reversed, the edges of
the shelf were free to cool, the plate acted as a heat
reservoir in the center.

There are two risk zones with the ceramic materials.
Around 573 C the quarz inversion and
Christobalite conversion somewhere near 240 C.
One should cross these zones carefully.

-lauri

charlie holden · Post by **charlie holden** » Mon Mar 22, 2004 11:15 am

The powdery spot on the surface is where a bubble came through all the way from the bottom of the glass. When bubbles rise through the glass they pull stuff, usually glass, up from underneath. Notice that the other bubbles have colors from down below in their centers.

I know that some people use powder under their pot melts but I'm not a fan of that. I think the glass eats the powder unless it is bound by something like plaster.

ch