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Please check out my blog post where I've got all the dirty details such as pictures and slumping schedule.

http://oakviewart.blogspot.com/2014/01/ ... redux.html

Your analysis and thoughts would be appreciated.

Thanks!

E

I have always been puzzled by firing schedules with an initial heatup like the one you used.

Why hold for 15 minutes at 250, then for another half an hour at 900, then increase the ramp speed from 900 to 1100? If the intention is to minimize the likelihood of thermal shock, then why not just go slower all the way up? Or, if you must hold, why not hold at around 500, where thermal shock is most likely? Just curious about the origination of this kind of schedule, which I have seen several times (most often with failed pieces, by the way). What was the logic behind the schedule? (I would have simply gone 200 dph from room temperature to 1100, rather than use the three steps you did.)

Setting that aside, the "tear" in your piece was most likely caused by thermal shock from one of three (perhaps all three) factors:

1. Top only elements can cause a "slit" to be formed in the top of the piece if you are too close to the elements. This is especially likely when slumping on a mold that has little opportunity for air to circulate beneath the piece. Because of this, the top of the piece gets a lot hotter than the bottom. That "slit", which looks like a knife cut at first, widens as the piece slumps.

2. You fired too fast from 900 to 1100, making that kind of "slit" more likely. For some reason, you increased the rate of firing in the range from 900 to 1100. Not sure why you did that.

3. Your piece was larger than the mold. My guess is that the overhang was greater on the edge near where the tear occurred.

Another thing to consider is that the particular mold you used is a difficult mold to use. The deep waves around the edges, couple with a long distance to slump, require a fairly high temperature or a very long hold. Having to change shape so much to conform to the mold means that you will almost always get stretch marks in the piece. Firing with a very long hold at a lower temp (or slumping multiple times) is probably the best solution and even that I've found that the kind of mold you used yields lousy results much more often than it yields good ones. Your piece didn't slump fully into the mold because it wasn't held long enough.

Bottom line, you hit on a lot of the answers in your blog post -- be more cautious with pieces that have been fired multiple times, slump longer at a lower temperature, and don't use a mold that's a lot smaller than your blank. Thanks for sharing your experience (and especially thanks for documenting it so well).

Are the sides of those holes all smooth and rounded?

Brad Walker wrote:I have always been puzzled by firing schedules with an initial heatup like the one you used.

Great Info! Thanks for the feedback.

Where did I get my firing schedule? From the Internet of course!

Google was my friend (or my enemy in this case). It's an amalgamation of schedules I found on different websites/forums. This was an experiment for me.
I used a pot melt that I considered to be a POS and added a border to try this mold out. I tend to be more of a "hands on" learner so this was a good experience for me.

E

You also mention that some of the glass was 'blown glass scrap'. Do you know the actual origins of what that glass was, and how much heatwork it may have already had?

Too fast for the mold and the size of the piece, likely not well-annealed going in, and a decent possibility of compatibility shifts, especially when you're putting a potmelt together with a lot of colors that could have had a lot of firings (especially hot colors).

The slit thing can happen anytime and with almost any element placement if it causes the glass to heat unevenly. Part of the glass will be trying to sag with gravity while another part is still too brittle to move (especially the part inside a closed mold), so it splits open as it's pushed down. It's a really good sign that you need to slow down and let all parts of the glass catch up on the heatwork.

As Brad said, that mold needs careful handling, and the thicker the glass, the more likely you are to get wacky results. I generally do that slump in two firings, the first in a ball mold a bit bigger than this one, just to get the general shape going. Then I put the slumped piece into the fluted mold. If I'm using an opaque glass (i.e., more prone to devit), I'll speed through the devit zone and slump at more like firepolish temps, for a fairly long time. The shape of the mold actually helps hold up the glass so it doesn't overfire. Preferably, though, I do a very low firing with a long process hold, to really settle the glass into the flutes. (BTW, you don't want to settle it TOO far into the flutes--it can stick a bit)

agreed it's a tricky mold. I have that mold and the 2 times I used it were not successful. my issue both times was an air bubble in the center probably caused by holding too long and letting the air under there heat up rather than escape. besides the bubble in the center the sides were perfect!

Apart from that, how was the play Mr. Lincoln?

David Jenkins wrote:Are the sides of those holes all smooth and rounded?

Yes.

Brock wrote:Apart from that, how was the play Mr. Lincoln?

Just fine Dr. Watson. Just fine...