another note on slumping

[Babette (Shawn)]

On my previous post "how to create a flat bottom on the slump with a ball mould?" I got some great advice and suggestions, thank you all for your time and generous help.

Now I have an interesting situation that has come up with that same 22' round bullseye (ball) mold...like many of you I have started using Boron Nitride spray on my molds and I find it to be wonderful stuff. But... a few times now I have had bubbles form (2-3 inch diameter) on the SIDES of my glass and this has only started since using BORON NITRIDE spray...so I thought about it and came up with the conclusion that the size of my vent holes had been diminished and I needed to go back after spraying the BORON NITRIDE, using a pin, and ream the hole to keep it fully open. OK now that problem is solved. Unfortunately before I figured this out I had a nice large bowl that I added-on to a list of pieces that I entered into to a National Juried event, added at the lat minute, hail Mary, no chance in hell. Sure enough this is the piece that got accepted. So I thought: "no problem", "I can fix this", "I have plenty of time"... I went back and reamed the hole and put the piece back in the kiln. Originally the thing fully slumped at 1100F holding 8 minutes, so I thought in order to flatten out the bubble I would bring it up to 1125F and hold for 15. FYI this bubble looks extremely subtle when viewed from above so when I looked in the kiln it looked good and I advanced the schedule to anneal. Some 40 hours later I open the kiln and the bubble did not change a bit. So I kept it in the mold and slumped it again, this time I went up to 1130F and held for 30. SAME THING! The tumor was still there! At this point I had just enough time to slump it once more (I have to deliver this piece to the gallery) So I slumped it for the fourth time, this time up to 1175F and held for 30. The sad news is that this tumor is resistant to treatment and it is going to the gallery as it is. Maybe it shrunk a bit, maybe I am being overly optimistic. The good news is that it will not grow.

I am afraid if I take it up to a higher temperature in the mold the bottom of the bowl will start to deform before the "tumor" flattens out? Or am I being over cautious? I am now thinking I should have flattened the whole thing to a pancake and re-slumped it all over again? I am glad I did not break or ruin the piece. I am learning that there is a fine line between fixing something and creating a new problem, or breaking it. The gallery wants me to put a price on this piece and because of this imperfection it has decreased in value and I am not happy about putting a piece out there that has an obvious defect. Such is life. Burp.

[Brad Walker]

First: what you think of as an imperfection may not appear to be one to the gallery or the potential buyer.

Second: I would have flatted the pancake and then reslumped. But even if you had done that the color of the bubbled area (now flattened) will be different than the surround area (especially if it was transparent glass, which gets stretched and looks lighter).

[Joe Wokovich]

Just wondering out loud, what would happen if the bubble were to be vented and then the piece reslumped? Would the tumor collapse on itself and thus the bubble eliminated?

Joe

[rosanna gusler]

Just wondering out loud, what would happen if the bubble were to be vented and then the piece reslumped? Would the tumor collapse on itself and thus the bubble eliminated?

Joe

bubble was between the mold and piece. rosanna edit. like brad said. call the 'added textural element' aka bubble a design feature and charge full value. it is art and it is what you say it is.

[twin vision glass]

What is the design like. Could you grind a lovely hole in the piece and insert a really wonderful design element in it so it is perhaps almost sculptural in nature. Just a thought.
Les

[Babette (Shawn)]

Thanks everyone! The piece is submitted and they either love it or leave it and REALLY some people will never notice the tumor. It was made with all opaque glass and it does not look to be stretched so if it does not sell I will pancake it and re-slump.

BUT am I correct in thinking that it is impossible to re-slump a piece with a bubble (between the mold and the glass) and get it to flatten to the mold? There really is no slumping schedule to do this that would save the piece without flattening it completely first?

Shawn

[Bert Weiss]

If the piece, simply bends, it can be unbent. If it really stretches, it will be impossible to "unstretch" it. You may be able to mitigate somewhat, but a trained eye will be able to detect the heatwork history.

[Morganica]

Thanks everyone! The piece is submitted and they either love it or leave it and REALLY some people will never notice the tumor. It was made with all opaque glass and it does not look to be stretched so if it does not sell I will pancake it and re-slump.

BUT am I correct in thinking that it is impossible to re-slump a piece with a bubble (between the mold and the glass) and get it to flatten to the mold? There really is no slumping schedule to do this that would save the piece without flattening it completely first?

Shawn

Well...think about what you're asking. A smallish bubble in the bottom of a piece, away from the surface, is really pretty stable, so it's going to need a fair amount of heat to get it juicy enough to fall. If you've got the glass soft and moving enough to completely flatten that bubble against the mold, it's probably also soft enough for deformation, i.e., for the steeper areas to slide down the mold and make a mess.

You could try heating the glass until it softens, then go into the kiln (wear protective equipment, obviously) and push the bubble area down into the mold. The problem, though, is that if there's less glass in the tumor than in surrounding glass, you'll wind up with a dimple instead of a tumor.

And this piece has already been through five firings, if my count is right. Depending on the colors you've used another firing might cause a shift in compatibility or devit issues. And you're increasing your investment in the bowl with every firing so it might get harder and harder to actually make a profit on this one. I think I'd let it go...

[Babette (Shawn)]

I get it Cynthia, thank you. I will give up on this piece. If this happens again I will flatten it right off and deal with the likely possibility that it did stretch and change the density of color...a learning lesson.

S.