Need Help with my mosaic bowl, it keeps cracking

dave laporta · Post by **dave laporta** » Fri Mar 14, 2003 8:36 am

Some of you may recall last week I posted a message about a mosaic bowl I tried to form and it cracked. The irony was that I had just said that I have never lost a piece. Well I tried it again and again it cracked. Now whats the deal? Cynthia suggested I slow down my ramp to 1100 and I did just that. My schedule was 225 to 1100 hold for 10, 75 to 1215 hold for 10. This is all Bullseye glass. The crack in both pieces split the piece in half then was fusedat the ends, as it formed the seam openned up in the center.

Has anyone had problems with this type of project or can anyone share their successes?

I'm a bit hesitant to try it again without some guidance. TIA

Dave

Barbara Muth · Post by **Barbara Muth** » Fri Mar 14, 2003 8:56 am

Dave your split fusing at the edges suggests a thermal shock on the way up. If you have varying thicknesses in the piece, that could account for your thermal shock. You are going up pretty slowly. Maybe a couple of soaks along the way?

Oh wait! I just looked at your schedule again. Was this a slump fuse? We need more details on the piece and the slump, into or over, bisque or steel. And pictures help.

dave laporta · Post by **dave laporta** » Fri Mar 14, 2003 9:41 am

the mold was a made bisque mold that I formed into. Overall the thickness is 6mm but the grout area sinks as it melts down so the result after the fuse is a slight ripple effect. I have other mosaic pieces that have not been formed so its not incompatibility. It is obvious thermal shock, i just don't understand why it happens to this type of piece.

I hope this is more information. Sorry no digital camera yet.

D

Barbara Muth · Post by **Barbara Muth** » Fri Mar 14, 2003 9:57 am

The variation in thicknesses may be your problem. In addition to slowing down, maybe try a few 15 minute soaks on the way up to 1100 to ensure evenivity of temperature throughout the piece. Anyway, that's what I would do.

Barbara

Cynthia · Post by **Cynthia** » Fri Mar 14, 2003 1:09 pm

Do you think it's possible that you have a shift in coe due to firing? Glass can become incompatible after firing. Or is it a possibility that you have inadvertantly combined some incompatible glasses? The incompatibility might not show up until you stress it by putting it back in the kiln. Another problem could be that it was initially annealed improperly and the stress again might not show up until you try to heat it up again.

Your schedule is plenty conservative, so these are my only guesses I have left.

charlie holden · Post by **charlie holden** » Fri Mar 14, 2003 2:29 pm

I don't think it's thermal shock. I think the piece is tearing along natural week spots at joints between mosaic elements before it gets soft enough to slump. This is why it cracks in the center instead of at the edge. The stress is greatest on the center underside when you slump into a bowl mold. Check the crack to see if it spread a little further on the underside than it did on the top.

As I understand it, you've fused mosaic pieces with a different kind or color of glass between them to give you the look of a grout. The joints between the pieces run completely through the piece from top to bottom and edge to edge. If this is the case, then you have to give the pieces a long, full fuse soak to eliminate any weekness between the pieces. If you have differences in thickness I think you're not fusing enough.

Another thing you could do is fuse on top of a solid, clear blank. That gives you a bridge across the joints.

The third option, if you really want the texture and the color through from top to bottom, is to slump on another piece of glass. Cut a solid piece of clear Bullseye slightly larger than your piece. Put it on the mold first, then sprinkle dry kiln wash on it. Put your piece on top and slump as low and slow as you can. The clear piece will support the delicate piece. I've slumped pieces with visible joints and open holes this way.

ch

dave laporta · Post by **dave laporta** » Fri Mar 14, 2003 4:40 pm

Maybe it is stress (whether its me or the piece is another story), cracks in my projects have only happened with these mosaic pieces. But I figured I could resolve the problem from the first try with the slower schedule. The assembly of the pieces consists of a clear double rolled(BE-30) base layer with .5 to .75" square mosaics mostly clear -30 (left over scrap) with color accent pieces made from single rolled (BE-50)

So if I understand you, I should flip the piece over to slump so that the base layer is now on top. In theory this will allow them to stretch into the form instead of being squeezed into the form. Does that sound right?

FYI-In the fuse I let it soak at 1420 for 25min.

Thanks for everyones help, I'm determined to work this out.

charlie · Post by **charlie** » Fri Mar 14, 2003 4:43 pm

dave laporta wrote:So if I understand you, I should flip the piece over to slump so that the base layer is now on top. In theory this will allow them to stretch into the form instead of being squeezed into the form. Does that sound right?

FYI-In the fuse I let it soak at 1420 for 25min.

Thanks for everyones help, I'm determined to work this out.

1420 may not be high enough to fuse all the way through. i hae to use 1470-1500 for 10-15 minutes in my kiln.

Cynthia · Post by **Cynthia** » Fri Mar 14, 2003 5:40 pm

dave laporta wrote:The assembly of the pieces consists of a clear double rolled(BE-30) base layer with .5 to .75" square mosaics mostly clear -30 (left over scrap) with color accent pieces made from single rolled (BE-50)

So if I understand you, I should flip the piece over to slump so that the base layer is now on top. In theory this will allow them to stretch into the form instead of being squeezed into the form. Does that sound right?

FYI-In the fuse I let it soak at 1420 for 25min.

Thanks for everyones help, I'm determined to work this out.

Okay. Charlie has a good point. I have had glass tear or split when bending when it is of different thicknesses or has holes in it. To combat that I use a super slow schedule and incorporate a soak too. Your schedule is slow though...

I either ramp up to 1000 at a reasonable rate (dependent upon my glass) and soak there for 30 minutes, then ramp up to 1200 at 100 dph. This allows the glass to be evenly heated throughout as well as giving it time to bend slowly. I think of cold modelling clay as an analogy. IF you bend it cold it tears, if you let it warm up it will bend and stretch without tearing.

The other schedule I use is to ramp up at two to three hundred dph to 1200. I do a mosaic pattern too that has been fired on machine rolled clear, then I flip and refire to have the clear act as a cap. I bend these pieces using the low and slow (second schedule listed) slumping schedule.
Go to this link to see a mosaic piece.

http://community.webshots.com/photo/647 ... 9323NLMgcz

I think Charlie has it right...so you will be able to conquer this if he's right.

Barbara Muth · Post by **Barbara Muth** » Fri Mar 14, 2003 6:32 pm

Cynthia, when you flip, do you have to blast the surface to clean off any residual kiln wash? I sometimes get scummy top when flipping.

B

Nice piece btw. Will I be seeing it in person next month?

Clifford Ross · Post by **Clifford Ross** » Fri Mar 14, 2003 8:18 pm

Cynthia- nice mosaic bowl, and who dresses you in the AM? Two different shoes in picture?