cabochon distortion

[Alma Rands]

I need help. I fired a batch of cabochons to be used for earrings.
I used 3/4" X 1" black 2mm glass for the bases. The dichro varied from thin to some thick rippled ones. I used 3mm glass tops for all of them-- I cut the dichro almost the same sizes as the base, as close as I could get them. Next I put 3mm clear glass for the toppings, cut a smidgen larger. The problem is that they all expanded considerably in size after firing, and the clear glass tops spread way out over them. They will require a tremendous amount of grinding to get them back to size.
I used my Paragon Xpress-E-12T, kiln, and I have never had a problem such as the one that happened yesterday.

However, this is the first time I have tried small earring sized pieces, as prior to this I made pendant sized pieces, each of which always came out beautifully. No problems.

I used the same full fuse program for the small cabochons that I used for the pendants.
600-1000-15
600-1475-30
full-900-30
full-200-2min. off
I have a couple of questions.
Is the problem caused by the fact that I used a 2mm black base, and a 3mm clear top? Should I have used a 2mm clear top?
The reason I used the 3mm clear was that some of the dichro was thick rippled and I was told that with thick rippled dichro one should use 3mm.
Is it bettert to use the same thickness for the black base and for the clear tops?

My other question is should I have used a different firing schedule for the small cabochons, and not the same one I have used successfully on my pendants?
I will appreciate help with this problem.
Alma

[Judd]

1) All that glass stacked up is 8mm, which is over 1/4". Melted glass tends to spread out to 1/4" thickness. I'm not sure why you bothered putting the clear on top of the dichro. Try it without the clear on top and you may have better results.
2) 600-1475-30 - this means you went up 600 degrees per hour to your process temp of 1475 and held it for 30 minutes? If so, why such a long hold at your process temp? My small kiln is less than a cubit foot and I hold at full temp between 12 - 14 minutes.
3) The black may have something to do with it. It does tend to run more in my kiln; however, I'd bet simply having two layers of glass would do it.

[Alma Rands]

Thanks Judd for the suggestion that I omit the glass over the dichro. The reason I was using it is that I was told that if I did not put glass over the dichro, the dichro would chip off when the pendant was being worn.

Regarding my kiln temp. I developed that firing schedule after a lot of trial and error It worked the best. Perfect fusing, no bubbles, no cracking, no dog boning, just really nice pendants. However, I use it for pendants and wonder if I need to modify it for small little earring sized cabs. Alma

[Morganica]

Use thinner glass, then. Unless you dam the pieces, anything you make thicker than a quarter-inch will (as Judd says) spread. You can certainly cut things smaller initially, to account for the spread, but that will take some figuring. And at the schedule you're using may not result in the shape you're looking for (unless that shape is round).

Another option is to do what I do when I'm making dichroic conglomerations: Gang them into a single sheet with all the dichroic, decorations and caps you want, full-fuse it and let it spread as much as it wants. Then cut the sheet into the correctly sized shape and fire again to a final form. I like to press it with a second kilnshelf, weighted down and edged with a quarter-inch of fiber paper, to get me to the right thickness. Then I can cut more easily, fire and get some nice stuff.

That method is extremely efficient if you're making a bunch--I find I can get some really interesting patterns, and use up dark transparent scrap, that way. Clear dichro over mixed-up violet striker and aventurine green irid is really nice...

[Alma Rands]

Thanks Morganica foe rhe helpful suggestions. I will try your method of making a large piece to be cut smaller. How do you cut them? I have heard that one could use a tile saw, and water, but would one need a special blade?
Alma

[Valerie Adams]

I can cut glass that's 3- to almost 4-layers thick by hand. Takes a little extra squeezing with the running pliers but easy enough. I cold work all the edges to shape them as I like. I also build my 'tiles' with and inch or so of sacrifice glass around the outside so I don't have to deal with rounded edges.