Tack Fusing Problem

[Pat K.]

I've been having problems lately with tack fusing. I'm using Spectrum 96 and a Skutt 14, and full fusing the blank which was clear with a Uro white streaky on top, then doing a decoration and tack fusing, then slumping. The first one completely shattered, so I changed the 2nd slump scheule from 100 an hour to 50 and lengthened the anneal by a half hour.
TAck Fuse Slump Fuse
100 400 5 50 400 5
100 900 30 50 900 15
100 1350 10 100 1175 30
9999 950 2.5 9999 950 3 hr
50 800 0 50 800 0
100 700 0 100 700 0
200 100 0 200 100 0

This one didn't shatter, but sim ply broke in half. So, now I made a third one, different decoration on top, changed my tack fuse to the slump schedule except for the process temp 1350 and the anneal to 3 hours. I just took it out of the kiln; it is sitting on the table and I heard the dreaded ping. When I look, there is now a crack on the bottom right corner. Can anyone tell me what I am doing wrong. These mistakes are costing me time and money. I'm guessing that I'm doing something wrong in the tack fuse. Also, since this one isn't broken, just cracked, can I refuse to a soft fuse, then slump. ANy sugguestions or help is greatly appreciated.
Patty

[Valerie Adams]

It sounds like you fused a two layer blank first, and then added details? And if so, what was the first schedule for firing the blank? How big is this piece?

[Morganica]

Not enough annealing. Tack fuses must be annealed extremely slowly--and an anneal is the WHOLE cooling process, not just the soak. The separate components in a tackfused piece start shrinking and pulling away from each other. If you cool too fast the glass doesn't have a chance to relax around those stresses.

Start by measuring the thickness of the glass at the tallest tack fuse, and double it. Then look at the annealing tables and find that doubled thickness--that's your baseline cool down schedule. You adjust from there by tacking on a new thickness multiple until the glass is stabilized.

Since we already know that your design has issues, you will likely wind up using a schedule for 3-4 times as thick as the nominal thickness of the piece, or more. And some designs just won't anneal well with any schedule--hard to tell about yours without seeing a picture.

On the re-fuse, yes, probably, although the heat will likely crack the piece apart early on. You may need to dam the work to keep it together.

[Pat K.]

Okay - I tried to upload a picture, but it said it was invalid. I have no idea how to fix that.

The pieces are 12" round (broken one) and 12" square (cracked one).

My first schedule was
300 900 15
500 1100 0
100 1250 30
9999 1465 10
9999 950 0
50 800 0
100 700
300 100 0

Morganica, by the whole annealing schedule, do you mean all the way to 100 or just to 700. What would you recommend for the temps and any holds? Since I don't have any holds at 800 or 700, what should they be minutes or hours.

I truly appreciate, Morganica and Valerie, your help.

Patty

[Morganica]

I wish I could give you a schedule but without seeing the work and having measurements, it's difficult (and I don't use Spectrum 96 so whatever I give you would probably be off anyway).

Annealing is basically the last half of the schedule after process, all the way down to room temperature. The kiln doesn't have to be on for all of that (especially in a firebrick kiln like the GM1414, since it tends to cool pretty slowly after 500F anyway)

The measurements I'm talking about involve the thickness of the plate/bowl, not the diameter. You look for the thickest part of the plate and measure that. So if your plate consists of a base of two 3mm pieces of sheet glass, topped with a 2mm piece of thin glass for color, and then you tacked a 4mm frit ball on top of that, you'd have a total thickness of 12mm, or nearly a half-inch of glass.

You'd multiply that by 2, or 3, or 4, whatever gives the glass enough time to de-stress, and use the schedule for that thickness. At 2X, then, that means you follow the annealing schedule for 1 inch-thick glass. If that still breaks (and if you are, for example, lightly tack-fusing a bunch of frit balls to a flat glass surface--frit balls are murder in a tack-fuse), you go to the next multiplier and try again.

Does that help?

[DonMcClennen]

On the way down (9999 to 950) may be your problem...(it's better to approach anneal more slowly).....try 9999 to 1000 then 80 to 950.... then continue anneal

[Thomas Decker]

it may not be the schedule, I have tack fused lots of glass all different colors thickness ext. but have always had problem with white wispy glass for some reason no matter what you do it all most allways breaks,

[Morganica]

Yeah, there are some designs (and, I think, some glasses) that just aren't going to anneal well as tack-fuses, no matter how slowly you go. I've run into a couple, certainly.