WarmGlass.com

I just slumped a customer's beautiful 10" bowl and discovered that it cracked during the slump. I've uploaded a photo to show that there's a 6" "Y" shaped crack on the underside. On the top side (inside of bowl), the crack is less pronounced and, since the pattern is very colorful and abstract, less noticeable. I've not used this mold before, it is 2" deep and maybe the stress from the deep slump caused the cracks. My question is: Can I fill in the cracks with clear frit (top and bottom), place in the mold again and slump again? Or, can should I full fuse first, then slump in mold again? The glass is Spectrum 96, I used the Auto Slump program in my Hot Start Pro, 1225 at 35 min. It was the only item in the kiln.

Thanks!
JaYing

You will see from this ( http://glasstips.blogspot.co.uk/2011/11 ... racks.html ) my view that you had too fast a heat up rate for the thickness of the piece. Also my view that you need to slow down from around 540ºC to allow the whole piece to be the same temperature from top to bottom. This will avoid the stress break on the back.

Solution? I would fuse to flat and then re-slump. I would not use any frit, as the colour (or lack of it) will emphasise the crack. I think the break will always be noticeable.

Second part of the solution is to avoid using pre-programmed schedules. I do not believe the shape, or depth of the mould is the problem. I think the pre-programmed schedule was too fast.

Now I will seem to be mean. Do you know enough about your kiln and appropriate schedules to be firing for others?

Thank you. I'll do as you recommended. I've actually fired and slumped hundreds of pieces for others; but they have mostly been small, less than 6" and shallow. Haven't had any problems with the pre-programmed schedules and have been too lazy to enter a custom one. I guess it's time to now!

JaYing

To add to Stephen's response...glass slumps before it fuses. If the frit gets hot enough to melt seamlessly into the crack, the rest of the glass will have changed shape. If it is on the kiln shelf, it will be flat. If it is in the mold, it will continue slumping until it is a puddle in the bottom.

Fuse it flat first. Given the design you could probably add some additional pieces to cover the scar once you've got a flat blank. I personally would drill the big bubbles and let the glass fuse hot enough to see if I got rid of a few of those, too, but that is a more advanced technique.

Which brings me to the hundreds of pieces you've already fired: Do not assume that something is well-annealed simply because it didn't break during firing. It is likely that some of those fired pieces will encounter a sunny window, being placed too near a heater vent, etc., and break. That will be the fault of the studio fusing the glass.

Your clients probably won't know that...but to be fair to them you need to learn more about the processes you are selling. What you are asking here is fairly basic stuff. Please invest in one or more kilnforming classes, or buy some books on the subject, study them and practice on your own glass. You will learn how to tailor a firing schedule to the glass, mold and kiln and avoid some real liabilities down the road.