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Does anyone have an answer as to why this is now happening?
The story:
1- Last year I made a plate slumping mold out of Ceramaguard, kiln washed it and successfully slumped 9 plates. Glass was and is BE #3501.
2- This year I had a request for 6 more plates. The first two were fine. Then like someone flipped a switch, the kiln wash is now sticking to the plates. Big chunky areas and it is really bonded. It takes diamond pads to get it off.
Does BE kiln wash go bad with age? Could it be increased humidity levels? Could it be gremlins?

What temperature do you slump at?

Jim

My slumping temperature is 1310 degrees. A little unique as the plate is only single thickness glass. It took many trials to find a temperature that would slump the glass into the mold but maintain the 1/8" thickness.

Unless you are slumping float 710C is a bit hot

I slump single layer dishes (even small ones) at 1150° with a 45 minute hold.

Won't the contraction of a single layer possibly cause the glass to gather up the KW as it (the glass) contracts?

Begs the question as to why it started doing that now though, after doing many other pieces successfully in the past. What's changed? Glass still the same? Schedule still the same? Any changes to kiln? Change the way you're holding your mouth?

how often are you re-applying the kiln wash?

Somewhere I read about a sticking problem with a primer. While in its dry powder form they shook it up to redistribute the particles. Something about the settling of the powder caused the sticking problem. Whether that was the only thing they changed; I can't remember. Wouldn't hurt to mix it up especially if it has been sitting around awhile.
Mary

Kilnwash gets old and tired, especially after multiple hot firings, and even more if you're firing pale opal glasses. It may just need to be scraped and rewashed--can you do that on Ceramaguard? (I've not used it, so I don't know).

I've also had a lot of sticking/flaking/failing problems with kilnwash I'd mixed up a couple of months before, and then decided to use up, or when I've diluted old kilnwash mix that's thickened up a bit. Nowadays I mix up as much as I'll need in a day or two, then discard the leftovers when I'm done. I've also had far better luck when I've made it relatively thin and applied multiple coats, letting it dry between coats, than when I've made it thicker so I could get away with only a couple of coats.

Slumping usually isn't a problem with thin glass, unless you give it so much heatwork that it starts to flow. Slower firing at lower temperatures will give you more control over the glass and also reduce the chance that it'll stick to the kilnwash.

I’m thinking Morganica has it right. “Kilnwash gets old and tired”
I didn’t know you needed to periodically replace kiln wash. This mold has been sitting on my shelf since I used it a year ago. I just pulled it off the shelf and put it in the kiln. Brad makes a similar statement in his book ‘Contemporary Fused Glass’ “Avoid firing on a damp kiln shelf ………..or firing on used kiln wash that should have been scraped and reapplied.”
Also Brad’s comment on “a damp kiln shelf” makes me wonder if humidity played a role in my problem. We have a very high humidity level in North Carolina and at this time of the year (fall) I have the air conditioning off so the humidity level in my shop is abnormally high.
Now I need to see if I can remove the old kiln wash from the Ceramaguard mold. The material is too soft to be scraped but maybe it can be sanded. Any suggestions?
I also need to see if I can lower the temperature with a longer hold time and still get the results I want.
Thanks to all for your input.
Mark

the humidity is not an issue. the water is all gone before the glass starts to soften. i use ceramiguard as molds alot and have no more sticking problems with them than with any other mold material. the main reason not to fire on wet kiln wash in my opinion is that the glass can form a perfect seal with it when soft and then trapped air can cause bubbles. minerals in the water used to mix kw can also react with glass. i always use distilled water to mix kw or anything else that contacts glass. rosanna

if the kilnwash is still smooth, just apply a new coat or two over the existing stuff.

If you use the molds for low-temp slumping, well-applied kilnwash should last a long time. I've had molds that haven't had a reapplication of kilnwash in 8 or 9 years and are still going strong.

I have other molds that need new kilnwash every year or so. Invariably, those are the ones that I go over 1300-1400F with, and/or that have frit or sharp pieces of glass pushed up against the wash regularly. I literally wear out the kilnwash--that's what I referred to when I meant "old and tired." And I've also had shorter-lived kilnwash applications when I dried the mold in the kiln on low heat (200F or less)--it just seems to flake and come off much faster than the ones I cured in a warm place for 2-3 days. Not sure why.

Is it possible your kiln temperature control is losing reliability? (running hotter than you think)

Because the glass plate is not "dog boned" I would guess the kiln is not running any hotter than it was previously.
Anyway, the mold has been cleaned up, new kiln wash is being applied and we'll see if the problem goes away.

I always use thin fire shelf paper in addition to the kiln wash. It leaves a nice smooth surface on the back of the glass, prevents the sticking, and allows the glass to slide and slump more consistently. You can also try sprinkling alumina hydrate powder although you may miss the sides on a steep mold.

SuzannePisano wrote:I always use thin fire shelf paper in addition to the kiln wash. It leaves a nice smooth surface on the back of the glass, prevents the sticking, and allows the glass to slide and slump more consistently. You can also try sprinkling alumina hydrate powder although you may miss the sides on a steep mold.

Does thin fire shelf paper work for slumping in a dish mold? Seems like the paper would bunch up, fold over and cause problems?