Clay texture plate/ kiln carving mold

[haleybach]

So today when I was buying clay to make myself some simple texture plates/molds the guy at the store told me he does not prefire the clay tiles, just dries them out then fires them for the first time with the glass on top.

I didn't think to question him more.

Now I'm wondering if I misunderstood him. I always thought I needed to fire the clay (to a temp higher than I needed to fire the glass) first. Although thinking about it, I'm not sure why for a flat tile (there really isn't the need for strength you would have if you were making a slumping mold. Right?)

Any reason not to just test this and find out? I have plenty of clay and I can make extra texture tiles pretty easily. However I'm new enough to playing with glass to not know where the danger lies. I don't mind failing, that is really how I learn best, but I don't want to do any real damage to the kiln or create a mess that is a total pain to clean up.

I love texture in glass and am always looking for ways to get unique textures.

[Laurie Spray]

The main issue will be that there can be absolutely no moisture in your clay slab. This time of year....when there is more moisture in the air......you need to dry clay longer. I put all of our clay pieces on a hot kiln over night while glass is firing. Sometimes for 2 nights. We do bisque fire everthing. If you only fire your clay to fusing temps they will never be as strong and as long lasting. That being said......i think that if your clay slab is totally dry you could probably get away with spraying some kiln wash on it and firing it with glass.
If you do it....let us know!

[Morganica]

Three problems with firing when the glass is on top:

--You'd either need to boil the glass by taking the clay up to bisque firing levels, or fire a second time to truly bisque the mold.
--The clay needs to completely dry warp-free and without cracks, which is hard to do when you're firing with glass on top--it will trap moisture in the spaces.
--Most important (and probably renders the first two moot): Bisqued clay will stick to fired glass unless you coat it with release. Dry clay will start to rehydrate if you apply kilnwash, which will promote cracking and the kilnwash may/may not stick. Boron nitride may not hold up to bisque temperatures.

[rosanna gusler]

i have fired on green clay with no wash and no sticking. rosanna

[Morganica]

i have fired on green clay with no wash and no sticking. rosanna

Really? It didn't work for me. Which clay were you using?

[rosanna gusler]

white paper clay low fire. rosanna

[Morganica]

Hmmm. Wonder if the paper clay has anything to do with it? I usually use a grogged earthen clay, cone 6, or a porcelain. I'll have to give it a shot. Thanks.

[rosanna gusler]

grog is already vitrified. might be part of the sticking? rosanna

[Peter Angel]

I prefired white eartheware paper clay to 820C.

I then added lead based glass to the prefired paper clay - with no kiln wash - and fired to 800C.

The result was a very clear casting with no sticking.

This was Rosanna's idea. There is an old post about it. It's got something to do with the paper in the paper clay.

Peter

[haleybach]

Paper Clay is great stuff. I need to play with some. I know a potter told me he uses it to repair his work sometimes because it doesn't shrink, or rather you can control shrinkage by fiber content, both % and length/type.

What schedule did you use with the Paper Clay?

The guy at the pottery shop said he fired glass over a slab that was not even dried bone dry (just carefully kept even leather dry), something about the moisture acting like a separator. I really wish I'd gotten his name so I could call and ask him exactly what he did. It was the sample tile that attracted me, lots of clear fine details. Better than anything I've gotten. His clay tile was also perfect.

I will post my experiments but it might be awhile. I need to rearrange my workspace and build a tile saw stand then I need to clear some space to roll out my slabs of clay. I'm thinking maybe he used a really slow ramp that effectively dried the clay out before the glass even softened. Maybe.

[la suisse]

I do my simple oneway molds I think in the same way as Rosanna does. Paper Clay, dry about two days, apply Boron Nitrid, let dry another day, then I fill with BE frits. For me it works very good and I love the result.

[haleybach]

Tested with earthenware, no grog. Longhorn White.

Dried to bone dry, glass placed on top with no seporator, just glass on bone dry clay, fired to 1350 with no hold.
The glass picked up the shape/texture and there was no sticking at all.

[Bert Weiss]

Interesting thread. My guess is that a high alumina content clay is used where there is no sticking. Grog is ground up firebrick, which we know is a nasty sticker. The grog adds strength and provides a pathway out for moisture. It does this in a refractory plaster mold just like it does in clay. If the grog is sufficiently coated with alumina the sticking can be mitigated.