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I do a lot of tiles and have always tossed slightly warped tiles before bisque into a recycle bucket (which I never recycle).

I had a couple of shelves of tiles that many had a hump to them due to not drying them the way I normally do (Sheetrock top and bottom for a couple of days) Because I wanted to try some test glazing I didn't toss these and to my surprise many of them (porcelain) came out of bisque perfectly flat and even more completely flattened out during glaze firing. Only a few remained warped and those were almost flat. Now they were not just way off to start but the hump was very noticeable, prob as much as an 1/8" and affected prob 70% of the tile.

This has me scratching my head and re-thinking culling much before bisque. I seldom recycle dry clay anyway. Drives my partner nuts so she makes me put it in buckets but I ignore it when pugging. 

Anyone else experience this with porcelain? It is JG-6 from clay art center (billed as a throwing porcelain) cone 4-6, bisque 05 and fired cone 5 with 20 minute hold in heat work. Makes me want to just fire flatware through before judging.

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Makes sense that porcelain tiles would flatten out in the glaze firing. Think of platter rims that slump and deform if too thin or fired too hot. If your clay and glaze can take it then firing a bit hotter could flatten / slump  even more of them out. That they flattened in the bisque is bringing up something I've experienced but in the opposite way. I once tried a few  ^6 porcelain plates that I bisque fired on edge to ^04 (Glacier Porcelain from Tacoma Clay Art Center). Every one of them had significant warping. It lead me to the conclusion that this particular clay is fluxing at a lower temp than my other ^6 bodies and as such plates and platters etc had to be bisqued flat on the shelves or I probably could get away with bisque firing them standing on edge if I went cooler.

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I work only with porcealin since 1985-lots of it-3 different kinds-the more glassy it is the  more it warps

Porcelain can move in the bisque (I do not see much of that) and much more in a glaze fire-I see more of this. Of couse my Temps are a lot higher than cone 6 but I think its most likely about the same with a lower fire cone 6 porcelain 

The other thought with any tile is the glaze pulls on the one side and can warp any clay body as its tension that is not equal .Pulls up usually.

Your leason is just because the tile warps in the green state it  can lay flat in the glaze fire and quit tossing greem warped tiles. This is old news for me . I see the most warping usually is the glaze  fire..Flat stuff wants to get flat again on a flat shelve( I fire advancers-always flat)

The other thought is your tile were all once flat and the clay has memory and can and will try to be flat again. The one sided glaze is the biggest reason they will not return to flat again.But since porcelain fluxes more than stoneware it can get flat easier than stonewares .

hope this all make some sense

Edited by Mark C.
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Porcelain has a tendency for pyroplasticity.  So if youre firing them flat (not in a tile setter) they'll want to do what gravity tells them to do, which is sink towards the earth.  Pretty handy for tiles, not so handy for everything else!  I have some slipcast porcelain forms that I've had to design around because of this pyroplasticity.  More evident in "translucent" porcelains I think, which is what my slip is.

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On 5/8/2020 at 10:56 AM, Mark C. said:

hope this all make some sense

It does! Thanks everyone. Great input. I do fire everything flat both bisque and glaze. In bisque though I stack them. My tiles are pressed and my routine so far has been to dry them for a couple of days between sheet rock and then a couple on solid ply shelf and then fire. Haven't had a lot of warping but those two racks of tiles referenced in this post were just left on a solid ply shelf for a few days and fired and they had that hump but they had been flat at one time so there is that. Been debating getting some wire racks for after the sheetrock but since mostly not many warping issues haven't done that and may not. I think it works ok because the sheetrock drying is pretty even and they are mostly dry after a few days. 

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