Plate issues

[Alice C]

Hi all,

I'm brand new here and also brand new to making plates! So far I'm having (predictably) little success, and wondered if anyone can help me troubleshoot. 

I'm trying to use a hump mould to make a large plate with a foot ring. The first attempt warped during firing - sinking slightly over the foot ring, which I put down to not being careful enough with the slab when lifting it onto the mould. Other than that it seemed like it would have worked fine.

The second attempt was slightly thinner, slightly larger, and not only did it warp in the kiln - it basically melted onto the kiln shelf and cracked right through the middle.

From research it seems like any of the following could be the problem...

- slab too thin

- fired too hot (the clay recommends a top temp of 1240C, but we fired at 1250 ... based on glaze experiments we're pretty sure our kiln fires a few degrees cooler than it reads, plus we usually fire this clay to this temp with absolutely no problems, including with thin-walled bowls)

- plate too large - it is pretty much the size of the kiln shelf, give or take a little - could it be that the edges are too close to the kiln walls, causing them to heat a lot faster than the rest causing major warping and the crack? If this is an issue, could it be alleviated by carving grooves / holes in the footring to let heat circulate during firing?

.... any thoughts on whether any of these may be the more likely culprit? It's hard to tell whether the crack was caused by surface tension as the plate warped or by differential heating. I can easily modify the slab thickness and firing temperature, but reluctant to reduce the size of the plate as it's for a commission.

Thank you!

[Sorcery]

I would try to fit it in the kiln so the rim is between 2 elements and not right on front of one. 

Sounds over fired, just yesterday, I became convinced it's overfiring, not building technique, that has been leading to minor cracks. Also true, the overfiring exposes the weakest parts of the build. Also true, the weakest part of the build, is sometimes the strongest.

The weakest part of the build is the strongest?

If your foot ring is extruded, for instance, it will have an entirely different set of compression and molecule direction, which can make it so strong, it doesn't allow the plate center to shrink. 

So the weak part of the build, is the strong foot.

By looking at things completely backward, sometimes the answer becomes straight forward.

I am also absolutely convinced that this shallow definition of "compression" we seem to throw around is quite disabling, especially for handbuilders. We should think more deeply into the direction in which we are actually orientating the molecules.

Even unwedged clay is going to end up running into pretty much the same direction on the wheel, it's going to homogenize, just with the action of throwing. So a trimmed foot ring is going to be perfectly in conjunction with it's plate.

If you cut your slab built foot ring from the same slab your plate is from, you are safer to place it, in the same orientation, on to the plate.

If you turn that peice of slab sideways, and attach it, they are going to be orientated to shrink at different rates because of how they connect. I found that to be the first place overfiring exposes mistakes. If it's a foot, the added drag across the shelf is going to expose that weakness for certain.

I reckon, the larger the foot ring, the more area this problem has within which to occur. So a smaller foot ring is safer.

If your shelf is recently flipped, and the plate is on the top of the "hill", it will have to climb up as it shrinks, so I'd use a shelf that is sagging, and use it in its sagging position, so the foot is climbing down.

In this deeper understanding of "compression", as moving molecules into certain directions, we can see that, if we "compress" or rib our plate from foot down to rim in one place, we must do so equally in all places. And if we go left to right, we must go left to right around the entire plate, then also back right to left.

Then we must remember by ribbing the inside, we can counter the outsides movement, or increase it, depending on if we first go up, down left or right.

I call this "counter-memory", because I also think "memory" is a ridiculous concept, since my pieces have yet to come out of the kiln, the same way they came out of the pugger!

Which is to say, "memory", is easily adjusted, if we stop giving human characteristics to our clay, and treat it for what it is!

So I don't believe your moving of the plate was it's downfall. 

If it was, you can use "counter-memory" measures to move molecules into unwarping positions again.

"If I swipe my finger just once down greenware, that place will shrink differently".

Sorce

 

 

 

 

[Min]

 

5 hours ago, Alice C said:

The first attempt warped during firing - sinking slightly over the foot ring, which I put down to not being careful enough with the slab when lifting it onto the mould. Other than that it seemed like it would have worked fine.

How wide a span across the base between the foot ring and is it a flat base or rounded? It could have slumped if unsupported for too wide an area. 

5 hours ago, Alice C said:

The second attempt was slightly thinner, slightly larger, and not only did it warp in the kiln - it basically melted onto the kiln shelf and cracked right through the middle.

Slumping could be from too wide a span again plus the thinner base. Is the crack sharp on the edges or smoothed over?

5 hours ago, Alice C said:

fired too hot (the clay recommends a top temp of 1240C, but we fired at 1250 ... based on glaze experiments we're pretty sure our kiln fires a few degrees cooler than it reads, plus we usually fire this clay to this temp with absolutely no problems, including with thin-walled bowls)

Have you used cones to verify what you reach? Going by temperature alone isn't measuring heatwork. Physics is different with bowls than with a wide footed platter although rims of thinly thrown bowls can definitely deform if one side is close to an element, porcelain especially.

5 hours ago, Alice C said:

plate too large - it is pretty much the size of the kiln shelf, give or take a little - could it be that the edges are too close to the kiln walls, causing them to heat a lot faster than the rest causing major warping and the crack? If this is an issue, could it be alleviated by carving grooves / holes in the footring to let heat circulate during firing?

If you can build a wall around the outside of the platter with square or triangular posts lying on their sides or even short small pots it will help avoid the rims firing hotter than the middle of the kiln shelf. Also,  don't fire platters on the bottom shelf of the kiln, unless you have a floor element, nor on the very top shelf. 

If you post a picture of the platters it could help with more clues as to the cause.

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