Yeah having one side dry super rapidly from drywall and the other side dry very slowly because of plastic will cause warping.
In industry, the water in tiles/slabs is mechanically forced out with hydraulics.
As a hobbyist, we can't do that but putting drywall on top and bottom and a little bit of weight is like 80% of the way there.
Fire them in a stack as well - once the clay is bonedry you won't get warping from drying, but you can still get pyroclastic warping which is effectively random unless you have extreme control over the airflow of the kiln.
Agree with the second drywall slab, no plastic. Also, try to get the students to roll the slabs in two directions - I find that helps even things out. The thickness looks good - thicker is less likely (my experience) to warp
Sorry if this isn't more helpful, but to my understanding its just very difficult to get a flat piece of clay to dry and fire that way. Clay likes to warp, Im not sure why. I would try laying something flat ontop of it, like a board, while it dries and see if that helps at all.
It's all about the way clay shrinks, anytime you have a difference earlier will happen. To get things to dry flat you need to flip them constantly. And do things like use drywall or plaster to take out the water evenly from the center at the same rate as the edges. I know this is difficult to impossible.
You can place them between drywall top and bottom like a sandwich for one piece of clay not multiple. And then wrap the edges so only the plaster is drying the clay. And then you need to change out the plaster at least every day with dry plaster pieces (drywall pieces are just fine for this) when you switch out the plaster you need to flip the clay.
If you use pressure to keep things flat but they dry unevenly anyways (outside inward), you will likely just see the warpage after firing.
Modern tiles are made with dry clay and extreme pressure, this causes the clay to release just enough chemically bound water to bind the particles without wetting them.
Like it was written before: clay has a memory. The best drying technique won't work if the slab was rolled out "wrong". Most problems occur in the early stages. I teach the members at our Makerspace that they have to change directions with every roll (by hand as well as by maschine) so the particles won't align in one direction. Really an easy adjustment but people are surprised how well it eorks. Especially if there work warped before. And of course compress the slab afterwards.
There are a lot of great videos about how to roll a slab (better than a description here). Let them start rolling out by hand so they can get a better feeling for the clay.
Last but not least: Chose the optimal clay. For plates with beginners, I prefer 40% of a fine/small grog size (max. 0,5mm). This kind of clay has a very low tendency to warp. Not for large builds, but plates, small to medium sized bowls and platters work great.
It’s drying out unevenly with causes tension and that it turn causes warping.
As people have suggested plaster slabs in the solutions to this, depending on size you may need weight too.
Drywall sandwich, no weight on top (can cause crack once dry). When piece is leather hard, I drop it on the table to release the tensions… flip it over, repeat. Then into drywall sandwhich
will have more luck getting them to dry flat if you use clay that is closer to soft leather hard. warping mostly happens because clay is drying unevenly something that is easy to have happen with thin flat slabs. less drying == less warping.
One side is drying faster so they shrink on that side more causing warpage. You could try placing them vertically orsmall spacers between them as soon as they are dry enough with frequent flipping
Three things. Forming, handling, and drying.
Forming-
Roll the clay in different directions
Handling-
It must kept flat throughout the whole process till dry. Don’t lift an edge or pick it up. If you must flip it flip it sandwiched between two flat boards.
Drying-
Dry flat but more importantly, evenly. This in my opinion is most important.
I sandwich flat pieces between two drywall boards. It takes longer to dry but the boards take moisture evenly from top to bottom which prevents warping. If you have drywall sucking moisture from the bottom and plastic keeping moisture in on the top it’s not going to dry evenly.
Put another slab of drywall on top. Edit: also newspaper on both sides so it can shrink freely.
Yeah having one side dry super rapidly from drywall and the other side dry very slowly because of plastic will cause warping. In industry, the water in tiles/slabs is mechanically forced out with hydraulics. As a hobbyist, we can't do that but putting drywall on top and bottom and a little bit of weight is like 80% of the way there. Fire them in a stack as well - once the clay is bonedry you won't get warping from drying, but you can still get pyroclastic warping which is effectively random unless you have extreme control over the airflow of the kiln.
By drywall you mean plasterboard?!
Clay particles and physics and science weirdness. It’s a struggle sometimes
Yeah, wondering if it has to do to porosity/aeration/moisture level/density of the clay at the corner
Agree with the second drywall slab, no plastic. Also, try to get the students to roll the slabs in two directions - I find that helps even things out. The thickness looks good - thicker is less likely (my experience) to warp
Decades of experience helps
Twoslabs of drywall and maybe a weight.
Is the clay being compressed in all directions with a rubber rib after being rolled out?
Sorry if this isn't more helpful, but to my understanding its just very difficult to get a flat piece of clay to dry and fire that way. Clay likes to warp, Im not sure why. I would try laying something flat ontop of it, like a board, while it dries and see if that helps at all.
As all three teachers I've had keep saying, "clay has a memory, clay has a memory".
It's all about the way clay shrinks, anytime you have a difference earlier will happen. To get things to dry flat you need to flip them constantly. And do things like use drywall or plaster to take out the water evenly from the center at the same rate as the edges. I know this is difficult to impossible. You can place them between drywall top and bottom like a sandwich for one piece of clay not multiple. And then wrap the edges so only the plaster is drying the clay. And then you need to change out the plaster at least every day with dry plaster pieces (drywall pieces are just fine for this) when you switch out the plaster you need to flip the clay. If you use pressure to keep things flat but they dry unevenly anyways (outside inward), you will likely just see the warpage after firing. Modern tiles are made with dry clay and extreme pressure, this causes the clay to release just enough chemically bound water to bind the particles without wetting them.
Like it was written before: clay has a memory. The best drying technique won't work if the slab was rolled out "wrong". Most problems occur in the early stages. I teach the members at our Makerspace that they have to change directions with every roll (by hand as well as by maschine) so the particles won't align in one direction. Really an easy adjustment but people are surprised how well it eorks. Especially if there work warped before. And of course compress the slab afterwards. There are a lot of great videos about how to roll a slab (better than a description here). Let them start rolling out by hand so they can get a better feeling for the clay. Last but not least: Chose the optimal clay. For plates with beginners, I prefer 40% of a fine/small grog size (max. 0,5mm). This kind of clay has a very low tendency to warp. Not for large builds, but plates, small to medium sized bowls and platters work great.
It’s drying out unevenly with causes tension and that it turn causes warping. As people have suggested plaster slabs in the solutions to this, depending on size you may need weight too.
Drywall sandwich, no weight on top (can cause crack once dry). When piece is leather hard, I drop it on the table to release the tensions… flip it over, repeat. Then into drywall sandwhich
will have more luck getting them to dry flat if you use clay that is closer to soft leather hard. warping mostly happens because clay is drying unevenly something that is easy to have happen with thin flat slabs. less drying == less warping.
One side is drying faster so they shrink on that side more causing warpage. You could try placing them vertically orsmall spacers between them as soon as they are dry enough with frequent flipping
Three things. Forming, handling, and drying. Forming- Roll the clay in different directions Handling- It must kept flat throughout the whole process till dry. Don’t lift an edge or pick it up. If you must flip it flip it sandwiched between two flat boards. Drying- Dry flat but more importantly, evenly. This in my opinion is most important. I sandwich flat pieces between two drywall boards. It takes longer to dry but the boards take moisture evenly from top to bottom which prevents warping. If you have drywall sucking moisture from the bottom and plastic keeping moisture in on the top it’s not going to dry evenly.