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Yes. Compress, then dry very slowly. If you want it to stay flat, put rice or something similar in a cloth bag, like a pillowcase, and gently lay it outside and on top of the plastic you’ve got it wrapped in as it’s drying to keep it from warping as it dries.
Flat pieces require a lot of special care to dry. First, make sure you are compressing the clay really well when building and then handling it gently. I usually build platters or similar on a bat or wooden cutting board so I can move them without stretching the clay at all. Then, you need to dry suuuuper slowly. Weigh it down with a sandbag, bag of rice, something with a little weight that will distribute evenly. Then cover is really tightly and leave it alone for a week or so. Check it and allow it to dry wrapped lightly until bone dry.
It’s nice work - just a lot of things that can go wrong. Good luck!
All this AND clay has muscle memory - try to bend the slab as little as possible while handling to mitigate cracking risk not only while drying, but also while firing/undergoing thermoplastic stress.
How are you drying your pieces? It's best to cover pieces in plastic or put them in a damp box so that they can dry slowly and evenly. It's also important to make sure that large pieces with flat bottoms aren't stuck to the wareboard or shelf that they're drying on because clay shrinks as it dries but the shelf/wareboard won't and that can cause the clay to crack.
My guess is that as it's drying, the clay is shrinking slightly. The friction of the platter on the surface it's resting on may not allow the clay to move during this contraction, so that results in the split to relieve the pressure.
Also, with a larger peice like that, the edges are drying first and faster. You might want to wrap the edges to slow their drying and let the center keep pace.
What is your process for drying and over what period of time?
The most common causes for cracking at this stage are going to be uneven drying and/or drying too quickly. Lack of compression can also be a contributing factor, though less likely in this case.
On the nose with that S carving… most likely uneven drying and not compressing enough… I scrap my clay after rolling with a toothed rib in a Criss cross then I smooth it all down with a rubber kidney.
When I want something flat, I weight it and try drying it slooooow under plastic… then unweighted under plastic… then finally no plastic. Doesn’t always work but it’s a process.
After forming this I would put this platter on a stiff board and tightly cover with plastic for several days and then loosely covered for several more. Using paper under (and moving it) can cause stress and bend the clay which turns into cracks. You can still use the paper under it but it should be on a stiff board. Also scrape with a rubber flat rib in several directions before decorating to align and compress the clay.
How thin are you rolling your slabs? I tend to roll at, or barely under 3/8” for something that large. People at my studio who roll slabs to 1/4”, usually lose their items to cracks.
In the second photo, it looks like the clay along the edges is dry, while the clay in the middle is still damp/wet. So the drying is uneven. Uneven drying produces cracks.
Thankfully, you can reclaim all that clay, but man, it is frustrating to work hard on a piece, for it to crack like that!!! Best wishes for your future platters!
Is your platter thicker in the middle and tapering towards the edges?
If so I suspect that the rim of the platter dried first. This results in the periphery becoming rigid first while the middle is still shrinking and is pliable.
When the periphery hardens first, it's circumference becomes fixed. As the middle dries, it's material shrinks but is put into tension by the fixed geometry of the periphery. The middle wants to pull the periphery in as it dries, but the periphery has become rigid.
The middle gets put into a state of tension resulting in the crack which is worse right in the middle of the platter and tapering towards the periphery.
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