kswan

If you heat air from 0°C to 1000°C it expands to less than five times its size. [It goes from 273°K to 1273°K and expands to 1273/273 its size.]

Water expands by about 1700 times when it turns into steam (over a much shorter temperature range). at's a lot.

You can also use your own body as an analogy about feeling the weight of gravity. If you hold heavy weights, you can hold them longer straight up as opposed to straight out. The same happens with clay, it needs support to not sag. An upward curve at an edge instead of horizontal will help. If your edge is thick, it is heavier and will sag down too. As Kelly said, thick in the support area like the base is important, and then thinner edges where possible.

That's called dunting. You'll be able to tell if it happened during the heating cycle or the cooling cycle by looking that edges of the cracks. If they are sharp, it happened in the cooling. If they are melted and rounded, it happened during the heating. Either way, the process needs to be slower. 
You kind of have a perfect storm for that to happen: glaze refiring, one side glaze application, maybe a too thick glaze layer. You may not have the best glaze fit for that clay too.
If you're set on using that glaze/clay combination, I'd add a foot ring to your plate to glaze both side and apply a thinner glaze layer. If you know there's a spot in your kiln that reaches the right temperature, make sure it goes there. 

Thank you  @kswan, I have never heard of dunting before, that is really helpful.

You can also use your own body as an analogy about feeling the weight of gravity. If you hold heavy weights, you can hold them longer straight up as opposed to straight out. The same happens with clay, it needs support to not sag. An upward curve at an edge instead of horizontal will help. If your edge is thick, it is heavier and will sag down too. As Kelly said, thick in the support area like the base is important, and then thinner edges where possible.

Chip and Dale!
We have a heat pump. I didn't know anything about them until we bought our house 12 years ago and it had one. The output doesn't get as warm as a gas furnace. I used to sit by a vent and read as a kid when we had a gas furnace. I called it the fireplace. Good times. 
I made the mistake of storing bird seed in the garage. Then I kept finding mice putting stashes of bird seed all around in the basement where my studio is. I'd open a drawer, look in a box, uncover a corner and there would be millet and sunflower seeds. I even found stashes in the kitchen around the stove. They found a little hole to get in there and were saving seed in the casserole dishes. Meanwhile the cat is just sleeping away oblivious to the work he needs to do. Sigh.

That's called dunting. You'll be able to tell if it happened during the heating cycle or the cooling cycle by looking that edges of the cracks. If they are sharp, it happened in the cooling. If they are melted and rounded, it happened during the heating. Either way, the process needs to be slower. 
You kind of have a perfect storm for that to happen: glaze refiring, one side glaze application, maybe a too thick glaze layer. You may not have the best glaze fit for that clay too.
If you're set on using that glaze/clay combination, I'd add a foot ring to your plate to glaze both side and apply a thinner glaze layer. If you know there's a spot in your kiln that reaches the right temperature, make sure it goes there. 

You can also use your own body as an analogy about feeling the weight of gravity. If you hold heavy weights, you can hold them longer straight up as opposed to straight out. The same happens with clay, it needs support to not sag. An upward curve at an edge instead of horizontal will help. If your edge is thick, it is heavier and will sag down too. As Kelly said, thick in the support area like the base is important, and then thinner edges where possible.

That's called dunting. You'll be able to tell if it happened during the heating cycle or the cooling cycle by looking that edges of the cracks. If they are sharp, it happened in the cooling. If they are melted and rounded, it happened during the heating. Either way, the process needs to be slower. 
You kind of have a perfect storm for that to happen: glaze refiring, one side glaze application, maybe a too thick glaze layer. You may not have the best glaze fit for that clay too.
If you're set on using that glaze/clay combination, I'd add a foot ring to your plate to glaze both side and apply a thinner glaze layer. If you know there's a spot in your kiln that reaches the right temperature, make sure it goes there. 

@Gazalposting a picture might help with your description problem leading to a more specific solution.
 
best,
Pres

You can also use the same material people use for stencils that are cut with a die cutting machine.  Oracel 651 permanent  vinyl is one brand, or the Cricut permanent vinyl.  There are others.  It's not permanent on bisqueware.  It peels off but gives you a crisp area.  After you glaze or underglaze, then yes, wax that area .  Once it is dry peel off the vinyl.  The wax burns off and gives you a nice crisp area.  Many people report success with contact paper as well. 

As clay reaches its maturation temperature (becomes vitreous, changes from bisque ware to stoneware/porcelain ) it becomes “pyroplastic.” Soft from the heat, in other words. I feel that has a significant bearing on your problem.
People solve this in various ways. One is to adjust the form to compensate for the sag, another is to add structure in the design to support wide flat areas. Still another is to calculate thin and thick areas for greater support. One very direct way would be to use a clay that matures a little higher than you’re firing it, or, better still, a clay known to keep its structure at maturity. I suppose even some kind of supports could be fashioned to make it work. 
Porcelain and highly vitrified clays are notoriously pyroplastic. The weight of a handle will pull a mug into an oval shape. 
Some peoples favorite ceramic word is eutectic. Mine is pyroplastic. 

Something else that may help you is to get the surface smooth after you trim. It looks like you have grog raised above the rest of the surface, maybe from wiping with a sponge. Trimming can also drag grog across the clay and leave grooves and holes. After you trim, smooth your piece with a flexible rib (metal or silicone) to even out the trimming lines and push grog back down into the clay. 

Chip and Dale!
We have a heat pump. I didn't know anything about them until we bought our house 12 years ago and it had one. The output doesn't get as warm as a gas furnace. I used to sit by a vent and read as a kid when we had a gas furnace. I called it the fireplace. Good times. 
I made the mistake of storing bird seed in the garage. Then I kept finding mice putting stashes of bird seed all around in the basement where my studio is. I'd open a drawer, look in a box, uncover a corner and there would be millet and sunflower seeds. I even found stashes in the kitchen around the stove. They found a little hole to get in there and were saving seed in the casserole dishes. Meanwhile the cat is just sleeping away oblivious to the work he needs to do. Sigh.

Happy new year to all!
Are you all using the same glaze inside and out when doing the magnet method? My liner glaze is different, and I've always hated when I dip something upside down and then it burps on pulling out of the glaze bucket. The burp slops glaze onto my liner glaze and I have to carefully get it off without also removing the liner. I tried practicing methods to reduce that effect, but I gave up. I know you can wax the lip, but I don't want to deal with that. Now I put my hand (or both if needed) inside the piece and lower into the bucket. It took practice to not let glaze flow inside, but it's better results for me since I can see what's happening. If there's a bare spot where the two glazes didn't overlap, I just dot glaze on with a brush, but most of the time it's fully glazed.
I could see the magnet method being really helpful for a wide flared bowl where I can't get good friction on the insides to hold it while dipping. In those cases, I hold it by the top of the rim to dip and then brush glaze near the rim afterward. That's not such a great method, though, I'd say. It doesn't look smooth, and it's time consuming. 
My current goal is to find a permanent dipping container for my main glaze. I would love to have a rectangular container wide and deep enough for platters to go in dipped straight down. Not too big, maybe 6-8 gallons, sturdy and with a lid. I should probably do a separate post on that. I'm sure people know about resources I haven't heard of! I want to save strain on my body not to lift my whole bucket and pour it into a pan for glazing plates and platters.

Chip and Dale!
We have a heat pump. I didn't know anything about them until we bought our house 12 years ago and it had one. The output doesn't get as warm as a gas furnace. I used to sit by a vent and read as a kid when we had a gas furnace. I called it the fireplace. Good times. 
I made the mistake of storing bird seed in the garage. Then I kept finding mice putting stashes of bird seed all around in the basement where my studio is. I'd open a drawer, look in a box, uncover a corner and there would be millet and sunflower seeds. I even found stashes in the kitchen around the stove. They found a little hole to get in there and were saving seed in the casserole dishes. Meanwhile the cat is just sleeping away oblivious to the work he needs to do. Sigh.

Chip and Dale!
We have a heat pump. I didn't know anything about them until we bought our house 12 years ago and it had one. The output doesn't get as warm as a gas furnace. I used to sit by a vent and read as a kid when we had a gas furnace. I called it the fireplace. Good times. 
I made the mistake of storing bird seed in the garage. Then I kept finding mice putting stashes of bird seed all around in the basement where my studio is. I'd open a drawer, look in a box, uncover a corner and there would be millet and sunflower seeds. I even found stashes in the kitchen around the stove. They found a little hole to get in there and were saving seed in the casserole dishes. Meanwhile the cat is just sleeping away oblivious to the work he needs to do. Sigh.

Chip and Dale!
We have a heat pump. I didn't know anything about them until we bought our house 12 years ago and it had one. The output doesn't get as warm as a gas furnace. I used to sit by a vent and read as a kid when we had a gas furnace. I called it the fireplace. Good times. 
I made the mistake of storing bird seed in the garage. Then I kept finding mice putting stashes of bird seed all around in the basement where my studio is. I'd open a drawer, look in a box, uncover a corner and there would be millet and sunflower seeds. I even found stashes in the kitchen around the stove. They found a little hole to get in there and were saving seed in the casserole dishes. Meanwhile the cat is just sleeping away oblivious to the work he needs to do. Sigh.

@Hulk, I can only attest to the complexities of starting a vehicle at -30° F. Sometimes batteries do not perform as the label leads you to believe. Usually the rest of the car isn’t cooperating either!
Also have to mention, no visible monks, but some squirrelly little hoarders here. Any pot left in the open will be mysteriously filled with spruce cones by late fall. And there are the not so secret critics. Magpies that disassemble any areas packed with ceramic fiber. They like to strew it all about, not happy with the neatness of my work. 

We had a xmas pair of chipmunks around the kiln for a spell this year.They liked the warmth of the chimney area where I have lots of  English dry pressed shelves stacked up from long ago
 

Something else that may help you is to get the surface smooth after you trim. It looks like you have grog raised above the rest of the surface, maybe from wiping with a sponge. Trimming can also drag grog across the clay and leave grooves and holes. After you trim, smooth your piece with a flexible rib (metal or silicone) to even out the trimming lines and push grog back down into the clay. 

Something else that may help you is to get the surface smooth after you trim. It looks like you have grog raised above the rest of the surface, maybe from wiping with a sponge. Trimming can also drag grog across the clay and leave grooves and holes. After you trim, smooth your piece with a flexible rib (metal or silicone) to even out the trimming lines and push grog back down into the clay. 

Something else that may help you is to get the surface smooth after you trim. It looks like you have grog raised above the rest of the surface, maybe from wiping with a sponge. Trimming can also drag grog across the clay and leave grooves and holes. After you trim, smooth your piece with a flexible rib (metal or silicone) to even out the trimming lines and push grog back down into the clay. 

Something else that may help you is to get the surface smooth after you trim. It looks like you have grog raised above the rest of the surface, maybe from wiping with a sponge. Trimming can also drag grog across the clay and leave grooves and holes. After you trim, smooth your piece with a flexible rib (metal or silicone) to even out the trimming lines and push grog back down into the clay. 

Something else that may help you is to get the surface smooth after you trim. It looks like you have grog raised above the rest of the surface, maybe from wiping with a sponge. Trimming can also drag grog across the clay and leave grooves and holes. After you trim, smooth your piece with a flexible rib (metal or silicone) to even out the trimming lines and push grog back down into the clay.