douglas

One other possible cause is too much glaze or using a runny glaze on the inside of the pot. Maybe when you are pouring the glaze out it is taking a while and the inside has a thicker than normal application? If the glaze is too thick at the bottom of the pot, and if the glaze doesn't shrink at the same rate as your clay body during firing, the thick glaze pooled at the foot is creating a stress crack and the sudden temperature change is causing it to fracture along the spot where the thick glaze ends and the clay wall begins.

Plastic bats seem to cause more cracking than plaster or masonite bats. If I have to use plastic, I wire it off immediately after throwing, and flip it onto it's rim when the rim is strong enough to support itself. If the base is too soft to flip and will sag, then I transfer the plate to a board with newspaper so the base can move easier as it dries and contracts. With plaster or masonite bats, I think they wick away enough moisture from the bottom of the plate to avoid the stress from uneven drying on the rim and the foot, and I don't have to be as careful.

If nothing comes out when you flip the mold over, then you might not be filling the mold up completely. If the walls are hollow that would explain why you are getting two halves instead of a solid bowl. You need to screen or thoroughly mix the slip to remove the lumps. You may also need more water in your slip if it is not able to run all the way into the mold before it dries at the base, blocking you from adding enough slip to fill the mold.

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If you have access to a ball mill, try running the glaze through that before using - that might break up the particle clumps.

Glaze fit means the glaze shrinks at a rate close enough to the clay body shrinkage, that it will not crack or shiver off the clay. Just because your glaze and clay are firing to the same cone, does not mean the glaze will fit the clay. Apologies if you know this, but it seemed like you are equating firing temperatures with whether the glaze should fit in your descriptions. The root of the problem you are experiencing is you are buying off-the-shelf products. Since you don't control the ingredients you don't have control over whether they play well together in the kiln. Learning glaze chemistry is an option, mixing your own, and adjusting to fit your clay body, but that takes most people a lot of time to work out. If you want to be able to just buy glazes and clay, then you should ask your vendors to recommend clay bodies or families of glazes that should work well. If your glaze vendor's response to using earthenware was don't use earthenware, then you either need to follow their advice and choose a different clay body, or keep your clay body and find a different glaze.

Coleman porcelain has warped quite a bit in wood fires for me (cone 10-12 with lots of ash). You might want to load fewer porcelain pieces since your space is limited.

Some general advice is to do a good amount of research before diving in with any ceramic project. The lusters you want to use are toxic to work with. If you use precautions it is not a problem, but if you don't it can lead to health problems. I don't know if luster glazes are safe after firing with prolonged skin contact. It's worth checking out before you make these. Lusters are not glazes. They are metal and binders that melt and adhere to glaze and I am guessing they would not hold up to daily wear and tear on jewelry. If this is just for you and friends that may not be a big deal, but if you sell them you want to test the durability so you don't get angry customers.

My guess is you are letting them dry on plastic or some other smooth surface. The rim might have adhered to the surface (lots of moisture trapped under plastic could make a small amount of slip where the rim touched the surface). When the foot and insides of the platter shrank, the rim stayed put and caused the cracking. One fix for this is to use paper or foam under the pot while it dries so the clay can easily move on the surface as it shrinks.

Use a cheese cutter wire with adjustable roller. Hold your thumb on the roller while you cut or glue it so that it won't roll. Hold the wire against the rim of the pot, and adjust the depth of the wire to the roller to 1/3 or 1/2 the width of the clay wall. Then when you cut the roller will prevent your from cutting too deep (assuming consistent clay wall thickness).

It might be possible to dip the thin delicate bisque in an engobe to build up the clay thickness and re-bisque the piece.

Sounds like your supplier may have sent you a bad mix of glazes. It sounds like they send you a premixed bag, so I would contact them and see if any of their materials may have changed. It does not sound like a user error on your part, but a problem with the glaze ingredients.

I am not a chemist, but from what I have read and observed, I think the risk of copper poisoning from ceramics use is overstated. If you drink a liter a day of water that has up to 1.3 mg in it, and that is considered safe, any copper leaching out of a mug would be far less than this. I doubt a mug would have 1.3 mg of copper in the whole glaze application, let alone a lethal dose. People have been drinking vodka mules out of 100% copper cups, and we have not seen the new stories about these deadly cocktails killing consumers. That is because in order to get copper poisoning in this fashion, you would drink yourself to death long before the copper got you.

You handled this well by moving on, this allowed the learning and creativity to continue. If you took the bait to engage with her pettiness it would have taken away from the other students who were there to learn and create. My advice is refuse to let her in the group again. She has nothing to contribute to your class and if she makes other accusations or complaints, it takes your time away from students that appreciate the efforts you are making to teach them. I am sure you can find a better student to sit in her seat. If she complains or asks why, say "you called me a liar, you didn't apologize, I refuse to teach someone who treats others the way you do."

Another approach to aging the handle is to take a walk outside with some sand paper and an x-acto knife and scratch and sand away some of the under glaze. Use fine sand paper at the end to get rid of any jagged spots. You can still add a wash to this afterwards if it needs it.