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Just double checking, you used part calcined and part not calcined epk right? A few drops of epsom salts solution won't have drastically changed the SG. of 1.43 Try it out on some test tiles and see if they have the cracking issue. If there are cracks in the raw dried glaze there will likely be crawling in the glaze fire. I'ld do what Peter suggested and try just a small amount of the glaze to test deflocculating with before doing the batch, or just put this glaze aside for now and start over.

I think this is one of those things that is partly dependant on how we learned to mix and apply glazes. I can't remember being taught to measure SG but had instructors to mix and demo the glazes so I learned through their experience. If someone hasn't had the guidance of more experienced potters I think that measuring the SG is a good way to dial in a glaze to a correct water to glaze materials ratio. I do measure SG when starting out with a new glaze, often because I don't bother to wait for it to slake down overnight so going by appearance alone can be deceiving.

Always!

I think planters would be okay but they won't be super durable as the blended clay won't be mature at cone 6. If you use a plastic pot and saucer inside the planters you wouldn't have to worry about moisture absorption leaving stains on the clay from soil / fertilizer. I wouldn't leave them outdoors during times of freezing weather though. Just put a hole in them so nobody uses them for cups down the road. Welcome to the forum.

Whatever works for you and your methods is the right way to do it. Spit, magic water alone or mixed with slip, magic water / slip / paper fibres, water, vinegar, casting slip... doesn't matter as long as it works.

@Malcolm, everyone has covered what I think is going on with your glaze already so I won't rehash what has been said about glaze application. What I would suggest is to start measuring the specific gravity of your glazes. Doing this will rule out one of the variables you have going on (ie glaze density / thickness). You need an accurate scale plus either a large syringe or a beaker shaped cup or a graduated cylinder. Basically what you are doing is comparing the weight of the wet glaze to that of water. John Britt video below showing how to do this, relevant part starts at the 3 minute mark. Another link here about specific gravity of glazes. (I find the large syringe method the quickest) Once you have the glaze working as it should then each time you go to use the glaze do a quick check of the specific gravity. You might find that over time the specific gravity increases as water is drawn off into the bisque or through evaporation. If you are using a dry glaze then checking you haven't added too much water is a good idea also. I'ld suggest if this is the case then mix the dry glaze with less water than the commercial glaze instructions give, measure the specific gravity and then top up the water only if necessary. You might find you need less water than what is called for. There is no one magic number for what specific gravity you need, each glaze type is going to be different dependant on glaze materials used. When you have the glaze working as you want it to then note the specific gravity for that glaze on the bucket and in your notes. I'ld also suggest taking some test tiles made from the same clay which are approx the same thickness as your pots and dipping once all over the tile, then half the tile with a second dip then dip one corner of the tile with a third layer of glaze. Wait until the wet shine is off the glaze before doing subsequent dips. Make 2 sets of tiles for each glaze, with one tile wait until the glaze is dry then scratch through the glaze but don't fire it. You want to be able to see how thick the dry glaze is. Fire the second test tile and compare results of raw unfired glaze thickness to fired tile. I'ld also do some small test cylinders, again make them approx the same wall thickness as your pots. Glaze the inside on one followed by the outside as soon as the liner glaze is dry to the touch. For the other cylinder glaze the inside then leave it overnight to dry then glaze the outside, fire and compare the results.

I don't think that would be necessary as bisque can withstand greater temperature variances far better than a matured claybody. My thought is it probably happened in the glaze cool down as the edges are sharp, if it was a crack that was there prior to glazing the glaze edge would be softened.

Hi and welcome to the forum. I wouldn't add feet to the wedging board. When you wedge there is a lot of force being pushed downwards, I think you run the risk of cracking it. Is the idea of the feet to stop the wedging board from moving around on the table?

I think my schedule for large or re-fired large pieces should be okay to try with your sinks. I'ld try just one to start with, put it in the middle of a kiln shelf in the middle of the kiln with shelves with pots both below and above it. - after the peak (drop and hold for your schedule) then 9999 to 1150F / no hold - 50 degrees hr / to 950 / no hold - 9999 degrees / to 480 / no hold -50 degrees hr / to 380 / kiln off I don't know how slow your kiln cools on it's own, if it cools slowly in the 480 - 380F range you might not need that second slow down in cooling to be scheduled in. If it's a thin walled kiln then schedule it in.

If you run your fingernail over the crack can you feel where the glaze is cracked and is it a sharp edge or rounded over? I think the cracks are from cooling too fast through the 1150 - 950 F range. I don't think the 100F drop and hold has anything to do with the cracks.

@AlachuaArtist, are you trying to join greenware tiles together that are quite dry or are the components on the tile what you need to join to the slab base? Hard to tell in your image if the additions sitting on top of the base or are attached. Joining fairly dry pieces together can be done with paperclay slip.

@Dragonfly Pottery, thanks for posting the pictures, always helpful. I think because of the fairly straight up and down wall of the espresso cups they have less of a chance of warping than the mug in the 2nd image. Also, if you can avoid that thickening of the handle to reduce the mass that should help. I'ld try a couple mugs with a lower handle placement also, having the weight close to the top is going to exacerbate the handle pulling the rim out of round. Thicker slabs would help too. Are you re-rounding the rims after attaching the handles? A bisqued former, (tapered like the top of an acorn) or a plastic cup helps to re round rims if necessary.

How are you rolling out the slabs and forming the mugs and cups? How heavy a handle is going on them? Are they warping oval towards the handle? How are you drying them? Are using witness cones to check for over firing? Could you post a picture or two of them?

@LeeU, I didn't find your posts regarding safety while selling pots to be controversial. We have (many) threads on the hazards of breathing in silica dust etc, firing of manganese containing clay and glaze, if contamination with the covid virus is possible with wet clay and so forth. How to go about selling pots in a safe manner follows along those threads on keeping people safe. It's a worthwhile discussion to have.

If you will be taking classes then working the same side of the wheel as the demo is being done it makes it easier for both you and the instructor. In Japan (and I think Korea) wheels go clockwise otherwise its counterclockwise. Doesn't matter if you are left or right handed since you use both while throwing.