It's not handbuilt from a slump/hump mold but from thrown and rolled pieces if that would work for you? I throw the wall with a small flange on the inside bottom edge and a rolled rim to help grip the pot when taken from the oven.These are quick to make up and not much trimming.
Throw a baseless wall with a flange on the inside of about 1/2". Form the rectangular shape of the wall while still quite soft. Throw or roll out a base and let it stiffen up slightly.
Foam batt on top of the wall piece, and flip upside down, I do this as soon as the wall is dry enough to support itself. Put the slab on and mark where the join will be. Flip back over so bottom of wall is again on top, remove slab, score and slip the wall and base.
Flip right side up again and secure and smooth flange on inside edge, trim outside edge and compress with a pony roller or rib along the outer edge plus where the wall and base meet.
Use low expansion clay and glaze and avoid sharp corners, round the inside edge as much as possible. The one in the picture is years old, it has stood up well with no cracking.
Clay and Glazes for the Potter by Daniel Rhodes, The Potters Dictionary of Materials and Techniques by Frank Hammer. The Val Cushing Handbook is good also but it might be hard to find. I'm sure John B knows of many more but those are my go to ones. You could try the Digitalfire site, I think you can access some of the info without buying level 2 access. Try this link, http://digitalfire.c...rial/index.html it should take you to the materials page.
It will be much easier for you if you have a really good knowledge base to work from when making a body from scratch. Otherwise it would be kind of like trying to bake a cake without knowing what the ingredients do and not having a recipe.
I would also suggest that you work only on the body first, and run it through a series of tests, slumping, porosity, plasticity etc before you start working on the glaze.
Pictures of the mugs. Also two pictures of a fragment of another "cracked" mug, showing the thickness of the inside and outside glaze. Looks like the inside glaze is thicker, but it is also much more stable. Three of the four outside glazes are really runny, which is obviously not helping.
Because I'm working with very small numbers of items and glaze, I'm pouring the inside glaze, and brushing the outside. The red in particular is a real **** to apply, but it's getting better with the addition of Epsom Salts.
What's weird is that two of the mugs have almost identical shaped cracks - one was hand-built, the other slip-cast.
I used to like playing with clay, it's the only craft material that you can keep re-using (until fired). With fabrics and other materials, once you've cut something out, you can only make something different if it's smaller. Now I'm getting frustrated.
Thanks for posting pictures, it does make it easier to try and figure out what happened. Your red glaze is gorgeous!
Agree with it looking like a cooling dunt. It's safer to wait until the pots are cool enough that you can pick them up with your bare hands when unloading. The kiln shelves retain a lot of heat, the top of pots might feel just a bit hot but the bases will be much hotter. The pyrometer is measuring the air temp in the kiln, ware and shelves will be hotter.
Ovenware is as much about the shape of the pot as it is about the clay and glaze fit.It can be made with any firing range clay.
Frustration and clay seem to go hand in hand. It's hard to do but if you can avoid getting attached to a piece until it has survived the final fire then it's a bit easier. I can't think of anyone I know that doesn't have issues with one process or another. It seems you can just get one problem solved and another will crop up due to materials change or a slightly different technique etc. I'm guessing that a failure rate of about 10% is not uncommon. This is a very humbling art to work in.
I almost always cut my cobalt with either red iron oxide, rutile or copper carbonate, sometimes more than one. Depends on the base glaze as to which one gives me the color I want. Most of the time I use 0.3%-0.5% cobalt, and about 2% iron, 2-4% rutile, 2-3% copper.
Thanks Neil, I'm guessing that gives richer, more three-dimensional colours?
I've used very few glazes over the last 20 years that looked good with just cobalt. It's too bright, intense, flat. Deeper, more complex colors are achieved by adding other oxides.
I agree with Neil on diluting the harshness of the cobalt with other colorants. Gosu, or asbolite, used in Japan is an impure cobalt. It had manganese and iron in it. Much more subtle blue. Used in brushwork with matcha green tea as the medium for brushwork but the same toned down blue is achieved in glaze or slips.
I have been making my own glaze for a little over a year now. I am no expert, but have had good instruction. It takes hours and hours of testing and firing. Here is a video that I made following the way several potters that I know as well as some noted potters on you tube, Hsin-Chin Lin, and Simon Leach mix their glazes.
In your first video I'm not quite sure about why you are dry mixing the glaze ingredients prior to adding water. You are making a lot of dust unnessasarily, the silica dust will stay airborne for hours. It's not just the dust you can see thats a problem, there will be fine dust floating around for a day or so. Your jiffy mixer and sieving twice will disperse all the ingredients well without dry mixing. (I put the clay component in the bucket first then the heavy settlers after that then wet mix and sieve)
I came across this idea from the Digitalfire site, using the BatMate to hold down plaster batts. It’s sold to be used sandwiched between the wheelhead with pins and a batt to stop wobbling from loose pins or warped batts.
Since I never use batt pins with my homemade batts I have always attached the batts to the wheelhead with a pancake (or usually a ring) of clay. I found that the BatMate works really well at holding down plaster batts and the smaller sizes of wood batts, no pins, no pancakes necessary. I have used it with plaster batts up to 16” diameter with no movement of the batt. When trying it out with wood batts I’m finding that I can only use it with batts up to 8”. Any bigger than that and I push them off center. I don’t have any masonite batts so I’m not sure if it would work with those.
I dip the BatMate in water, squeeze it out so it’s fairly dry and put it on the wheelhead. If I don’t use too much water to throw with then I can throw for a couple hours before squeezing it out. If the BatMate gets too wet it’s harder to remove the batts. It seems like a fairly durable material that it’s made from, I had to pull up on a few times to remove the batts and there is no stretching or tearing of it.
The lid in the picture is on a 10” plaster batt, 14” wheelhead. (please excuse the mess, cleaning isn’t one of my strong points)
Just unloaded a bunch this morning. I agree with throwing them thicker than usual, the ones in my attachment are from 2lb 12 oz of clay. I do the cut-outs straight after trimming the bowl and dry them upside down. Cut-outs done with a scalpel (one of my favorite tools), holes with drill bits. ^6, smooth white clay, shrinks 14= 15%
I use the tapered ones. For sizing what I did was get one of those drafting circle templates that go up by 1mm diameter increments, roll out a slab of clay and cut out a series of holes (and mark the diameter) then fire slab to maturity. Plunk your stopper in the holes to determine the right size neck opening to throw, allowing a bit for glaze. When throwing the necks I taper it to the same angle as the stopper and measure the top.
When I first started spraying glazes I often put them on to thinly. I came across the site linked below; they take a different approach to determining how much glaze to put on a pot. They use volume of glaze per surface area of pot. I tried this for the first few kiln loads I sprayed glazes on to get a proper feel for what the glaze should look like to get the correct coverage.