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Hi everyone, Hobbyist ceramist with a few years of experience here, but not so much glaze chemistry knowledge. I make work at a studio in Germany since a few years and since the summer, we noticed a lot of new issues with various glazes used at the studio. Nothing seems to have changed a priory with the way we do things. We fire with 2 different Rhode ecotop kilns at 1220 celcius. All ours work is made using a white stoneware clay from one distributor. We order commercial glazes from the same distributor and a few other glazes from another one. We've been using the same glazes for years and the same firing cycle as well. We received a new order of clay and glazes in August from our main distributor and since that, we started to notice an increasing amount of dunting. The issues are mainly with work from beginner students that are taking classes at the studio and that are perhaps applying glazes a bit too thick. This is something that happens regularly but didn't usually result in dunting before. Then recently, I also started to have shelling or slight shivering issue at the rims of cups on some of my own pieces when using one particular glaze. It's a matte transparent glaze I've been using for years and never had issues with. Is it possible that it is the clay body that is creating all these new issues? What is the best way to trouble solve from here? I emailed both distributors so ask if they know of any changes with their products already ( waiting on an answer) but any other tips I can get from someone with more experience would be much appreciated!
Me again! I recently posted a topic (here) about work cracking. A different issue (the absorption levels of the clay and possibly the use of a commercial liner glaze) was identified and so my focus was on fixing this and hopefully, in turn, my cracking problem would disappear. But it's back! After shock testing test pieces (freezer overnight and then boiling water), the image shows a hairline crack that wraps around at leats half the cylinder on the outside with smaller cracks coming off at right angles (highlighted with ink). The ink has been painted on to the gloss liner in the interior but the cracks haven't materialised on the inside. I am using Earthstone ES5 clay from Scarva that I tested in the post above and know that the absorption for this clay is on average 2.1% at 1230˚c/2246˚f in electric oxidation. The inside is a high calcium gloss and the outside is a satin matte white, Pike's Oatmeal, from John Britt's Mid Range Glazes book (see below for recipes...I tried to attach insight images but to no avail). In the last post Min helpfully made me aware that particularly with calcium mattes, the CTE of glazes can be misleading so I don't want to rely too much on judging the outer glaze by its CTE values. Another cylinder, the same clay and inner glaze but with a different outer glaze in black, has odd ridges that have formed after firing, tried my best to show this in the photo. The glaze info for that particular glaze has also been included (see black satin matte glaze info). My questions are - is this crazing? On either? The white exterior glaze shows a CTE of 5.9 and inner gloss of 6.3. I would have expected crazing from the gloss rather than the satin exterior. Is there too big a mismatch between the CTE of the inner and outer? If not crazing then by looking at the cracks and surface issues here is there another problem? Perhaps in the making process? How do I fix this? I'm going a bit craze-y here Any help appreciated!!! Satin Matte White (Pikes Oatmeal): f-4 feldspar 37.400 silica 22.700 kaolin 4.500 dolomite 6.400 talc 13.600 Gerstley Borate 12.700 zinc oxide 2.700 +tin oxide 10% +rutile 0.2% Black Satin Matte: Soda Feldspar 5.900 5.41% Silica 12.700 11.65% Wollastonite 19.600 17.98% China Clay 36.300 33.30% Talc 13.700 12.57% ferro frit 3124 11.800 10.83% + mason stain 6600 3% +rutile 3% + red iron oxide 1% High Calcium Gloss: nepheline syenite - 4.000 quartz - 17.000 wollastonite - 29.000 china clay - 30.000 frit 3195 - 20.000