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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
A few quick questions for the glaze experts out there. Background: I'm very excited to be creating my first base glazes - a simple cone 6 clear for a white (talc free) clay body. I also have a dark red clay body with obviously a high iron content. And then after that I will be going for a cream color glaze and then a temmoku. I have no real interest at the moment in bright glossy colors. I have 2 different options for silica - sand (white) and flint (270M) - Which is the best starting point or should I just get some of both? I want to give myself the most possibilities for testing and using in secondary glaze recipes. For the base white recipe I want to use it calls for Tin Oxide Tin Oxide vs Zircopax - Tin Oxide is REALLY expensive and I am reading a lot that Zircopax although it can be substituted at around 2:1 for Tin Oxide, it tends to kill the colors that oxides produce a bit. Not too big a problem for me I wouldn't think at the moment and none of the Temmoku I want to test calls for it. Does Zircopax work in a wide range of recipes and does it have any caveats to watch out for? Thanks much! Very excited to get going and will post results as I go.