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Hello all! I am a potter of several years but know that I still have a lifetime of learning to do. I have a very particular aesthetic I try to accomplish with my mugs-- essentially they are liner glazed interior with a thick coating of multiple glazes on the exterior. Many potters do this, amiright? I have been creating these intricate glazes myself for 3 years now, without issue. It seems like all of a sudden (last 4 months) I have had customers come to me saying that their piece has been fine during multiple uses, when all of a sudden they pour their hot liquid in, get a ping, and a horizontal crack appears on the vessel moving along in a ring. To me, this sounds like thermal shock dunting. Right now, I am feeling lost among the vast amount of variables and don't know where to begin trying to solve the problem. I will list all of the variables/info I have collected, and hope someone out there can tell me where to focus my attention. I appreciate this community, immensely-- Thank you in advance!! -I fire with Laguna Bmix 5 and Standard 225. Both mature at cone 6. I fire to cone 6. Majority of issues have been with the 225, with one or two bmix pieces. I worry it is a clay body issue. -My firing cycle is quite standard and the same each time. I fire at a medium rate with no slow cool (I can provide the schedule if needed). -I throw my mugs thin. -My liner glaze is always the same. Black Licorice, Cone 6 (Mastering Cone 6 glazes). Recipe below. I do add Epsom Salt to this to prevent settling. -The exterior glaze always has a bit of the glossy liner glaze above and a matte as the undercoat. Then-- there are SO MANY different glazes I use from there in my laying techniques-- application and amounts differ from piece to piece. -Each time the issue has happened with a mug, the glaze has been thick and the issue arose when the customer added freshly boiled water. My glaze is alwayyyys thick on the exterior though... What does everyone recommend? If I had my biggest wish come true, there would be an adjustment to the Liner Glaze (better COE) that would allow a more balanced thickness of glaze overall in the piece. Can you see an adjustment I can make in the recipe below? Is there a better black recipe for this? Or, Should I throw thicker to ease the tension of exterior vs interior glazing? Orrrr... Am I missing something entirely, hah! Black Licorice Material Amount Ferro Frit 3134 26 Silica 26 Custer Feldspar 22 EP Kaolin 17 Talc 5 Whiting 4 Total base recipe 100 Red Iron Oxide 9 Cobalt Oxide 2
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
I am perplexed why I get this crawling in about 1 in 10 mugs. The mugs are poured in an old hand built mold, from large batches of slip. At leatherhard cleaned up and sanded when dry. I rinse them all well with a clean sponge after bisquing and allow to dry 24 hours before glazing. The glaze is a large batch I’ve been using for a year. I’ve never seen this on anything but these mugs.
using 2 diff. commercial glazes for inside and outsides of bowls / mugs etc - losing MANY with dunting ! These glazes are supposed to be compatible. Sometimes dunt occurs 2 weeks later after leaving the kiln. Advise ? thx