Crooked Lawyer Potter Posted February 5 Report Share Posted February 5 I need help figuring out this glaze defect. I've gotten these mugs just where I want them except for this issue. The defects do not appear anywhere except in the interior of mugs or glasses. So, for instance, it is never a problem with plates or bowls. I'm thinking it must be related to the different conditions that arise in the tight confines of the mugs interior (higher heat, perhaps?). The mugs are dipped first in an ash glaze, allowed to dry, and then second dipped in David Leach II. The ash glaze is 50% ash, 50% red slip. David Leach II is : potash feldspar 45 dolomite 15 frit 3124 5 EPK 15 Silica 10 Talc 10 colorant (iron ochre, or copper carb, or cobalt carb) The clay is Laguna WC436. Its an oxidation firing in an electric kiln at cone 6 with a slow cool. The bisque was to cone 08. Anyone have any ideas as to what is going on here? (The hole looks to me as if it goes all the way to the clay.) Quote Link to comment Share on other sites More sharing options...
PeterH Posted February 6 Report Share Posted February 6 11 hours ago, Crooked Lawyer Potter said: I'm thinking it must be related to the different conditions that arise in the tight confines of the mugs interior (higher heat, perhaps?). The "higher heat" idea was explored in this thread ... but I wouldn't be surprised if your problem was related to differences in inside and outside glaze application. Hulk 1 Quote Link to comment Share on other sites More sharing options...
Hulk Posted February 6 Report Share Posted February 6 (edited) "...I wouldn't be surprised if your problem was related to differences in inside and outside glaze application." Perhaps sequence and/or timing involved there. Edited February 7 by Hulk edit PeterH 1 Quote Link to comment Share on other sites More sharing options...
Babs Posted February 6 Report Share Posted February 6 If not dopping bisque or wiping down with damp sponge then if liner glaze will be thicker than outside glaze if applied first. Perhaps a test with a play with this aspect may help. The flaws are beautifully round though Quote Link to comment Share on other sites More sharing options...
PeterH Posted February 7 Report Share Posted February 7 (edited) On 2/5/2024 at 9:42 PM, Crooked Lawyer Potter said: The defects do not appear anywhere except in the interior of mugs or glasses. Are there any differences in glaze application methods/timing between your mugs/glasses and the other pots? Edited February 7 by PeterH Quote Link to comment Share on other sites More sharing options...
Crooked Lawyer Potter Posted February 7 Author Report Share Posted February 7 No differences. Mugs (and other pots) are dipped using tongs. First glaze (ash) is allowed to dry before 2nd dip in David Leach II. Quote Link to comment Share on other sites More sharing options...
Callie Beller Diesel Posted February 7 Report Share Posted February 7 @Crooked Lawyer Potteris your kiln vented? I’m not sure if it’s the atmosphere or the temperature that’s different, but if you’ve ever had a chance to look inside a pot that’s at temperature, usually either a raku or wood fire where you could see inside the kiln, you can see that there is indeed a difference in the glow on the inside bowls and cups, and it moves with the kiln’s atmosphere. (My optometrist would like a word with 20 yo me.) If that fluctuation is from gasses from the kiln atmosphere, or even from materials decomposing and being contained in more upright forms, venting might take care of it. I notice that your bisque temp is quite low as well. Have you tried going up to cone 06 for the additional burn off? Quote Link to comment Share on other sites More sharing options...
Crooked Lawyer Potter Posted February 7 Author Report Share Posted February 7 Callie Yes the kiln is vented. I have always bisqued to cone 06 but changed just recently to 08, just to see if it made a difference in the glaze take-up. I think the current problem appears in both instances -- cone 06 and cone 08 but I am going to make sure by further experiment. Thanks for your input. Quote Link to comment Share on other sites More sharing options...
Babs Posted February 7 Report Share Posted February 7 Are your mugs and affected bowls thinner walled than your other ware? Are some or all affected? Could some be drier before the second dip? Quote Link to comment Share on other sites More sharing options...
Min Posted February 7 Report Share Posted February 7 If the first glaze saturates a thin wall then you apply a second coat or glaze the clay can't absorb the extra moisture without pulling away from the clay. I'ld try adding some gum to the ash glaze and then apply the Leach glaze when the ash one is just dry enough to handle. Don't wait too long before applying the Leach glaze. BTW the David Leach II glaze looks like a cone 10 recipe that someone has added just a titch of boron frit to. Does't look like a cone 6 recipe. Babs and Kelly in AK 2 Quote Link to comment Share on other sites More sharing options...
Babs Posted February 7 Report Share Posted February 7 1 hour ago, Min said: If the first glaze saturates a thin wall then you apply a second coat or glaze the clay can't absorb the extra moisture without pulling away from the clay. I'ld try adding some gum to the ash glaze and then apply the Leach glaze when the ash one is just dry enough to handle. Don't wait too long before applying the Leach glaze. Concisely put! What I was alluding to...not as articulately..... Quote Link to comment Share on other sites More sharing options...
Crooked Lawyer Potter Posted February 7 Author Report Share Posted February 7 I got the Leach glaze from Vaughn Smith. Its works beautifully for me (other than this issue) Quote Link to comment Share on other sites More sharing options...
Kelly in AK Posted February 9 Report Share Posted February 9 Random armchair speculation, two cents, worth every penny. The ash glaze is bubbling. The defect is happening in the cups because, even in a vented kiln, the atmosphere inside a more closed form is different than everywhere else. The iron in your “red slip” is decomposing at the same time your Leach glaze is melting, and it’s doing it more vigorously inside the cups and mugs because there’s a slightly more reduced atmosphere there. On a different note, I have to agree with @Min about the Leach glaze. It’s melted and gorgeous, but some people may take issue with its durability based on the recipe. Quote Link to comment Share on other sites More sharing options...
Mark C. Posted February 9 Report Share Posted February 9 The easy fix is use a liner glaze on inside and ash glaze on outside-issue is done. If that sound to hard (an inner and outer glaze job) try- below Since you are using a iron clay a hotter bisque any help by making sure all organics are burnt out yes it looks like the glaze is peeled back clean to me as well. burnouts or ash peeling it back is my guess. Even in oxidation as Kelly said the inners are a slight more reduction or lets say not as oxidized. One nice liner glaze inside will fix this in a heartbeat Kelly in AK, Babs, Hulk and 1 other 4 Quote Link to comment Share on other sites More sharing options...
Crooked Lawyer Potter Posted February 9 Author Report Share Posted February 9 Gonna try the higher bique and see what happens. I'll let you know. Quote Link to comment Share on other sites More sharing options...
Mark C. Posted February 9 Report Share Posted February 9 Its just one of many things to rule out. Quote Link to comment Share on other sites More sharing options...
neilestrick Posted February 10 Report Share Posted February 10 I think it's probably an application issue. How long are you waiting between applying the two glazes? My guess is that the inside is still too wet when you apply the second glaze, and it's failing to get good adhesion, which is causing the crawling. I imagine the ash glaze takes forever to dry? Pres 1 Quote Link to comment Share on other sites More sharing options...
Denice Posted February 11 Report Share Posted February 11 Marc commented on using liner glaze, I have had this happen to me in the past. I like the looks of liner glaze it gives the piece a higher quality look and the liner gives it a visual break that separates your inner space from the outward spacial design. Denice Mark C. and Hulk 2 Quote Link to comment Share on other sites More sharing options...
Mark C. Posted February 11 Report Share Posted February 11 1 hour ago, Denice said: Marc commented on using liner glaze, I have had this happen to me in the past. I like the looks of liner glaze it gives the piece a higher quality look and the liner gives it a visual break that separates your inner space from the outward spacial design. Denice Absolutely a different liner looks better in most instances. Its been so long since I even considered that as my stuff always has a different liner glaze Hulk 1 Quote Link to comment Share on other sites More sharing options...
Crooked Lawyer Potter Posted February 11 Author Report Share Posted February 11 12 hours ago, Mark C. said: Absolutely a different liner looks better in most instances. Its been so long since I even considered that as my stuff always has a different liner glaze What are the characteristics of a liner glaze? Does the term simply denote its position and function in the interior or are there certain qualities distinct from regular glazes? Quote Link to comment Share on other sites More sharing options...
PeterH Posted February 11 Report Share Posted February 11 22 minutes ago, Crooked Lawyer Potter said: What are the characteristics of a liner glaze? Does the term simply denote its position and function in the interior or are there certain qualities distinct from regular glazes? https://digitalfire.com/glossary/liner+glaze Liner Glaze The term "liner glaze" refers to two things. First, it is a technique (links below), where the inside and outside of a piece have different glazes that meet at the rim. Second, it refers to the practice of choosing a glaze for the inside food surfaces of utilitarian ware based more on its durability and resistance to leaching, running, crawling, blistering and crazing (problems common with reactive glazes). Glossy whites, transparents or modestly colored glazes are most common as liners. Liner glazes can have other practical purposes also. An example is their use in combination with intensely colored or variegated glazes, such are often runny and form a lake in the bottoms of vessels - this can lead to glaze compression failure. ... Hulk and Pres 2 Quote Link to comment Share on other sites More sharing options...
Kelly in AK Posted February 12 Report Share Posted February 12 A liner is a highly reliable unquestionably safe glaze that goes on the interior of pottery, allowing artists to use more outlandish combinations on the exterior. I must confess, I habitually overthink in my imagination of what clay and glazes do in firing. I’m strongly leaning towards what @PeterH, @Hulk, and @neilestricksaid about application and crawling bearing significance to your problem. Quote Link to comment Share on other sites More sharing options...
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