Nadrali Posted May 12, 2023 Report Share Posted May 12, 2023 Hello, We use a high alkaline glaze regularly and today opened the kiln to find all items on one shelf only, had what appears to be scumming on them. (We have seen this before but normally on a kiln load rather than one shelf of items) Any ideas of what caused this_ All items where made and glazed at the same time with the same batch of glaze. Photo 1 shows the pieces with defects and photo 2 the results from the shelf directly below which have no scum... Quote Link to comment Share on other sites More sharing options...
Callie Beller Diesel Posted May 12, 2023 Report Share Posted May 12, 2023 A few questions to help us troubleshoot: Is what you’re referring to as scumming pinholes/blisters, or is it some kind of solid crystal in the glaze? The image isn’t super clear. If you experienced it in an entire kiln load previously, do you know what caused it in that instance? Also, can you tell us if the firing may have been cooler/hotter on that shelf for any reason? And what is the glaze recipe if you have it? If you don’t, please provide the brand name and we can look up details. Nadrali 1 Quote Link to comment Share on other sites More sharing options...
Bill Kielb Posted May 14, 2023 Report Share Posted May 14, 2023 (edited) From the pictures it also looks crazed on those wares, not sure that is intentional or not. Edited May 14, 2023 by Bill Kielb Quote Link to comment Share on other sites More sharing options...
Babs Posted May 14, 2023 Report Share Posted May 14, 2023 Think the scumming pots have been fired at a bit lower temp. Can you wipe it iff and does it return? Quote Link to comment Share on other sites More sharing options...
Mark C. Posted May 14, 2023 Report Share Posted May 14, 2023 scumming for me needs more explanation ? its not a word I use in relation to glazes. Bubbles ,pitting.crawling. overfired ,underfired,cracked. shivering,dunted,rough surface smooth surface . shinny surface matt surface??Dull.bright?? scumming is what happens on a bucket of funky water or a pond surface what is scumming?? and what are we looking for in the photos- as both are very close looking as as Babs said one looks to be cooler fired Roberta12 1 Quote Link to comment Share on other sites More sharing options...
Hulk Posted May 14, 2023 Report Share Posted May 14, 2023 (edited) If scumming where solubles in the clay are driven to the surface of bare clay - say the inside of a ware is glazed, hence the bisque is soaked with water, some of which migrates to the other side, bringing with it solubles, which are left as "scum" when the moisture evaporates (aka efflorescence, I guess?). See Sulfate Scum (digitalfire.com) and related links, e.g. the soluble salts one... From there, maybe the unmarred ware were glazed just enough thicker that the solubles were melted in, or maybe the unmarred ware dried faster after glazing, hence less solubles were dissolved, or maybe the unmarred ware were dried faster at the green stage, or maybe the unmarred ware were glazed when the glaze bucket had been adequately stirred up (or opposite?), or maybe the unmarred ware was from a different bag of clay - or from the edge of a block of clay that had migrating solubles, or idk! Edited May 14, 2023 by Hulk Italics added Nadrali 1 Quote Link to comment Share on other sites More sharing options...
Babs Posted May 14, 2023 Report Share Posted May 14, 2023 A cure used to be adding..Barium Carb to clay body if "proper" scumming. This one I'd be purting cones on shelves to get an idea of temp differences. Refiring higher would sort this I think. What is your glaze recipe? What is the temp you're firing to? Water supply? Quote Link to comment Share on other sites More sharing options...
Min Posted May 15, 2023 Report Share Posted May 15, 2023 What is the glaze recipe? I'm wondering about how balanced it is and also the cooling cycle and the possibility of devitrification. Roberta12 1 Quote Link to comment Share on other sites More sharing options...
Nadrali Posted May 16, 2023 Author Report Share Posted May 16, 2023 Hello, thank you so much for all the replies! Please see the recipe and answers to the questions below. Recipe High Alkaline frit - 100 High soda frit - 100 China Clay - 20 Copper carbonate - 8 Zirconium - 6 Bentonite - 4 cmc 4 Water 300 We also add a tbls of vinegar ................................................................................................ It appears to be some kind of solid crystal in the glaze or on the surface of the glaze. I don't know what caused it in the whole kiln loads. We have three kilns and in one the glaze was coming out fine so I just used that one kin for this particular glaze. It seems to disappear when wiped with a wet cloth and then it is visible again when dry. (Even when wiped with vinegar it does not go) In the 1st photo there is this whitish crystal thing in some areas. I will try to take a clearer photo. The firing temperature is around 1020 c. It is a low fire raku glaze No controlled cooling cycle, the kiln was left to cool on it own. Items are glazed from green All pieces where made, glazed and then fired at the same time in the same kiln. All pieces on one shelf only had the problem, the rest were fine. Not sure why or if that particular shelf was hotter/cooler. I can't seem to upload anymore photos!?! Quote Link to comment Share on other sites More sharing options...
Nadrali Posted May 16, 2023 Author Report Share Posted May 16, 2023 Photos Quote Link to comment Share on other sites More sharing options...
Min Posted May 16, 2023 Report Share Posted May 16, 2023 (edited) There are a lot of possibilities of what is causing it. I can't recall seeing this issue before. It's almost like an Egyptian paste where salts form on the surface but at a much larger and localized scale. Could be soluble salts precipitating to the surface or a firing issue. It looks like the problem is worse on the rims which would make sense. We don't know the firing cone reached on the shelves with this problem versus those that fired okay so thats one variable, size of the kiln, how fast the problem ones cooled compared to ones that didn't. Was the shelf the problem ones on in a small or large kiln, shelf placement in the kiln? Is there a pattern with the firing and the results you can ascertain? Glaze will be extremely high in R2O fluxes (sodium in particular) and very low in clay. This won't be a durable glaze insofar as being suitable for functional ware. May I ask what the items are for? Non functional work? If you want to see how badly this glaze will leach out due to being a very unstable base with this amount of copper put a slice of lime or lemon on it (cut side facing the glaze) then cover with plastic wrap for a couple days or submerge part of a piece in household vinegar for a couple days then rinse and dry it and see how the glaze has changed. Also, with such heavy crazing and I'm assuming a claybody with a high absorption rate this would also be a very poor choice of glaze for functional ware. Edited May 17, 2023 by Min clarity Quote Link to comment Share on other sites More sharing options...
Babs Posted May 16, 2023 Report Share Posted May 16, 2023 3 hours ago, Min said: There are a lot of possibilities of what is causing it. I can't recall seeing this issue before. It's almost like an Egyptian paste where salts form on the surface but at a much larger and localized scale. Could be soluble salts precipitating to the surface or a firing issue. It looks like the problem is worse on the rims which would make sense. We don't know the firing cone reached on the shelves with this problem versus those that fired okay so thats one variable, size of the kiln, how fast the problem ones cooled compared to ones that didn't. Was the shelf the problem ones on in a small or large kiln, shelf placement in the kiln? Is there a pattern with the firing and the results you can ascertain? Glaze will be extremely high in R2O fluxes (sodium in particular) and very low in clay. This won't be a durable glaze insofar as being suitable for functional ware. Copper is extremely high also, again indicating this glaze won't be suitable for functional ware. May I ask what the items are for? Non functional work? If you want to see how badly this glaze will leach out due to being a very unstable base with high amounts of copper put a slice of lime or lemon on it (cut side facing the glaze) then cover with plastic wrap for a couple days or submerge part of a piece in household vinegar for a couple days then rinse and dry it and see how the glaze has changed. Also, with such heavy crazing and I'm assuming a claybody with a high absorption rate this would also be a very poor choice of glaze for functional ware. I have had a similar thing happen. I actually posted years ago. I live coastally so the rainwater added to the sodium. I took the glaze higher and added a soak . Prob solved . My glaze was a frit ,nephsy high C03 glaze, Didn't craze . Nadrali 1 Quote Link to comment Share on other sites More sharing options...
Denice Posted May 17, 2023 Report Share Posted May 17, 2023 The scumming I had problems with was more of a white smear on the glaze, it didn't scrub off and I couldn't get all of it to disappear refiring it. I did figure out what was causing it. I have always gave my bisque ware a good washing the day before I glazed. When I started working in my current studio, I started having problems with scumming. I did a little research and decided it was my bisque or the water. After a couple of test I was positive it was the water, I use a microfiber towel now or distilled water to clean my bisque ware, no more scumming. Denice Babs and Nadrali 2 Quote Link to comment Share on other sites More sharing options...
Callie Beller Diesel Posted May 17, 2023 Report Share Posted May 17, 2023 I’ve had something similar happen in a glaze that had dry epsom salts added to the batch. The salt crystals didn’t dissolve fully, so the bits left almost identical white patches. So my question is, was your glaze lumpy, and could this have been caused by chunks of something (maybe the soda frit?) that weren’t fully dispersed? Did someone brush the lumps off of the pieces on one layer and not the other? Quote Link to comment Share on other sites More sharing options...
neilestrick Posted May 18, 2023 Report Share Posted May 18, 2023 On 5/16/2023 at 5:03 AM, Nadrali said: It is a low fire raku glaze Definitely not a good glaze for pieces that could be used for food, due to the crazing and poor durability. Raku glazes are generally formulated for a certain look, not for functionality. With low fire work you really want a glaze that fits really well if you're making functional work, or anything that is used around liquids, since the clay body doesn't vitrify. Roberta12 1 Quote Link to comment Share on other sites More sharing options...
Nadrali Posted May 18, 2023 Author Report Share Posted May 18, 2023 On 5/16/2023 at 7:21 PM, Min said: There are a lot of possibilities of what is causing it. I can't recall seeing this issue before. It's almost like an Egyptian paste where salts form on the surface but at a much larger and localized scale. Could be soluble salts precipitating to the surface or a firing issue. It looks like the problem is worse on the rims which would make sense. We don't know the firing cone reached on the shelves with this problem versus those that fired okay so thats one variable, size of the kiln, how fast the problem ones cooled compared to ones that didn't. Was the shelf the problem ones on in a small or large kiln, shelf placement in the kiln? Is there a pattern with the firing and the results you can ascertain? Glaze will be extremely high in R2O fluxes (sodium in particular) and very low in clay. This won't be a durable glaze insofar as being suitable for functional ware. May I ask what the items are for? Non functional work? If you want to see how badly this glaze will leach out due to being a very unstable base with this amount of copper put a slice of lime or lemon on it (cut side facing the glaze) then cover with plastic wrap for a couple days or submerge part of a piece in household vinegar for a couple days then rinse and dry it and see how the glaze has changed. Also, with such heavy crazing and I'm assuming a claybody with a high absorption rate this would also be a very poor choice of glaze for functional ware. Hi Mims, thank you for the reply, I will need to do some more firings to see if I can ascertain if any of the above mentioned factors may have affected that shelf in the kiln. The heavy crazing is because it is a raku glaze. We use it for that bight colour it gives. It is used for decorative pieces only! Quote Link to comment Share on other sites More sharing options...
Nadrali Posted May 18, 2023 Author Report Share Posted May 18, 2023 On 5/16/2023 at 11:08 PM, Babs said: I have had a similar thing happen. I actually posted years ago. I live coastally so the rainwater added to the sodium. I took the glaze higher and added a soak . Prob solved . My glaze was a frit ,nephsy high C03 glaze, Didn't craze . Hi Babs, thank you for your reply. Did the higher temperature affect the colour at all? The glaze tends to go greener the higher it is fired. Do you remember what temperature and how long a soak? Maybe you could share the recipe? Quote Link to comment Share on other sites More sharing options...
Nadrali Posted May 18, 2023 Author Report Share Posted May 18, 2023 17 hours ago, Callie Beller Diesel said: I’ve had something similar happen in a glaze that had dry epsom salts added to the batch. The salt crystals didn’t dissolve fully, so the bits left almost identical white patches. So my question is, was your glaze lumpy, and could this have been caused by chunks of something (maybe the soda frit?) that weren’t fully dispersed? Did someone brush the lumps off of the pieces on one layer and not the other? Hi Callie, no chunks in the glaze and only appeared on that one shelf, which is the curious thing. Thanks for the reply. Quote Link to comment Share on other sites More sharing options...
Callie Beller Diesel Posted May 18, 2023 Report Share Posted May 18, 2023 4 hours ago, Nadrali said: only appeared on that one shelf, Can you tell if that shelf was hotter or cooler than the others? It could be as simple as that. Magnolia Mud Research and Babs 2 Quote Link to comment Share on other sites More sharing options...
Babs Posted May 18, 2023 Report Share Posted May 18, 2023 Is your kiln toploading? Do you use test cones?. Top shelves of top loaders can be cooler. Before altering everything, if have no cones, add a soak to top temperature, Try 10 mins to start with. Refire a failed pot in next glaze firing, on a lower shelf. Quote Link to comment Share on other sites More sharing options...
Nadrali Posted May 23, 2023 Author Report Share Posted May 23, 2023 On 5/18/2023 at 9:33 PM, Babs said: Is your kiln toploading? Do you use test cones?. Top shelves of top loaders can be cooler. Before altering everything, if have no cones, add a soak to top temperature, Try 10 mins to start with. Refire a failed pot in next glaze firing, on a lower shelf. I did another firing without changing anything and all pieces where fine. The kiln is a top loader and the shelf was near the bottom of the kiln. Going to have to get some more cones to test the temperatures. If it happens again I will try the soak! Thank you Callie Beller Diesel, Babs and Hulk 3 Quote Link to comment Share on other sites More sharing options...
Callie Beller Diesel Posted May 24, 2023 Report Share Posted May 24, 2023 If you wanted to confirm if the shelf was cooler or hotter, another way to do that is re-fire one of the scummed pieces and see if it improves or gets worse. If it goes worse, the shelf was likely hotter. If it improves, the original shelf was likely a bit cooler. PeterH and Nadrali 2 Quote Link to comment Share on other sites More sharing options...
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