Chrissa Posted March 17, 2021 Report Share Posted March 17, 2021 Hello everyone! I use a low fire white clay with a commercial clear glaze. I tried several low fire clays and glazes and had the same problem: Wheelthrown pieces turn yellowish with occasionally some reddish areas, as if there is iron oxide in the glaze. The slab-made and pinched pieces do not have that problem, they turn a normal white. The discoloration is not consistent though, as some random pieces turn whiter than others. I usually fire at 1030 Celsius. I tested several firing temperatures and noticed (but not 100% sure) that as I lower the temperature the pieces are whiter, but the yellow is still there, in comparison to the slab-made pieces. The discoloration is visible in bisque firing but it it is really noticeable after glaze firing. I have some ideas but I haven't found a solution: As wheelthrown pieces absorb much more water, the local water would be the culprit? Also, my wheel turns rusty and I have to clean it with W40 pretty often, but could this amount of iron oxide be sufficient to color the clay? My firing schedule: 65C/hr- 120C 200C/hr- 900C 50C/hr -1030C - 10min hold time Any help would be appreciated! Thank you! Link to comment Share on other sites More sharing options...
Callie Beller Diesel Posted March 17, 2021 Report Share Posted March 17, 2021 Because you state that you’re experiencing the same problem with several low fire glazes and clays, and that your wheel rusts readily, I think your suspicion about your water being the culprit could be the most likely one. I can’t see it having anything to do with your firing if it’s not happening to all your pieces, and if it’s getting worse in the glaze. Although I don’t know what’s available in Greece, most wheelheads in North America are aluminum. That form of oxidation goes grey, usually, not rusty. Is your local water known for high iron? Link to comment Share on other sites More sharing options...
Hulk Posted March 17, 2021 Report Share Posted March 17, 2021 Hi Chrissa! Perhaps try running a the metal rib over the outside at the finish, hence removing the slip and somewhat burnishing the surface; you might leave a band un-ribbed surface as well. If the coloration is associated with your throwing water and/or what it is picking up, ribbing off the slip may ameliorate. I typically run the metal rib over the outside of every piece, and the inside of open pieces (bowls) as well - I like the smooth surface. For the more closed forms - mugs, vases, jars - I open a bit more than I'm looking for, then close to the finished shape with the rib; the flexible edge really helps the smooth curve. For more open forms - bowls - I pull a bit less open than I'm looking for, then open to the finished shape with the rib, which gives me the smooth interior curve. My guess is it's the coating of slip generated in throwing that's at issue more than the water. To test that, perhaps pick up a gallon of distilled water for your throwing bucket - make (and mark) a few pieces to compare... Post some pics of slab built and thrown ware, and your wheel, if you get a chance? Please do post a follow up of your findings. Link to comment Share on other sites More sharing options...
neilestrick Posted March 18, 2021 Report Share Posted March 18, 2021 9 hours ago, Hulk said: To test that, perhaps pick up a gallon of distilled water for your throwing bucket - make (and mark) a few pieces to compare... Hulk beat me to it. That's exactly what I was thinking. Link to comment Share on other sites More sharing options...
Chrissa Posted March 20, 2021 Author Report Share Posted March 20, 2021 Thank you all for your replies! After reading @Callie Beller Diesel's reply I did a facepalm, because up until now I -stupidly- thought "My wheelhead cannot be the culprit or else it would be a common problem and I can't find a similar case on the internet". So, yeah my wheelhead is not aluminium as most wheelheads are (I didn't know that), it is iron with elecrostatic painting as the manufacturer informed me yesterday, and naturally the paint has worn off over time... Even if I had a totally clean wheelhead before starting throwing, obviously rust was sneakily "contaminating" my throwing clay little by little, so that it wasn't visible clearly. Long story short, I called the manufacturer and he is going to send me an aluminium one... (sigh) @Hulk, yes I always run the metal rib but this wasn't solving the problem as the iron oxide was already all over the clay body not just the slip. Thank you for the suggestion to test distilled water, now I found about the wheelhead I'm sure the water is fine, thank you all! Link to comment Share on other sites More sharing options...
oldlady Posted March 20, 2021 Report Share Posted March 20, 2021 chrissa, wonderful to find you have a simple solution to the problem. it is good to hear the result, sometimes the question is never answered and sometimes the soultion is found but the person who asked goes away and we never hear what happened. thanks for the follow-up. Link to comment Share on other sites More sharing options...
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