Dave K Posted April 20, 2016 Report Share Posted April 20, 2016 I have several white stoneware muds that I have used and have discolored on the bottom where it isn't glazed. It kind of has a dirty look to it , maybe like mold. What is it and how do I get it off? I have tried soft scrub and sanding but it doesn't help. Link to comment Share on other sites More sharing options...
Roberta12 Posted April 20, 2016 Report Share Posted April 20, 2016 Dave, Are they mugs you made? If so, did you fire them? If so, are you certain of the firing temp? I ask because this was happening to me years ago. The kiln in the community studio was not firing either bisque or glaze to the proper temp. I didn't know enough about it to realize what was going on. Pretty soon, everything I am making has this dirty look on the bottom. And I also noticed that if I put water in a piece and let it sit, the water would seep out. After research, I deduced that it was indeed mold. Because things were not vitrified. This was my experience, perhaps one of the pros out there might have another response for you. Maybe there are other reasons for this. Roberta Link to comment Share on other sites More sharing options...
Min Posted April 20, 2016 Report Share Posted April 20, 2016 If you refire the mugs it will burn out. I had the same thing happen with Georgies White Salmon. I was lazy and didn’t do a porosity test before starting to us it. Friends recommended the clay and I just went with it. I fired to the proper cone and still had leaky pots that discoloured at the bottom, t-pots and mugs the worst. I contacted Krista, the tech at Georgies, and she gave me a long story of how the kaolin they use in that body has a large particle size and it needed a very long (like 20 hr) bisque to tighten the body up. Seemed nonsense to me but I had a ton of the clay so I tried it plus added a slow glaze fire. Pots still leaked. Expensive lesson since I tossed a lot of bisque and finished pots. I used the rest of that clay just for yarn bowls where they don’t need to be water tight. Don’t know if I just got a bad batch but for Krista to admit there was an issue with water absorption with this clay I don’t think so. Doesn’t matter what the spec sheets say for a clay, do a porosity test for new clay bodies being used for functional pots that will hold liquid. I do a boiling water test plus leave an unglazed pot with water in it on newspaper for a week then check for no wrinkling of the paper. Then like Roberta said make sure the pots are actually getting to the proper cone. Link to comment Share on other sites More sharing options...
bciskepottery Posted April 20, 2016 Report Share Posted April 20, 2016 Why type of clay body was used for the mugs? If it was a Cone 6 to 10 clay body fired to cone 6, then the clay is not vitrified and slowly accumulating mold, etc. from water retention. I have a vase that has that "dirty" bottom look due to under-firing. Scrubbing will not help; maybe sit in a pan of bleach water -- but that will only be a temporary fix. Link to comment Share on other sites More sharing options...
glazenerd Posted April 21, 2016 Report Share Posted April 21, 2016 Min: In the future; wedge in 3-5% molochite to close up an open body clay. Molochite is super fine particle size, and will not add significant amounts of magnesium or iron. Bruce has pointed out the usual problem associated with a range of cone temp firings. Several clays are sold as cone 6-10, which is nearly impossible. Try adding 2-3% Nep Sy if you are firing to cone 6 without achieving vitrification.(Add more after testing the original additions.) If coffee cups or other functional ware begins to show crazing rings after several uses: it is more likely the clay reacting to thermal shock instead of the glaze. In these cases you can wedge in 3-5% mullite to increase resistance to thermal shock. These fixes also depend on the moisture content of the clay: may need to deal with that as well before wedging in fixes. Nerd Link to comment Share on other sites More sharing options...
Min Posted April 21, 2016 Report Share Posted April 21, 2016 Hi Nerd, Thanks for the suggestions but I don't think I would ever want to wedge in tiny amounts of materials to fix many hundreds of pounds of a lousy clay body. I think (some of) the clay manufactures' need to take more responsibility in their quality control. Link to comment Share on other sites More sharing options...
glazenerd Posted April 21, 2016 Report Share Posted April 21, 2016 Min: Fixes will get you through a box or so, but not truck loads. It would take me several very long posts to finish my rants about quality control. The paint. plastic, and rubber industries are the biggest users of clay, silica, and some fluxes. They require analysis with every shipment they receive:have my doubts if that protocol is stringently followed in the pottery clay biz. In fact if they were, we would not see the variances in products. Many of the variables the pottery biz has accepted as the norm: IMO are not variables at all, but rather poor quality materials. I enjoy the experimenting and testing more than making pieces; so these issues are high on my radar. Then again I do not make my living with pottery: so I am not under the pressure of production. Nerd Link to comment Share on other sites More sharing options...
Biglou13 Posted April 21, 2016 Report Share Posted April 21, 2016 i have same issue.... im going to fire higher then go from there.... Link to comment Share on other sites More sharing options...
Mark C. Posted April 21, 2016 Report Share Posted April 21, 2016 Never had this exact issue with cone 10 clays but I have had some crappy tons of bad clay before. I like Min just switched bodies, Life is to short for bad clay When manufactures start taking about 20 hour bisque cycles you need to ask them if Casper the ghost is in the clay as well and maybe that's why the long cycle is needed? Link to comment Share on other sites More sharing options...
Dave K Posted April 21, 2016 Author Report Share Posted April 21, 2016 Thanks for all of your replies. The pieces I am have this problem with are older and probably at that time I was using Standard 182, cone 6-10, and firing it to cone 6. I have changed to 240 ,cone 6, and have not noticed any discoloration. Thanks again, lesson learned. Link to comment Share on other sites More sharing options...
Mark C. Posted April 23, 2016 Report Share Posted April 23, 2016 Clay that is has that large of range of temps is most likely not vitrifing at that low end of cone cone 6 temp.sounds like you cured it by switching to a clay that is vitrifing. Link to comment Share on other sites More sharing options...
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