Cadaola Posted October 22, 2014 Report Share Posted October 22, 2014 hello everybody! I have tried the Copper-chrome green glaze from Linda Bloomfield book "Colors in glaze" and had 2 problems that probably are connected one ti the other. Here the glaze receipe (cone 04) High alcaline frit 75 China clay 15 Flint 10 + copper oxide 1,5 + chromium oxide 0,4 I post some pictures to better understand, but basically, the pot came out of the kiln in good shape, but after 5 or 6 days small white stains appeared on the surface. Those stains disappear if washed with water, but reapper later. The other problem is that there is a kind of dirty ring between glaze and clay, like if it was wet. This problem you can see in the picture that shows the bottom of the pot. Do you have any idea about the cause of those problems? Maybe the receipe contains to much flux? Sorry of the bad quality of images... Thanks a lot! Paola Link to comment Share on other sites More sharing options...
bciskepottery Posted October 22, 2014 Report Share Posted October 22, 2014 What clay body are you using? What is the clay body firing range? Link to comment Share on other sites More sharing options...
neilestrick Posted October 22, 2014 Report Share Posted October 22, 2014 At low fire temps the clay is still porous. Because the glaze is crazed, water can seem in the cracks and into the wall of the pot. The white stuff is most likely minerals (salts) from the clay or glaze or water precipitating out as the pot dries. It could also be mold forming under the glaze. The discoloring at the bottom is from wetness. In order to for low fire, porous clay bodies to be water tight and hygienic, the glaze must fit perfectly. No crazing. You're going to have to work on that glaze or switch to another one. Try different frit, for starters. I'd go with Ferro 3124. A simple low fire clear is 90% 3124 + 10% kaolin. Link to comment Share on other sites More sharing options...
Cadaola Posted October 26, 2014 Author Report Share Posted October 26, 2014 I'm using a grès 1100-1300°C (2012-2372°F), this could be the cause of the problem, because the clay remains porous? Do you think that increasing Silica content in the glaze could make disappear crazing and all the glaze problems? Link to comment Share on other sites More sharing options...
bciskepottery Posted October 26, 2014 Report Share Posted October 26, 2014 Are you bisque and glaze firing at low-fire temperatures? Your clay body seems to have a wide firing range, from mid- to high-fire; if so, it is not vitrifying and remaining more porous and will be more likely to seep as you are glazing at low fire temperatures of Cone 04. And unless you fire the current clay body to its mature temperature (1300C), you may still have seeping. You can try to modify the glaze to work through trial and error; but it will be easier if you get a better match between clay body and glaze. Link to comment Share on other sites More sharing options...
Cadaola Posted October 28, 2014 Author Report Share Posted October 28, 2014 Ok, you recommend me to change clay body. Do you think it is a good idea to bisque my current clay body to its mature temperature and then glazing it with this cone 04 glaze? I ask, because I really like this glaze, but the ceramic suppliers around here sell earthenware whose highest firing temperature is 1020 °C (1868 F) so I think it is risky to fire it to cone 04 (1040 °C /1904 F). But maybe I'm wrong, and it is better to change claybody and slightly underfire the glaze...? Link to comment Share on other sites More sharing options...
neilestrick Posted October 28, 2014 Report Share Posted October 28, 2014 If you're using low fire glaze, use low fire clay. The earthenware that goes to 1020 will probably handle 1040 just fine. Ask them. Sometimes they just say cone 05 as a general recommendation. Link to comment Share on other sites More sharing options...
Bob Coyle Posted October 28, 2014 Report Share Posted October 28, 2014 I'll go along with Neil... the range you give for your clay is way out of line, it is so wide it is almost meaningless. Link to comment Share on other sites More sharing options...
enbarro Posted October 30, 2014 Report Share Posted October 30, 2014 Is not only a good idea to change to a low fire clay, checking the frit's composition would be beneficial too... Its hard to avoid crazing with a High soda frit % , like Ferro 3110, on a translucent glaze even when using a low fire clay... Link to comment Share on other sites More sharing options...
Cadaola Posted November 2, 2014 Author Report Share Posted November 2, 2014 I asked the supplier, but he is not sure that his earthenware that goes to 1020 will handle 1040. I'm going to test his clay and hope it will work! If not, do you think that bisquing my clay body to its mature temperature and then glazing it with this cone 04 glaze will give good results? I heard that this is a process often used with porcelain.... Thanks a lot for your suggestions! Link to comment Share on other sites More sharing options...
Min Posted November 2, 2014 Report Share Posted November 2, 2014 I ask, because I really like this glaze, but the ceramic suppliers around here sell earthenware whose highest firing temperature is 1020 °C (1868 F) so I think it is risky to fire it to cone 04 (1040 °C /1904 F). But maybe I'm wrong, and it is better to change claybody and slightly underfire the glaze...? ^04 is 1060C, 1046 is ^05 (given the kiln is heating at correct climbing rate). I asked the supplier, but he is not sure that his earthenware that goes to 1020 will handle 1040. I'm going to test his clay and hope it will work! If not, do you think that bisquing my clay body to its mature temperature and then glazing it with this cone 04 glaze will give good results? I heard that this is a process often used with porcelain.... Thanks a lot for your suggestions! Earthenwares are porous, fired to their top temp or not. You need a glaze that fits well without any crazing to make something water tight with earthenware.The recipe you posted is a high alkaline glaze, those are notorious for crazing. So, if you really want to use your posted glaze recipe I would use a non crazing liner glaze on the inside of pots with your posted glaze on the outside. I would be surprised if a clay that can do 1020 ( approx ^ 06 1/2) couldn't go to 1040 (just a titch under ^05). Maybe post the name of the clay, someone here might be using it and can comment. You could high fire your other clay to maturity then glaze with the low fire glaze but that would be a major pain. It's very hard to glaze vitrified clay and have it turn out decent. It's also a heck of a lot of work that you don't need to do. Link to comment Share on other sites More sharing options...
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