kevinmansfield Posted January 16, 2014 Report Share Posted January 16, 2014 Hi Folks, I create earthenware tiles. I bisque fire to 998º and then glaze them using commercial glazes such as Duncan Envision and fire them again at 998º. I soak them for 10 minutes and never take them out of the kiln until they are cool. Now I have been doing this for some time and never had a problem but recently they have started crazing (see attached pic) and I don't understand why, I'm using the same clay (a standard ungrogged white earthenware) and the same kiln (a large modern electric Paragon kiln). The kiln still reaches the right temp and the colours and the pottery is all fine, it's just the crazing. The actual gazes are all in good condition.Has anyone got any ideas what could be the problem? Many thanks in advance, kind regards, Kevin. Link to comment Share on other sites More sharing options...
bciskepottery Posted January 16, 2014 Report Share Posted January 16, 2014 Are these tiles from a different batch of clay (or box)? There might have been a change -- even slight -- in the clay body due to variations in the composition of the clay body ingredients. If you have some older tiles, check them for delayed crazing, too. Hard to suggest a fix to a commercial glaze. Is the crazing only apparent in the transparent colors? or all colors? Link to comment Share on other sites More sharing options...
kevinmansfield Posted January 16, 2014 Author Report Share Posted January 16, 2014 Are these tiles from a different batch of clay (or box)? There might have been a change -- even slight -- in the clay body due to variations in the composition of the clay body ingredients. If you have some older tiles, check them for delayed crazing, too. Hard to suggest a fix to a commercial glaze. Is the crazing only apparent in the transparent colors? or all colors? Hi bciskepottery and thank you for your reply. The clay is a new batch, so it could be that. Older tiles seem ok and I generally only use transparent glazes. It tends worse with greens if that means anything? Link to comment Share on other sites More sharing options...
PeterH Posted January 16, 2014 Report Share Posted January 16, 2014 Essentially the same point as Kevin's. If you have any spare "good" tiles - and they have not crazed on standing - it might be worthwhile trying one of the freeze-heat torture tests to see if they were on the borderline of crazing. Indian ink is wonderful in making fine crackle visible. Regards, Peter Link to comment Share on other sites More sharing options...
mregecko Posted January 16, 2014 Report Share Posted January 16, 2014 Another option could be your pyrometer being miscalibrated. You're firing to 998o but if the pyrometer has aged or something has affected the voltage across it, then that could be inaccurate. Worth checking on too. It could also be a clay body difference. Contact the manufacturer and see if anything has been reformulated recently. Link to comment Share on other sites More sharing options...
kevinmansfield Posted January 16, 2014 Author Report Share Posted January 16, 2014 Another option could be your pyrometer being miscalibrated. You're firing to 998o but if the pyrometer has aged or something has affected the voltage across it, then that could be inaccurate. Worth checking on too. It could also be a clay body difference. Contact the manufacturer and see if anything has been reformulated recently. Thanks mregecko, Yikes 'pyrometer being miscalibrated' how do I check if this is the case? I am afraid I am quite ignorant because I have only ever fired earthenware to 998º I don't have any formal training. )o: If the problem is a kiln problem could it be to do with the elements? As far as the clay body goes I have some over the counter bisque and some old tile bisque which I have made myself so I can test that. Link to comment Share on other sites More sharing options...
bciskepottery Posted January 16, 2014 Report Share Posted January 16, 2014 If the problem is a kiln problem could it be to do with the elements? As far as the clay body goes I have some over the counter bisque and some old tile bisque which I have made myself so I can test that. Crazing is the result of a mismatch in the expansion/fit of clay bodies and glazes. You generally fix the problem by modifying either the clay body (or choosing another) or the glaze; most potters adjust the glaze (often by adding to the percent of silica) as that is the variable they have more control over. That is not easily done with commercial glazes, though. So, the problem is not with the elements. From the colors in your thumbnail, it looks like everything is melting -- nice shiny transparent glaze surfaces. So, my guess is you are hitting temperature and the kiln/elements are not the cause. If the glazes were coming out more matte and non-glossy, that would be an indication of under-firing -- which could be elements and/or thermocouple. Are your over-the-counter bisque items made with the same clay as your own-made work? You could have two clay bodies with differing expansion properties, one of which crazes and the other not. Link to comment Share on other sites More sharing options...
NancyAmores Posted January 17, 2014 Report Share Posted January 17, 2014 Hi Kevin, I've had the same problem with some of the Envision glazes in the past, especially the blues and greens as you noted. I alleviated the problem by firing to Cone 05 1888F, and then firing down 250F/hr. to 1500F before turning off. It has stopped the crazing, at least short term, I haven't had the improved pieces long enough to tell if it is permanent or if they will craze later. Link to comment Share on other sites More sharing options...
Guest JBaymore Posted January 17, 2014 Report Share Posted January 17, 2014 Firing CAN affect the crazing of a glaze........ IF............ the firings in which the crazing happens are sl;ightly different in the level of heat work that is being accomplished on the ceramic raw materials in the glaze or body. As an example, if a glaze is "fired" and melted looking when they open the kiln (the usual way studio potters assume that a glaze is "done"), but is in fact only partially finished all the chemical reactions that will eventually take place if allowed to go to completion (and what would be reflected in glaze chem software), then some of the raw mateials that went into the glaze batch are not yet inviolved fully in the melt. Hence the COE is not going to be the same COE as that for the fully melted glaze. So it is possible that this not-quite-melted glaze either shivers or crazes on a given body. Something like a change in the mesh size of a particular raw material in the recipe can cause the particles of that material to not be in as intimate contact with the other glaze ingredients, and hence not become involved in the melt the same way at the same exact cone response as it used to be. Speed of the firing to the given cone can impact this. Cones look the same... but the glaze looks different. Lots of variables here folks. best, ..................john Link to comment Share on other sites More sharing options...
kevinmansfield Posted January 17, 2014 Author Report Share Posted January 17, 2014 Wow, thank you all so much for your help! My kiln is in my garage so it's pretty cold, nothing like the crazy cold weather you good people on the over side of the Atlantic have had to deal with but perhaps cold enough to accelerate the cooling time. I'm going to have to look at my kiln manual and figure out how I can slow down the cooling. I have a nice modern Paragon kiln so it must be possible. Thanks very much to SheAmores for the suggestion of firing Cone 05 1888F, and then firing down 250F/hr. to 1500F before turning off. I'll give this a go. I am slightly nervous about it though because I've only ever fired Cone 06 for both my bisque and glaze firings. Link to comment Share on other sites More sharing options...
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