itsallabouttiger Posted February 13, 2021 Report Share Posted February 13, 2021 Hi Guys, I hope someone can help, my friend is an amazing potter, but shes having problems with glazes. she has her own kiln, but recently shes been firing mugs and a few of them seem fine, but as soon as you pour hot water in them the start clinking and cracking the glaze. Any ideas why this could be happening? John Link to comment Share on other sites More sharing options...
Bill Kielb Posted February 13, 2021 Report Share Posted February 13, 2021 The glaze coefficient of expansion does not match her clay coefficient of expansion. That glaze will not fit with her clay. Assuming the glaze and clay both mature at the same cone (temperature) and she fired to maturity, she will need to find a glaze that matches better. Often only figured out by firing. There is no real predictor of expansion by composition and firing together and testing is the truly the only way to have confidence they fit reasonably well. Link to comment Share on other sites More sharing options...
itsallabouttiger Posted February 13, 2021 Author Report Share Posted February 13, 2021 ok, cool. so you re saying she just has to try different glazes til she gets one that doesnt clink and crack? Link to comment Share on other sites More sharing options...
Bill Kielb Posted February 13, 2021 Report Share Posted February 13, 2021 Yes, pretty much. Google crazing and you will find a bunch she can read. Crazing can happen immediately or be delayed by weeks, months etc.... there are some stress tests for serious potters which entail freezing then pouring boiling water etc... to try and make something craze. Unfortunately there is no accurate test that says a pot will craze in X amount of time. For most if things were fired appropriately and they do not craze fairly soon then they stay that way for a long long time. I mention the qualifiers of firing to maturity because if each component is not, then crazing can occur and it’s hard to assign fault. . So a common example is someone fires a cone 9 clay to cone 6 with a cone 6 glaze and it crazes. Well to be fair, the clay really needed to be fired to its finish point to have its finished characteristics. So maybe that cone 6 glaze will be fine for other cone 6 claybodies but just not suitable for the cone 9 claybody fired to cone 6. Matching clay and glaze maturity firing temps is always a thing and from there determining if they fit is appropriate. But yes in general without qualification she needs to find another glaze or even claybody if she really likes that glaze. Link to comment Share on other sites More sharing options...
Min Posted February 14, 2021 Report Share Posted February 14, 2021 @itsallabouttiger, I edited the title of your post to better reflect the contents. Makes it clearer to understand what the thread is about and also to search. I notice you are in England, there seem to be quite a number of broad range claybodies available there. What Bill said about firing to the clays maturity conflicts with many of the broad firing ranges clays that are available in England but it is a very wise thing to do. Another point I've noticed in England is it's not commonplace to verify the firing of electronic kilns with witness cones. This is fine if the thermocouple is calibrated well but they do tend to drift as they age making the readout inaccurate. It's a good idea to verify with witness cones that the kiln is indeed reaching the desired cone rather than just going with a temperature. Regarding crazing, the closer the craze lines are together the greater the mismatch in fit between the clay and the glaze. If the craze lines are widely spaced (like about 1.5cm apart) it might be possible to fix the glaze by adding silica, sometimes this simple fix is enough to remove crazing. It would take some accurate measuring and testing though. Welcome to the forum. Link to comment Share on other sites More sharing options...
PeterH Posted February 17, 2021 Report Share Posted February 17, 2021 On 2/13/2021 at 8:25 PM, itsallabouttiger said: ok, cool. so you re saying she just has to try different glazes til she gets one that doesnt clink and crack? Sorry to see that your enquiry isn't progressing. The glaze is crazing because it doesn't "fit" the body well. Which means that your friend needs to change at least one of: - the glaze - the clay body - the firing schedule If your friend posts details of these, preferably with a clear photo of the crazing, the experts here will have something to respond to. Link to comment Share on other sites More sharing options...
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