ashhorth Posted September 10, 2021 Report Share Posted September 10, 2021 Hi I have recently had a mug returned after the pour boiling water into the mug and the glaze cracked around the outside , it is cosmetic but not nice to see, can anyone suggest the cause? And how to fix? Thankyo Link to comment Share on other sites More sharing options...
Bill Kielb Posted September 10, 2021 Report Share Posted September 10, 2021 This would indicate that the coefficient of expansion of the fired clay and fired glaze are not close enough or compatible enough and delayed crazing will likely occur in items using this clay and glaze. It generally means adjust the glaze chemistry for a better fit or in the case of a commercial glaze use a different or more compatible one. Most mechanical or other solutions such as glaze thickness alleged will not really fix this and the combination will eventually craze. Most folks would consider this not suitable for food products and this would be a glaze defect when used with this claybody. Food and bacteria will accumulate in these cracks and the cup will be weaker than a non crazed equal. There are folks that consider crazing to be aesthetic and ok to use though. Most potters out of conservative caution will not sell crazed products for potential food use. Here is a good read IMO on glaze defects. The safety issues are summed up in paragraph seven. https://digitalfire.com/glossary/crazing Link to comment Share on other sites More sharing options...
Mark C. Posted September 11, 2021 Report Share Posted September 11, 2021 You need to change glaze or clay body as noted in above post next time test your mugs with boiling water before selling them Link to comment Share on other sites More sharing options...
ashhorth Posted September 11, 2021 Author Report Share Posted September 11, 2021 Thankyou for the advice! Struggling to understand how to change the glaze. It’s all very confusing! But yes until I’ve got it sorted I won’t be selling any more, thankfully was just a kind friend Link to comment Share on other sites More sharing options...
Denice Posted September 11, 2021 Report Share Posted September 11, 2021 If you make your own glaze, you need to make up test samples of some different formulas. Purchased glaze cannot be easily change, trying a different brand might help. It is probably the clay, if you are firing C5/6 glazes you need a clay that is C5/6 not one that can fire to 7,8,9,10. Denice Link to comment Share on other sites More sharing options...
Babs Posted September 11, 2021 Report Share Posted September 11, 2021 Are you throwing and firing these mugs? Sometimes very thinly thrown , or unevenly thrown work can accentuate this prob. Though clay "gives " in these situation Link to comment Share on other sites More sharing options...
ashhorth Posted September 12, 2021 Author Report Share Posted September 12, 2021 Firing I think I’m gonna have to contact the supplier as I can’t actually find info on what cone they fire to! Link to comment Share on other sites More sharing options...
Min Posted September 12, 2021 Report Share Posted September 12, 2021 12 hours ago, ashhorth said: Firing I think I’m gonna have to contact the supplier as I can’t actually find info on what cone they fire to! @ashhorth, are these commercially made mugs that come glazed then you do the decorative work on? Could you give a bit more information on your process? Link to comment Share on other sites More sharing options...
ashhorth Posted September 12, 2021 Author Report Share Posted September 12, 2021 Yes I’ve bought the bisque mugs from bisque direct, then I’ve painted with Marco underglaze, then I’ve dipped them in a clear glaze ( that I mix from a powder , the powder is premade from my local pottery supplier) then fired them Link to comment Share on other sites More sharing options...
Min Posted September 12, 2021 Report Share Posted September 12, 2021 Okay, so the clear glaze doesn't fit the clay. For commercial glazes it's really a matter of trial and error to get one that fits. I'ld start with contacting the supplier of the bisque and asking which clear glaze they recommend. If you use any green underglazes it's best to use a zinc free clear. Zinc can mess up any underglaze colours that contain chrome and turn it an ugly brown. Are you using cones to verify the glaze is getting to the correct cone? Underfiring can cause a glaze to craze too. Link to comment Share on other sites More sharing options...
ashhorth Posted September 12, 2021 Author Report Share Posted September 12, 2021 Thankyou for this! Seems less scary! I have contacted the mug supplier to see firing info as I’ve just noticed it’s not on the website, I haven’t used my cones yet but thinking now I may have to really appreciate this response Link to comment Share on other sites More sharing options...
PeterH Posted September 12, 2021 Report Share Posted September 12, 2021 From Bisque Direct's mug page https://bisquedirect.com/bisque/mugs.html Earthenware Clay - Glazing and Firing. The recommended glaze firing temperature is 1000˚C to 1080˚C and soak (hold) 30 minutes + for all our Earthenware bisque items. For tableware items firing 1030˚C to 1080˚C produces a more durable product. Which is repeated on their dipping glaze page https://bisquedirect.com/glaze/clear-gloss.html They offer three glazes all firing "Approximately cone 06 to 05." Also gives Bullers ring ranges for each glaze. Link to comment Share on other sites More sharing options...
ashhorth Posted September 12, 2021 Author Report Share Posted September 12, 2021 Ah thankyou I mustn’t have seen that at all! I know my glaze default setting only goes to 998 maybe that’s part of the issue Link to comment Share on other sites More sharing options...
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