flowerdry Posted November 20, 2015 Report Share Posted November 20, 2015 This was really embarrassing, and the first time this has happened to me. I took some mugs to a friend and let her pick one out as a gift. The next morning she poured boiling water into it for tea and the water came pouring out the bottom onto the floor! I brought the box of mugs back and she settled for her 2nd choice. I'm so grateful this was not a sale. The mug was made with standard 112, using studio glazes, fired at the community studio which usually runs a high cone 6. My mugs have a glaze catcher channel around the bottom rim and I use runny glazes. The crack occured right at the glaze channel and ran halfway around the mug. Liquid in the mug literally pours out. Some folks at the studio looked at the mug and one thought was that the problem could be that the walls at the channel were very thin and a lot of glaze ran in there creating some tension. The thermal shock was too much for it. One person suggested that people should never pour boiling water into a room temperature hand thrown mug, but I feel like this is expecting too much from tea drinkers. If they can't pour boiling water into the mug, then it's not a useful piece for them. I have always poured boiling water into my and others' mugs without any problems. Has anyone had this or similar problems? I would appreciate any thoughts. Link to comment Share on other sites More sharing options...
GEP Posted November 20, 2015 Report Share Posted November 20, 2015 Yes. I once got a phone call from one of my galleries ... a customer reported that my mug cracked when it met boiling water. I was mortified! I sent out a free replacement, and the gallery sent back the broken mug. The crack occurred along a latitude near the bottom of the mug where I had trimmed it too thin. Until then, my priority was to make mugs as light as possible, because I think that makes a mug more comfortable to use. But I changed my mind. Now my priority is "evenly thick throughout." My mugs are a little heavier now. I agree with you that a mug should be able to withstand boiling water, so I am not willing to use that excuse. I can't really comment about tension caused by the extra glaze in your glaze catcher. My opinion is that the thinner wall was the sole cause. Link to comment Share on other sites More sharing options...
Mark C. Posted November 20, 2015 Report Share Posted November 20, 2015 I pour boiling water in my mugs every week-they should take this well. You have a thin spot in wall or foot or the wrong clay body or a tight glaze or something else. You may want to try this on some other mugs you have. I once did the same thing with hot water in a mug made with bad scrape clay from a friend the bottom blew out-that clay could not take the shock. Mark Link to comment Share on other sites More sharing options...
dhPotter Posted November 20, 2015 Report Share Posted November 20, 2015 One of the tests for glazes, I do, is put the mug in the freezer overnight, next day pour boiling water into the frozen mug. If cracking or crazing or pinging sounds - back to the glaze formula for another test. Link to comment Share on other sites More sharing options...
Babs Posted November 20, 2015 Report Share Posted November 20, 2015 Thinness, a word??, will do this. Have had this happen when trimming a higher that my normal foot on a mug and got tooo thin on the floor of the mug, crack first cuppa. Link to comment Share on other sites More sharing options...
flowerdry Posted November 21, 2015 Author Report Share Posted November 21, 2015 Yes, from now on all my mugs are going to get the boiling water test. I tested all the ones I have and found one more that didn't pass. Not as dramatic as the other failure but I heard the cracking as the water went in and although I couldn't find the crack, there was a very slow leak if I left the water in the cup. My mugs are also going to get a bit heavier. Keeps the tea or coffee hotter longer anyway, right? Link to comment Share on other sites More sharing options...
Lampman Posted November 25, 2015 Report Share Posted November 25, 2015 I've only made one cup, using it for years. Spirals is what I do, I wanted to make a spiral cup. This cup was made from the lower portion of a spiral lamp. Link to comment Share on other sites More sharing options...
Babs Posted November 25, 2015 Report Share Posted November 25, 2015 maybe not thinness, maybe unevenness of clay wall Link to comment Share on other sites More sharing options...
yss Posted July 8, 2020 Report Share Posted July 8, 2020 I bought a beautiful mug and a few weeks later, it would develop a sandy feeling all over the mug about 10 -15 minutes after pouring boiling water into it for tea. About a week later I noticed the sandy feeling was sticky, as if the honey in the tea was leaking through. What causes this odd thing? Link to comment Share on other sites More sharing options...
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