Crackling coffee

February 17, 2009 6:30 AM

I bought two hand-painted ceramic mugs that appear to have been glazed. When I put my first cup of (very hot) coffee in one of them, I heard a faint crackling sound. The coffee also smelled a bit chemically. What's going on?

Are the mugs no good for hot drinks? Do I need to return them?

posted by anonymous78 to Grab Bag (11 answers total)

I can't explain the smell but I would return them. My understanding is that mugs need to be made of proper porcelain. I heard that until the British were able to able to produce domestic porcelain, they always added the milk to their cup before pouring the tea (and many still do).
posted by bonobothegreat at 6:42 AM on February 17, 2009

I would return them. I'm sure a coffee mug isn't suppose to crackle, snapple, or pop!
posted by Mastercheddaar at 6:46 AM on February 17, 2009

The motivation for milk first is up for debate.

If there's no visible damage to the mugs I'd try giving them a thorough wash and run through a few more mugs of boiling water and see if the problem persists.
posted by edd at 6:50 AM on February 17, 2009

Don't mess around with this--many mugs (especially hand-thrown ones) are glazed with lead glazes. If the cup cracked, it could be because it was improperly fired. It could leach lead.
posted by hardcore taters at 7:44 AM on February 17, 2009

I can't explain the chemical smell, but the sound may come from a hairline crack or air bubble someplace in the clay. I have mugs that "sing" when you pour hot drinks into them, and it's the adjustment caused by the heated air and clay expanding that makes the noise.
posted by Pallas Athena at 7:45 AM on February 17, 2009

Best answer: I'm a potter and I experience this all the time. What this means is that the clay and glaze are not compatible. In other words, the clay was fired to vitrification at a higher temp than the glaze, so they expand at different rates. It should have no side effects other than the fact the cracks will stain if you drink coffee or tea out of them on a regular basis. I would also advise against washing the mugs in a dishwasher, as the drying cycle will cause the glaze to craze even more. The smell supports my theory that the glaze doesn't have a good fix on the clay, and will probably not last very long, so keep an eye out for chips. In extreme cases, if he cup was in a cold cabinet and you poured boiling water into it, it might shatter, but even that will be a rare occurrence, I've had it happen maybe once in one thousand mugs.

The glaze is probably not lead based, because lead tends to be flexible enough not to have this problem and no legit potter would sell a mug fired with lead, it's very illegal and will open you up to lots of problems if you sell lead based glazes without proper markings.

Personally, if a mug I made did this, I'd take it back for a return and replacement, so contact the creator directly and check with them.
posted by 1f2frfbf at 8:26 AM on February 17, 2009 [1 favorite]

The glaze and the clay body expand at different rates, this causes crackled glaze.
posted by hortense at 8:30 AM on February 17, 2009

The coffee also smelled a bit chemically. What's going on?

Did you wash the mugs first? You really should.
posted by yohko at 8:51 AM on February 17, 2009

The same thing happened to me- a mug I'd had for years and had had no problem with developed a fine crack. I don't know if it happened in the microwave or if the crack was invisible and was made worse by the microwave. Anyway, I saw brown liquid bubbling from the crack when it was in the microwave, and it smelled HORRID. The mug was one of these. This product has no lead glazing, according to the website, but that smell....!
posted by Piscean at 9:14 PM on February 17, 2009

I´ve found that soaking handmade mugs in the sink for a while and running them through the microwave a day or two later does not have good results. Probably something to do with absorbed water turning to steam. Piscean, if your mug was unglazed on the bottom, that could be what happened.
posted by yohko at 9:37 PM on February 17, 2009

I can't explain the smell but I would return them. My understanding is that mugs need to be made of proper porcelain.

I think the best answer above covers what happened in this case, but I wanted to chime in to say that the statement I quoted, from the first answer in the thread, is completely incorrect. It may be true that fine china cups must be made out of porcelain to resist shattering, but it certainly isn't true about pottery in general. I drink hot black coffee out of an earthenware mug all the time.
posted by OmieWise at 8:24 AM on February 18, 2009