What is this powdery mineral on my glazed ceramics?

May 18, 2020 12:45 AM

Last week I repotted some plants and put one into a red glazed flower pot. Today I noticed that the pot had a spiderweb of white lines all over one side. It looked exactly like the glaze had crazed due to heat from the sun or something. But in fact the "cracks" were made of a powdery mineral. It rubbed off easily, leaving the glazed surface perfectly smooth.

A while back I noticed something similar on a glazed teapot I've been using. Except with the teapot, the lines are long and straight as if someone had scored the teapot with a sharp knife. And the mineral on the teapot is much harder and takes a lot of effort to remove.

I suspect it's salt or calcium from my tap water. What is it really, and why would it take on these patterns?

posted by henuani to Science & Nature (5 answers total)

I'd vote for calcium. The different patterns could be because in the open air the water on the flower pot dried more quickly and uniformly, while in the teapot it had time to run down first, drying from the top down. I've noticed that hard water deposits tend to form most clearly at the edges of the water spots - you can see it clearly on faucet tops, where they retain the shape of water drops. Diluted white vinegar will remove it easily, left to soak if it's a particularly stubborn deposit. Too much scrubbing may damage the glaze.
posted by I claim sanctuary at 4:18 AM on May 18, 2020

+1 vote for hard water. Get used to it, it’ll be a part of your life. Sigh.
Particularly annoying when it starts clogging up the shower head.
posted by Neekee at 6:07 AM on May 18, 2020

Hey, that happened to me with an African violet pot that I bought off Etsy! It was almost certainly something in either the ceramic or the glaze leaching out. We have a water softener, and I've never seen anything like that happen on any other item in my home, so I don't think it was anything to do with the quality of our tap water.

In any case, the pot quit producing the powdery stuff after a few weeks --- maybe after whatever it was finished leaching? There was no long-term change to the finish of the pot, as far as I could tell.
posted by slenderloris at 10:36 AM on May 18, 2020

Upon googling, I think what I (and probably you) saw was efflorescence.
posted by slenderloris at 10:39 AM on May 18, 2020

Is it possible that it was, in fact, a spiderweb that had fallen/been blown onto the pot, and then some ambient dust had settled on it, and a subsequent breeze or breezes had removed the dust that wasn't stuck to the spiderweb? I've actually seen that happen.
posted by bricoleur at 3:15 PM on May 18, 2020