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I bought a cup on Etsy (that's in the likeness of the one John Dutton uses in the show Yellowstone.) It looks awesome but I noticed tiny pinholes in the glaze on the inside.

I know nothing about pottery but read a little online. The concern is that the holes are deep enough to connect to the inside of the cup and either draw water in or leach something out.

I guessed that if I were to weigh the cup, fill the cup with water for some time, empty and dry it, and weigh it again, then an increase in weight might confirm the issue.

I weighed the cup last night and it was 343.5 g. I left it filled with water overnight. I emptied and dried it thoroughly this morning. Now the cup weighs 360.6 g. That's a significant increase of 17.1 g.

But like I said, I don't know enough about pottery to know for certain. Is this normal for ceramic? Is there something I haven't considered?

I intend to use this cup daily and wasn't just looking to buy it for looks, unfortunately.

Below are images of the holes found in the glaze after opening.

tiny gap on side of inside pinholes on base of inside
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  • Hi Korgan, have you already brought it up with the creator on Etsy? Don't they have a policy for these kinds of (possible) shortcomings?
    – Joachim
    Dec 2, 2021 at 14:50
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    @Joachim Not before I confirm that it is, in fact, a shortcoming. I want to get unbiased opinions first so I can decide whether or not I should return it. Dec 2, 2021 at 14:56

2 Answers 2

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Should a ceramic cup gain this much weight after this test?

Good ceramic, probably not. Properly coated good ceramic, surely not.

Bad ceramic with a joke of a coat? Maybe possible. Especially if the cup is on the big-ish size, and if the coat is broken in really many places (it seems that it is), and if the clay is very porous (the underside might give a clue, but not necessarily).

To use the words of the song (see Elvis): "Return to sender". It might not even be usable as a pot for some flowers. Even flowers demand a minimum of quality from their environment.

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Ceramics are made of clay and inherently clay is porous. When the clay is first fired it goes from greenware to biqueware. At this point your mug would still be porous and allow water to be absorbed. Once you glaze a piece and fire it again, then this is called glazedware.

Depending on if the clay/glaze is low fire (cone 5/6) or high fire (cone 8/10) this can lead to water absorption and retention or a lack there of. I would ask the etsy artist who you purchased this mug from what type of clay/glaze they used. Is it low fire or high fire?

If they used low fire, then absorption is possible and normal. If you want a mug that is almost completely impervious to water then you would need a high fired mug.

So if you aren't happy that your mug isn't completely water proof then I would consider returning it. But if you're ok with drinking from a mug that is still porous and it isn't leaking all over the place while you have a cup of tea or coffee then I would keep it.

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    HI @Joachim I have edited my answer. Hopefully this is more useful Dec 14, 2021 at 21:38

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