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My glazed ceramic teapot has exhibited hairline cracks since I thrifted it. Tiny droplets of tea tend to bleed through, which probably means it isn’t foodsafe.
After a small shock to the pot’s exterior, two long thin shards have fallen out, leaving a larger cavity (image below).
I’m wondering what I can do to render this teapot usable, foodsafe, and leakproof. Is there a coating I can apply to the inside to prevent water from contacting the cracks and to prevent further damage?
Potter here. Those cracks are very bad, and are the kind that will continue to spread each time the teapot heats and cools; I doubt you have too many uses before it falls apart entirely. You should consider whether it's worth trying to salvage that teapot at all. Assuming you want to, though, I'll outline the methods.
Those cracks, by themselves, are not a food safety issue. Tea is fairly antiseptic, and just having some in the cracks doesn't really expose you to anything. It's pretty common to make teapots out of porous earthenware because nobody is worried about bacteria in tea.
You have three ways to seal those cracks: epoxy, silicone, and milk.
Epoxy: various food-safe epoxies can be used to fill in the broken areas. This will make your strongest and most durable bond, and will fill that divot nicely. However, it's going to be difficult to seal the hairline cracks, because there's no way to get the epoxy into the crack. You could consider cracking the teapot the rest of the way, and then epoxying the whole thing back together. Some people even do this with decorative elements in a technique known as kintsugi.
Silicone: there are also various food-safe silicone sealants. While these do not have as much binding strength as epoxy, they will fill that divot well, and possibly work better for smearing on the hairline cracks to seal them. You can also use a dilute silicone sealant to try to coat the inside of the pot. However, such sealants don't bind well to glazed surfaces, and may affect the flavor of the tea.
Milk: the casein in cow's milk has been used as a sealant for earthenware pottery, and for crazed glazes. While this won't bind the crack (you'll still need epoxy for that), it's obviously a cheap and easy way to seal the inside. However, the sealing ability of milk is disputed, and the milk technique is not generally used for vessels that will hold hot liquids.
There really isn't a foodsafe practical repair method.
If you had a pottery kiln you could attempt re-firing it, but if you had a kiln your time would be better spent making a new teapot and letting this one go on to future archeologists. And letting this one go on to future archeologists is definitely the practical, economical and food-safe approach without a kiln.
Take it to a dentist and have them fill the crack with tooth repair epoxy, the kind that's UV cured. A good dentist can even match most off-white colors, the repair should be invisible. I don't know how thermally stabile it will be, but it sure will be food safe, if the stuff can stay in your mouth for years.
Of course, the cost will be much more that the cost of a new pot.
Foodsafe isn't the issue. Burn-safe is.
Cracks in ceramic tend to spread, first slowly and almost invisibly, then catastrophically. There's a stress concentration at the tip of the crack, which you can't get glue into. Every time the pottery changes temperature, it expands or contracts, and that will tend to advance the tip of the crack.
One day, the pot will disintegrate while you are carrying it or pouring from it, and you may get scalded. Possibly very seriously.
A repair might be acceptable for a container of cold drinks, or even warm drinks. But a teapot full of almost-boiling water? I would strongly suggest its had its day. Buy a new teapot, and throw this one away, or keep it as a non-functional ornament if that's why you want to fix it.
I'm not a potter though I've fixed ceramics using epoxy resin before, and I'm facing a similar situation with a couple of pieces with cracks of varying sizes.
I'd love to get comments on this idea: you could coat the entirety of the inside of the pot with epoxy, and then use whatever glue works best on the cracks (e.g. runny superglue on hairline cracks, or more epoxy on the bigger ones). The epoxy 'lining' will be leak-proof, with the drawback of there being a lot of epoxy in contact with the liquid if that's a concern.
Just a suggestion: Stalactites and stalacmites.
stalactite water evaporates and fills microcracks with limestone.
You can search for someone online who has made artificial stalactites by dripping water through cement, and copy their process.
Perhaps it will take a few months depending on temperature, but a drip drip of stalactite water into the cracks will should seal it as well as a stalactite is glued to a ceiling. perhaps research stalactite hardness chemistry too lol.
