Found my favorite kind of mug at a shop yesterday. It has a few hairline cracks that run through the wall, inside to out, but I did an overnight leak-test and there's no problem (yet). But I figure, since dried coffee/water mineral "seasoning" (residue) builds up fairly quickly in cups that aren't scrubbed, and is pretty tough stuff, why not use it as a possible crack filler? The ugliness pictured is a thick paste of instant coffee and water. I'll let it dry, scape off the excess without soaking or scrubbing, then reapply it a few times. Maybe the hardened "seasoning" will help fill in the surface of the cracks and keep them from expanding, or at least from developing leaks as quickly. Or not. I'll find out eventually.
Thanks, but updates won't be frequent unless it self-destructs. For an investment of 50ยข, I might as well give it a shot. Edit: Scraped off the hardened dried paste then gave a damp wipedown. All the cracks are now darker (but not any wider). Even a tiny pinprick chip in one of them is sealed over. Maybe this will work.
The best way to find if the cracks will leak is to clean and dry it, inside and out. Then pour boiling hot water inside. After a few minutes, wipe around the cracks, with a paper towel, to see if there is any moisture. Or just sit the mug on a paper towel with the boiling hot water, and see if there is moisture on the towel, after a while.
Milk protein is another method... https://coffeecherish.com/how-to-repair-hairline-crack-in-coffee-mug/
