Post #334584 by The Gnomon on Mon, Sep 24, 2007 10:09 AM

Yet another approach...

Here's what I think I'd try.

First, a soak in Oxalic Acid.

Oxalic acid is used by rock hounds and people who mine their own quartz to clean the crystals that have been extracted from clay. Between the clay and the crystal a layer of yellow iron forms. To clean that off you let it sit in a bath of oxalic acid, sometimes two baths. I don't know what kind of crud is coating the inside of your mugs, but I have a feeling that oxalic acid might give it a run for its money.

Oxalic acid, BTW, occurs in tea leaves, spinach, black pepper, and lots of other things you consume on a daily basis. Of course, you wouldn't want to be directly exposed to it at the concentration necessary for use as a cleaner. It is also used as a rust remover, sterilizer, bleaching agent, purifying agent, and a host of other things, including cleaning ceramic tiles. That's why I think it would be safe to try it on a cermic mug. Wear gloves. Do this outside unless you want the inside of your house to smell skunky after your mugs are clean and sweet. The irony of it all. Are rust jokes considered irony?

Any cleaning products you find in the grocery store that specialize in rust removal probably contain oxalic acid. Anything that is a wood bleach/rust remover is probably oxalic acid. Crystal miners make up 5 gallon buckets of the stuff to soak their finds. A one pound bag of the stuff miners use (± $10-15) mixes with 2½ gal. of hot water.

Second, a soak in Sea Salt Brine.

I would do this regardless of whether or not I did an oxalic acid bath first. I'd rinse the mugs thoroughly, fill them to the rim with coarse sea salt, fill them to the rim with warm water, and let them sit for a few days adding water as it evaporates. If I didn't do the oxalic bath first, I'd let them sit for a week replacing all of the sea salt and water about halfway through.

Considering all you've done so far, I'm not sure a week's worth of soaking in salt will do the trick. Soaking for months probably would. Salt has a very cleansing effect. Good luck anyway.