I have a set of green, cracked glaze Japane tea cups. A totally unused one is pictured in Figure 1.
Figure 1: A clean, unused tea cup.
As I have used one of these cups several times with African red tea, the cup has begun to take on a stain, as shown in Figure 2.
Figure 2: A used and reddish/brown stained tea cup.
What method can remove this stain while preserving all parts of the cup, including the glaze?
For these types of cups with a cracked finish is the purpose is actually that over time, the tea fills up the cracks. This sounds a bit strange, but the same happens with porous Yixing clay teapots. The pores are filled with tea over time, which helps to improve the brew.
Given that you want to remove the strains I wouldn't use any detergent or soap. If that is sucked in the cracks it can be released when you serve tea in the future. This might negatively affect the flavor of tea.
To clean, I suggest the same method that's used to 'season' Yixing clay pots. You can cook the cup in hot water until the strains are gone.
A few suggestions that might help with the tea stains include rubbing with baking soda and water, or soaking in hot water and dishwasher detergent several hours, even overnight, before washing. Alternately, it may help to lightly scrub with salt or a paste of salt and vinegar (as salt may be able to get into cracks, supposedly will not harm ceramics, and can be dissolved out afterwards). Or a paste of corn starch and vinegar, which feels like a work around for just soaking in vinegar.
In any case, once you have cleaned the cups to your satisfaction, you might want to take steps to prevent this happening again - not just for aesthetic purposes, but from the way the color concentrated along the lines of the cracked glaze, it looks like maybe a potential safety issue if tea residues can't be cleanly washed out of the crazes, whether they can be seen or not. Of course, it depends on what you are willing to risk. A very thin layer of transparent or clear food grade epoxy might work, without spoiling the beauty of your cups (you should be able to see the pattern through them).
I would try bleach, at varying concentrations of bleach in water. I'd start with something like 1 part bleach in 50 parts water. The trick is that you want to bleach out the red color while not otherwise harming the appearance of the ceramic and glaze. Test with your worst-stained (and hopefully chipped) bowl until you get it right or give up.
I've had success in removing tea stains (although not Rooibos) from pottery mugs by using baking soda and vinegar. Put about a teaspoon of baking soda in the cup, and add white vinegar mixed with water (half and half). Swirl gently, and let sit for about 30 minutes. Toothpaste might also work, although I've not tried this method; use a non-gel type.
