How to clean red tea stains from cracked glaze Japanese tea cups 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.