トラブルシューティング8If you don't know how to take care of it and leave it, it turns black!
From "Leather in Japan No. 4" published in 2011
Most stains on leather
washed away with water!
If you leave it without care, the dirt will accumulate and turn black.... Don't give up and think that you have no choice but to throw it away. According to Yasutomi, "In most cases, leather stains can be cleaned by washing with water.
People tend to think that wetting leather causes water stains. However, he says this is a big mistake. When leather is wetted with water, dirt floats to the surface. Therefore, it is true that if you wet a part of the leather, the floating dirt will collect in the creases and adhere to the leather, causing water spots. However, if the entire surface is wetted and the floating dirt is washed away, it will not become a stain. However, washing with water is not effective on leather that has been treated to prevent water from staining. Test the leather in an inconspicuous place to see if the water will stain the leather before you try it.
I have been using nubuck gloves for many years for my motorcycle. The owner says, "I don't know how to do it, and I have never taken care of them. The blackened stains not only look bad, but are also a concern from a hygiene standpoint.