YEMA bronze watches develop a darker appearance due to the natural process of patina, they can create a build up of dark and/or greenish coating in areas of the watch depending to the degree of exposure to atmospheric conditions (heat, humidity, pollution) and when used during aquatic activities on the seaside (salty water) and swimming pools (chemicals). Most customers love this, as it adds character to the watch that becomes unique to each wearer.
However, one can easily reverse the patina and bring your bronze watch back to a healthy shine in just a few minutes. To do so, create a paste out of baking soda and lemon juice, apply an even coat of the paste to the bronze watch, then rub it away with a soft toothbrush or cloth after and allow it to sit for 20-30 minutes with the paste still on it. Remove the paste and rinse the watch under warm water. Dry and polish with a cotton cloth ensuring that all of the paste has been removed. If any patina still remains then repeat the process. Your bronze watch will now be cleaned and should look almost as good as new.