Bronze tag badges are seemingly created automatically and awarded once the requirements are met. The requirements are described in the HelpCenter as:
You must have a total score of 100 in at least 20 non-community wiki answers to achieve this badge.
The mechanism has worked flawlessly on a couple of other tags but, unless I oversaw something, has failed for photochemistry despite a current total score of 111 on 42 non-wiki-answers.
Whether this results from some disturbances during site graduation or has other reasons is beyond my knowledge. Anyway, I'd like to see the issue solved.