A good way to clean bronze coins.. http://www.romanorum.com/docs/A Method for cleaning ancient coins. Romanorum.com.pdf
Based on the image in the OP, I'd say the seller of these coins is baking them in mud after they are found in order to generate sales (unsearched, unattributed) and curiosity. LOL. PS Looks like fun and nice work!
Lovely! Middle bronzes of this empress depict Vesta standing left, holding a simpulum and Palladium with and without an altar at her feet. The variety without the altar is the scarcer of the two. Here are the examples from my collection: