vitamin C test for staining

It’s likely a copper stain. If it starts off as light brown in color or tan and then, upon exposure to ascorbic or citric acid, it gets darker brown or black, then it’s likely copper. Unlike iron which has it’s solubility in water enhanced by the presence of ascorbate ions, copper actually oxidizes faster in the presence of ascorbate. There are other organic acids that are better at dissolving copper, namely sulfamic acid.

To be 100% sure, you should get a trichlor puck or put some dichlor in a sock and let it sit on the stain for a few minutes (NOT 30 mins) to see if it lightens. Using a potent chlorine source near a stain helps to differentiate between organic/biological and metal staining.
 
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