I picked up a low use Presto 6-quart aluminum pressure cooker last weekend. To test it out, I used plain tap water. It left a dark stain. What causes this? When I cook stuff in a PC, it doesn't stain like plain water did.
Is it a rule of thumb to add cream of tartar when running this type of test? I presume cream of tartar is how I'll get rid of the stain, but I'm open to other suggestions.

Is it a rule of thumb to add cream of tartar when running this type of test? I presume cream of tartar is how I'll get rid of the stain, but I'm open to other suggestions.
