This answer is an edited version of an answer provided by Minds in Motion Concrete Contractors in Huston, Texas.
Companies pay large sums of money for acid stain recipes. A lot of times home-brew stains won't retain their color very long. Acid stain is not that expensive. Why don't you just buy some, thereby paying for someone else's trial and error?
If you are determined to make your own, you already have the building blocks with your knowledge of pottery glazes. Search with Google for Material Safety Data Sheets (MSDS) for acid stain and they will give you the approximate proportions to start with. For instance, a blue stain sold by Lambert Southwest in Texas lists phosphoric acid and cupric chloride as main ingredients.
BEFORE you even open the HCl bottle or heavy metals container, you had better read and understand fully the dangers and safe procedure for handling them, then FOLLOW THEM. Nobody has made a good blue acid stain that I am aware of. There are blue dyes and integral color available.
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