White crystalline compound, made by purifying argol, used in baking (5,2,6)

I believe the answer is:

cream of tartar

(Other definitions for cream of tartar that I've seen before include "An ingredient of baking powder" , "Salt used in baking powder" , "Cooking ingredient" , "White powder used in baking" , "Substance used in baking powder" .)

I've seen this clue in The Independent.

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