The yellow/blue cube in f102v2

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It is also interesting to note that ( winestone ) tartar solution was used in the Middle Ages to produce azure blue:

15th century Ink recipes by Jehan le Begue ( You are not allowed to view links. Register or Login to view. ).

29. To make excellent azure.—Take of sal ammoniac oz. iij., and of verdigris oz. vi., mix them together and make them into a paste with solution of tartar, and put them into a glass jar,
which you must stop up, and lute, and place in warm dung, and let it stand there for some days, and when you take it up you will find the green changed to excellent azure.