Little Memorandum of Tincture for ye use of Mr. Banks drawn by his friend &c Ct. Lauragais [sic]', ca July 1772 (Series 06.125) - No. 0004

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     It is the alum which serves in almost all Colours.  or if you don't make use of alum, it will be the vitriolic salt. which you make either in begining by the Vitriolic acid or by the dissolution of an alkaly which forms the Basis of your Tincture

     You must observe to leave the colored Dissolution, wth a Point of acid, as ye - natural dissolution of alum is constantly so.  For that dissolution Makes ye Violet Tincture Red --