Prout's Ink from Roberson and Co, water-based ink for use with Roberson's Automatic Pens.
Brown, semitransparent ink whose intensity, darker or lighter, is controlled by the porosity of the paper to which it is applied. The transparency can also be increased as desired by tapping the ink with water.
Prout's Ink consists of a mixture of VanDyke crystals, organic dyes and extracted from walnut shell, and ammonium carbonate in an aqueous solution. The ink is lightfast, but not waterproof. Delivered in a 30ml glass bottle.