Activated Carbon: Pine Sawdust Digested with Zinc Chloride and Phosphoric Acid, Activated at 500 C (McCullough, Darco, US 2,146,024)

Zinc chloride soaks into wood and pulls out its water as it is heated, so the wood chars into a sponge of carbon instead of shrinking into dense charcoal. McCullough found that a little phosphoric acid in the zinc chloride made the carbon far more active. Washed free of the salts, it is a strong decolorizing carbon. Result: activated carbon, relative efficiency 600 at 90 % decolorization. Source: McCullough, US 2,146,024 (Darco, 1939), Example 1. Picture drawn by Youblob.