Malachite Green: Fischer's Process, as the Oxalate (Thorpe, 1913)

The works process for malachite green as Thorpe's dictionary gives it: benzaldehyde and dimethylaniline condensed with zinc chloride to the colourless 'leuco base', oxidised cold with lead peroxide in hydrochloric and acetic acid to a deep green, salted out, freed of lead, and crystallised from oxalic acid as the glittering green crystals of commerce. Both acids must be present in the oxidation, and the peroxide must be exactly one molecule per molecule of base; too strong an oxidation gives a green that will not crystallise. Source: T. E. Thorpe, Dictionary of Applied Chemistry vol. V (1913), pp. 533-534. Picture: malachite green oxalate, photo W. Oelen (CC BY-SA 3.0).