Making Viridian — The Permanent Transparent Chromium Oxide Green

Viridian (hydrated chromium oxide, Cr₂O₃·2H₂O) is a cool, transparent, blue-green pigment first developed by Pannetier and Binet in Paris in 1838. It was the answer to a long-standing problem in painting: how to get a permanent, transparent green that would not darken, fade, or turn brown over time. Copper resinate turned brown; sap green faded; emerald green was lethally toxic; verdigris was unstable. Viridian solved all of these problems — it is completely lightfast, chemically stable, non-toxic as a finished pigment, and has a beautiful cool transparency perfect for glazing.

The pigment is made by heating potassium dichromate with boric acid. The boric acid acts as a reducing flux, converting the hexavalent chromium (Cr⁶⁺, toxic and orange) to trivalent chromium (Cr³⁺, green and stable). The resulting chromium oxide-borate glass is then dissolved in water to remove the borates, leaving pure hydrated chromium oxide — the vivid green pigment. The 'hydrated' form is crucial: anhydrous chromium oxide (Cr₂O₃) is a dull, opaque olive green used as an abrasive; the hydrated form has a completely different crystal structure that produces the vivid, transparent blue-green.

SAFETY WARNING: The starting material — potassium dichromate — is a powerful oxidiser and known human carcinogen (Cr⁶⁺). It is corrosive to skin and mucous membranes. Boric acid is a mild irritant. HOWEVER, the finished pigment (Cr³⁺ oxide) is non-toxic. Full PPE (gloves, goggles, respirator) is mandatory during preparation. Work in a fume hood or well-ventilated area.