Making Orpiment Yellow Pigment — Grinding the Golden Arsenic Mineral of the Ancient World

MANDATORY SAFETY: Orpiment is an arsenic compound. Arsenic is a potent carcinogen and cumulative systemic poison — there is no safe level of chronic exposure. Before handling any orpiment, equip yourself with a P100/FFP3 respirator with organic vapour cartridges, chemical splash goggles, and chemical-resistant nitrile or neoprene gloves. Work OUTDOORS or under a fume hood. Prepare a wet-cleaning station with damp cloths — never sweep or use compressed air, as this raises toxic dust. All waste water from grinding and washing must be collected and disposed of as hazardous waste. Never allow orpiment dust to contaminate food preparation areas.

Obtain orpiment mineral specimens from a geological or mineral supplier. Good pigment-grade orpiment is a bright, warm golden-yellow with a distinctive pearly or resinous lustre on cleavage surfaces. It often occurs in thin, flexible sheets (orpiment has perfect cleavage in one direction). Avoid specimens that are heavily mixed with grey-white matrix rock or that show orange-red realgar (the closely related arsenic sulfide As₄S₄) — while realgar produces an orange pigment, it slowly degrades in light to yellow orpiment powder, making it unstable as a pigment.

Place the orpiment pieces in a stone mortar and add a small amount of water — just enough to keep the material damp. Grind WET at all times. Never grind orpiment dry — the arsenic-containing dust is extremely dangerous to inhale. Pound and grind firmly but carefully, breaking the mineral down to a coarse paste. Orpiment is relatively soft (Mohs hardness 1.5-2) and cleaves easily, so it grinds quickly. Continue until the paste is a uniform, bright yellow slurry with no large fragments.

Transfer the wet ground orpiment to a glass settling jar and add water. Stir gently, then let settle for 30 seconds — the coarse sand and matrix material sinks first. Carefully pour the bright yellow suspension through muslin cloth into a second jar. Let the second jar settle for 2-4 hours. The finest, most uniformly coloured particles settle last. Collect the settled pigment, discarding the wash water as hazardous waste (it contains dissolved arsenic). Unlike azurite, orpiment retains good colour even at fine particle sizes, so a relatively fine grind is acceptable.

Grind the settled wet paste on a glass muller with a small amount of water until perfectly smooth. Keep the pigment wet throughout — never let it dry on the grinding surface where it could become airborne. Transfer the wet paste to sealed glass jars for storage. When ready to use, the wet pigment can be mixed directly with binder (egg yolk, gum arabic). If drying for storage, do so in a sealed container in a well-ventilated area, and wear full protective equipment when opening. INCOMPATIBILITY WARNING: orpiment reacts with lead-based pigments (lead white, Naples yellow) and copper-based pigments (verdigris, malachite, azurite) to form dark-coloured arsenic compounds. It must NEVER be mixed with or layered adjacent to these pigments.