Realgar

Realgar is an intense red arsenic sulphide, highly sensitive to light and the environment, that progressively alters to pararealgar.

A brittle mineral, valued for its lustre but delicate to preserve. Light converts it over time into its yellow‑orange polymorph, pararealgar. Sun exposure degrades the original crystal structure and can end as yellow powder in the final stage. It does not alter to orpiment, as was long asserted. It occurs in hydrothermal parageneses.

Realgar from Baia Sprie is recognised by very red, lustrous, well‑formed prismatic crystals on pale grey matrix. Not all specimens show that habit, but those lots fixed a visual reference. Major historical finds, including the 2005 hoard, consolidated the canon for European cabinet realgar.

For centuries it was a rare and costly material, known as “the poison of kings”. With the Industrial Revolution, arsenopyrite made it common and cheap, and the poison moved from palaces to pantries to kill rats — and sometimes people. In the nineteenth century, arsenical compounds were widely used in dyes and bright green wallpapers, with numerous deaths linked to their toxicity. In 1836, James Marsh developed an unfailing method —the Marsh test— that transformed arsenic into gas and made it reappear as a metallic mirror. Since then, the poison stopped hiding.

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