Luna Cornea was made by treating silver, or a silver salt in solution, with spirit of salt or with common salt, precipitating a heavy, curd-like white solid that, when gathered and gently melted or fused, takes on a distinctive waxy, semi-translucent, horn-like texture entirely unlike the hard, bright, reflective metal it came from, a transformation striking enough to earn the substance its descriptive name in Latin and in several European vernacular languages alike. Alchemists recognized this dramatic change of character, a lustrous metal becoming a dull, pliable, horn-like solid, as a genuinely notable demonstration of how thoroughly a chemical treatment could alter a substance's observable properties, a point of real interest within a tradition that took such transformations as evidence about the deeper, hidden nature of matter.
A property of luna cornea recorded by careful early observers, including Renaissance and early modern writers on minerals and chemistry, is that it darkens visibly on exposure to sunlight, turning from white or pale grey to a deep violet-grey or black over time, one of the earliest documented instances of a chemical reaction driven specifically by light rather than by heat or by contact with another reagent. This property went largely unexplained for centuries, simply recorded as a curious behavior of the substance, until it became, much later, the essential chemistry underlying the earliest practical photographic processes, silver halide sensitivity to light being the foundational reaction on which all silver-based photography would eventually depend.