Asphaltum Varnish

Asphaltum, or bitumen, is a naturally occurring dark hydrocarbon. Ancient builders used bituminous materials for waterproofing and construction; painters later prized their transparent brown-black depth. Natural gilsonite from the Uintah Basin became a notable American form, identified in the 1860s and marketed at scale by the late 1880s.

Historic artists prepared asphaltum in more than one way. Some recipes dissolved it in turpentine and combined it with drying oil, resins, balsams, or wax. Others heated it first, then ground the residue into oil. Natural Pigments’ review of Mérimée and A. H. Church shows why the distinction mattered: raw bitumen could remain soft, stain nearby paint, and move on the canvas.

The nineteenth-century glass sign writer more often reached for fast-drying japan black. Callingham advised separating that dark backing from the leaf when it dulled the gold, then waiting until the coating hardened before cutting the edges crisp with a small chisel and straightedge.

Modern reverse-glass shops give asphaltum other jobs: removable resist for glue chipping and frosting, shaded warmth over leaf, and protective backing behind gilded or silvered work. Hand Lettering Forum archives also record screen-printing the resist, hand-filling missed “holidays,” and isolating asphaltum with shellac before solvent paint is added above it.