gassu

Gypsum — calcium sulfate dihydrate, CaSO₄·2H₂O — is one of the oldest building materials in human history. The Mesopotamians had extensive access to gypsum deposits in the river valley sediments and surrounding hills, and Akkadian texts record the substance as gassu — a plaster or calcined mineral used in construction and finishing. The root is connected to the Semitic *gbs* or *jbs*, meaning 'to plaster' or 'to coat,' which appears across related Semitic languages: Aramaic gipsa (גִּפְסָא), Hebrew geves (גֶּבֶס), and Arabic jiṣṣ (جِصّ). The Sumerian-Akkadian building tradition used gypsum plaster extensively: palace floors, wall finishes, and sculptural casting in the great cities of Ur, Nineveh, and Babylon were all executed in gypsum plaster, some of it surviving to be excavated by 19th-century archaeologists.

The critical chemical discovery embedded in the word's history is this: gypsum, when heated to approximately 150 degrees Celsius, loses three-quarters of its water of crystallization and becomes calcium sulfate hemihydrate — the powder known as plaster of Paris. When water is added back, this powder re-crystallizes, expanding slightly and setting into a solid mass in under an hour. Mesopotamian builders discovered this property empirically, likely by accident when gypsum-bearing rocks were used in kiln construction and the calcined powder was noticed to harden when wet. The discovery predates any chemistry — it is pure applied observation, a material science breakthrough made by anonymous Akkadian craftsmen.

The word passed from Akkadian through Aramaic into Greek as gypsos (γύψος), where it already carried both meanings current in English: the raw mineral and the calcined plaster. Latin borrowed it as gypsum without alteration, making gypsum one of the most direct Semitic-to-English transmissions in scientific vocabulary. The Parisian plaster trade — centred on the gypsum-rich hills of Montmartre — gave English 'plaster of Paris' in the 18th century, but the word gypsum had been in English since the 14th century, used in medical and alchemical texts. Medieval bone-setters used gypsum plaster for immobilizing fractures by the same principle that Akkadian builders used it for immobilizing stone blocks.

Today, gypsum is the world's most commonly used mineral in construction: global production exceeds 150 million tonnes annually. Drywall (known as gypsum board, plasterboard, or Sheetrock) is manufactured by sandwiching a gypsum plaster core between paper facings; it lines virtually every interior wall in the United States and most of Europe. Agricultural gypsum is applied to fields to improve soil structure and supply calcium and sulfur to crops. Medical plaster of Paris — though increasingly replaced by fiberglass casting tape — remains in use for orthopedic immobilization worldwide. The Akkadian gassu applied to palace floors in Nineveh is chemically identical to the gypsum board being nailed to wall studs in a suburban housing development today.