Pure Calcined Gypsum Plaster Mold Casting & Cantilevering in Karbandi

Across the hyper-arid, treeless plains of the central Iranian plateau, monumental architecture had to be erected without access to the vast temperate timber reserves of northern Europe or the dense oak forests of the Alps. Timber was an expensive, scarce commodity reserved for doors, windows, and structural lintels. To span colossal bazaar arcades and palatial chambers, Persian builders looked down at the mineral soil beneath their feet: gach (calcined gypsum plaster).

The geology of Iran is blessed with boundless deposits of ultra-pure crystalline gypsum rock (calcium sulfate dihydrate, CaSO₄·2H₂O), formed by the evaporation of ancient inland seas. When this rock is burned in simple wood or brush-fired kilns at modest temperatures (between 150°C and 180°C), three-quarters of its chemically bound water is driven off, producing plaster of Paris (calcium sulfate hemihydrate, CaSO₄·½H₂O). When recombined with water, this mineral reconstitutes into solid crystalline rock within minutes, releasing gentle exothermic heat. This rapid chemical reconstitution became the technological cornerstone of the Persian vaulted universe.

In standard European masonry, mortar hardens slowly over days or weeks, requiring carpenters to erect massive, intricate timber frameworks (centering or falsework) to support stone arches until the mortar cures. In Iran, the absence of timber made full falsework impossible. Persian builders resolved this through gach-e zendeh (living plaster).

Gach-e zendeh is mixed quickly with minimal agitation. It begins setting within ninety seconds and hardens into load-bearing stone within three to five minutes. Crucially, as the gypsum crystals interlock, the plaster undergoes a microscopic volumetric expansion of approximately 0.5 percent, wedging itself tightly into surrounding brickwork. Using gach-e zendeh, a mason can stand on a ladder, butter the edge of a brick with wet plaster, press it against an existing arch in mid-air, and hold it with his bare hand. Within two minutes, the plaster sets, locking the brick firmly in place. The mason immediately steps onto the new brick to lay the next, cantilevering monumental vaults across open space without a single stick of timber centering beneath.

For the intricate three-dimensional ribs and curved facet panels of Karbandi vaults, master builders avoided the slow, difficult process of laying individual bricks overhead. Instead, they pioneered an ancient form of pre-cast fiber-reinforced composite engineering: gach-o-ney (plaster and reed).

On a flat floor in the workshop or courtyard, artisans constructed full-scale wooden and sheet-metal molds (ghaleb) matching the exact curved profile of each Karbandi arch segment. The craftsman poured liquid living gypsum into the mold, immediately embedding long, split marsh reeds (ney) and twisted palm or hemp fibers along the tensile axis of the rib. Within ten minutes, the exothermic reaction subsided, and the artisans demolded a lightweight, rigid, structurally reinforced arch segment. Hoisted up to the ceiling on ropes, these pre-cast ribs were slotted together like an enormous three-dimensional puzzle, joined permanently with dabs of fresh gach-e zendeh to form the soaring skeletal rib cage of the vault.

While living plaster provided the fast-setting structural skeleton, it was completely useless for fine surface art; its rapid set and brittle crystalline hardness caused steel carving chisels to chip and fracture the surface. For the breathtaking floral scrolls, geometric arabesques, and calligraphic inscriptions that adorn Persian ceilings, masters deployed gach-e koshteh (killed or mortified plaster).

To create gach-e koshteh, the artisan mixes plaster with an excess of water and kneads, whips, and rolls the slurry continuously with a flat wooden float for twenty to thirty minutes. This relentless mechanical shearing physically breaks and grinds down the nascent gypsum crystal needles as they attempt to form, preventing them from interlocking. The result is a miraculous, non-hydraulic paste that never chemically sets. It remains as soft, smooth, and spreadable as dairy butter for forty-five to sixty minutes. Applied as a five-millimeter skim coat across the vault facets, it allows the master stuccoworker (gachbar) to carve deep, fluid, shadow-catching relief with effortless precision before it dries into an unalterable porcelain-smooth finish.