Calcined Raw Gypsum Slab Casting and Wet-Knife Carving for Qamariya Windows

In the high desert plateau surrounding Sana'a, geology has provided vernacular builders with a material of extraordinary purity: crystalline alabaster and gypsum. For millennia, Yemeni craftsmen have practiced the pyrotechnology of al-qasab (calcined gypsum plaster). Unlike Portland cement, which requires industrial rotary kilns firing at 1450°C, raw gypsum rock (calcium sulfate dihydrate, CaSO4·2H2O) requires only gentle calcination between 150°C and 180°C to drive off three-quarters of its bound water, converting into the workable hemihydrate (CaSO4·0.5H2O).

Traditional calcination takes place in conical stone kilns fueled by mountain scrub brush and dried cattle dung. The artisan stacks large alabaster boulders into corbelled vaults over a central fire chamber, firing the kiln continuously for 24 hours. The resulting calcined stones are pulverized by hand with heavy wooden pestles and sifted through fine brass sieves. Because the kiln temperature fluctuates naturally across the boulder stack, the resulting powder contains a diverse mix of fast-setting and slow-setting crystal fractions, creating a plaster that sets with unprecedented plasticity and workability.

Unlike European stained glass, where glass facets are held by pliable lead cames measuring barely 5 mm in thickness, a Yemeni Qamariya is built upon a monumental monolithic foundation: a hand-cast gypsum slab measuring 45 mm to 60 mm in depth. To cast this slab, the artisan constructs a semicircular timber formwork ring bolted atop a dead-flat basalt casting bed lubricated with animal fat.

Mixing al-qasab requires intimate tactile knowledge. The powder is sifted into a basin of cold well water until it forms a saturated mound. The craftsman stirs the slurry vigorously by hand, feeling for the exact moment when the mix achieves the viscosity of heavy cream with zero air bubbles. The plaster is poured into the mold in a single continuous lift and screeded level. As the chemical hydration reaction commences, the plaster warms significantly (exothermic crystal formation) and expands slightly (0.3% to 0.5%), locking tightly against the formwork perimeter and creating a dense, structurally sound stone blank.

The defining technical constraint of Qamariya craftsmanship is the fleeting green state. As the exothermic hydration heat peaks and begins to subside, the gypsum enters a magical intermediate phase lasting between 2 and 4 hours. In this condition, the plaster is no longer a liquid slurry, yet it has not fully cross-linked into rigid crystalline rock; it possesses the firm, cohesive consistency of hard cheese or cold beeswax.

If the artisan attempts to carve too early while the mix is wet, the plaster slumps and tears into liquid mush. If the artisan waits too late until full hydration cures the slab, the gypsum becomes brittle, glass-hard stone that shatters and chips under tool impact. The master carver must seize this brief working window, scribing the complex geometric layout and carving out thousands of delicate apertures with lightning speed and absolute precision.

To carve the green plaster cleanly through its full 50 mm thickness, traditional carvers utilize the minqash. Hand-forged by local highland blacksmiths from high-carbon tool steel, the minqash features a curved, double-beveled blade shaped like an elongated leaf or narrow spoon. The tool is honed on river stones to a razor edge.

Holding the minqash vertically like a dagger, the craftsman plunges the curved tip directly through the 50 mm slab. By rotating the wrist, the curved blade slices through the damp gypsum with zero lateral wedging pressure, following the scribed geometric layout lines flawlessly. The severed core of waste plaster is popped out from below, dropping free onto the casting bed. Cell by cell, the solid slab is transformed into a breathtaking, gossamer filigree of freestanding structural ribs, setting the stage for the optical beveling that will channel the brilliant Arabian sunlight.