Calcined Gypsum Cast Tracery Framework and Wet Channelling in Ottoman Revzen

The foundation of Ottoman plaster stained glass craftsmanship is alçı, a high-purity plaster of Paris derived from natural alabaster or gypsum rock (calcium sulfate dihydrate, CaSO4·2H2O). In the historic workshops of Istanbul, raw gypsum was calcined in wood-fired reverberatory kilns under strictly regulated temperatures ranging from 130°C to 170°C. This controlled thermal processing drives off three-quarters of the chemically bound water, yielding calcium sulfate hemihydrate (CaSO4·0.5H2O).

When re-gauged with water, the hemihydrate undergoes rapid dissolution and exothermic crystallization. Microscopic, needle-shaped dihydrate crystals radiate outward and interlock in three dimensions, creating an open, microporous solid that exhibits a unique physical property: upon setting, it expands slightly (between 0.2% and 0.5% by volume). This subtle expansion ensures that the curing plaster grips perimeter casting frames with zero shrinkage voids, producing crisp, razor-sharp tracery edges that never pull away from their structural perimeters.

While unreinforced gypsum exhibits impressive compressive strength (typically 12 to 18 MPa), its flexural and tensile capacities are notoriously low. Slender tracery ribs measuring only 15 mm across would shatter under the lateral wind gusts and seismic vibrations common to the Sea of Marmara region. To overcome this structural limitation, the imperial revzenzenân engineered an early composite material by embedding organic fibers directly into the plaster matrix.

The most prized reinforcement was keten elyafı (washed flax tow), followed by combed hemp and fine goat hair. Artisans combed the raw fibers to remove woody shives and dust, then washed them in mild alkaline potash water to eliminate natural resins that could inhibit gypsum bonding. When dispersed into the mixing water before the dry plaster was added, the fibers formed an omnidirectional fibrous lattice. This network bridges microscopic tensile stresses, granting the slender plaster tracery remarkable toughness, elasticity, and crack resistance.

Carving a revzen window is an exercise in precise temporal choreography. The craftsman cannot carve the plaster while it is still a liquid slurry, nor can they wait until it reaches its final, stone-like cure. The entire carving operation must occur during the fleeting cheese-hard phase—a window of approximately twenty to forty-five minutes after initial setting, when the plaster has lost its surface sheen, developed structural coherence, but remains soft and buttery to the touch.

Working swiftly with an assortment of curved steel gouges (oluklu ıskarpela) and flat scrapers, the revzenzen carves out the negative spaces marked by the cartoon. The cuts are executed with fluid, continuous strokes that remove clean ribbons of moist plaster without micro-fracturing the adjacent ribs. The artisan sculpts each rib with two distinct geometric zones: an inward-facing 45-degree chamfer designed to capture and scatter daylight, and a flat structural spine that provides depth and rigidity to the window assembly.

The final subtractive operation on the wet plaster bed is the excavation of the cam yuvası—the stepped rebate carved into the rear face of the tracery framework to seat the colored glass panes. Using a specialized L-shaped rebate plane or custom-ground scraper with an adjustable depth stop, the artisan routes a continuous channel 4 to 6 mm wide and 3 to 5 mm deep around the entire perimeter of each glass opening.

This stepped configuration is crucial to the window's mechanical stability. By recessing the glass within the depth of the plaster, the individual panes are shielded from direct shear stresses and structural racking. Furthermore, the rebate provides a clean, continuous ledge to receive the secondary bedding paste of fine gypsum, which seals each glass pane against air infiltration and prevents the rattle of glass during heavy city winds or palace ceremonies.