Baked Brick Arch Rib Springing and Quick-Setting Gach Gypsum Mortar Construction

Unlike thick modern modular bricks, traditional Persian architectural bricks (khesht-e pokhteh or aajor-e nezami) are thin, broad, and square (typically 200mm × 200mm × 35mm). Sourced from alluvial clay beds in Varamin or Yazd, the clay is thoroughly washed to remove destructive sulfate salts.

Fired in traditional wood or oil kilns (koureh-ye aajor-pazi) at 1000°C, the bricks vitrify into a warm pale-yellow or buff-ochre stone. Their high surface porosity (12% to 16% water absorption) is intentional: it allows dry bricks to absorb water instantaneously from wet gypsum paste, creating a powerful capillary suction lock within seconds of contact.

In standard building trades, gypsum plaster is considered a finishing material. In Persian architecture, it is a primary structural mortar. Pure alabaster rock (CaSO4 · 2 H2O) is calcined in kiln pits at 150°C to drive off three-quarters of its chemically combined water, yielding dry bassanite (hemihydrate).

When mixed with water in small rubber bowls, Gach-e Zendeh (living gypsum) undergoes rapid, violent re-crystallization back into interlocking needle-like gypsum dihydrate crystals. The reaction is strongly exothermic, heating the mortar in the mason’s hand. Within 90 to 150 seconds, the paste transforms from a liquid cream into a rigid structural stone capable of bearing tensile shear stresses.

To spring an arch rib into open air, the mason begins at the wall-mounted Pa-kar footing. The mason scoops a dollop of Gach-e Zendeh onto the brick edge and presses it against the previous brick, angling the unit slightly back toward the springing point.

The mason holds the brick immobile with hand pressure for roughly 60 seconds. As the exothermic gypsum reaction peaks, the mortar achieves immediate adhesive strength. The mason releases the brick, which hangs unsupported in space. Repeating this sequence from opposing springing points, two parabolic brick cantilevers climb toward the vault center until meeting at the crown key brick (kelid-e tooyzeh).

Where two arch ribs cross at a Sar-kar node, bricks must be carefully notched or cross-lapped using specialized brick chisels. Masons pack the junction with high-density gypsum mortar reinforced with chopped flax fibers or goat hair to absorb dynamic shear stresses.

Because gypsum is water-soluble, historic Karbandi vaults must be strictly protected from roof leaks. If water penetrates the outer dome shell, gypsum mortar dissolves, causing instant catastrophic de-bonding. Conservation protocols mandate hydrophobic clay-straw (kahgel) waterproofing or lead-sheet flashing across exterior vault surfaces.