Kelaniya traditional wood-fired brick clamp kilns: paddy husk insulation and draft holes?

Artisanal brickmakers along the Kelani River floodplain construct temporary clamp kilns holding 20,000 to 50,000 bricks without permanent walls. Sun-dried raw clay bricks are stacked in alternating lattice courses over arched combustion tunnels. The tunnels are packed with split coconut tree trunks and rubber firewood. The outer walls are sealed with a 10cm sacrificial layer of under-fired bricks plastered with wet mud and thick paddy husks (Dahayya). Once fired, the paddy husks smolder slowly, acting as an insulating blanket that retains heat up to 900°C across four days of continuous firing.