What the wisdom is made of

Subsoil with the right clay content, shaped wet as cob or dried into adobe blocks. The oldest building material there is: most humans who ever lived slept inside earth walls.

Moist sandy-clay soil compacted in layers inside formwork until it sets rock-firm, leaving the strata visible like geology made by hand.

Limestone burnt and slaked into a putty that cures by slowly turning back into stone. The binder of most of the world's pre-cement architecture.

Iron-rich tropical soil that cuts soft from the ground and hardens in air into a rust-red porous stone. Quarried in blocks like giant bricks.

The igneous heavyweights: granite in the south, basalt in the Deccan. Quarried in slabs and blocks that outlast every other part of the building.

The grey-black limestone of Andhra's Kadapa district, split into large economical slabs; South India's workhorse shelf, floor, and lining stone.

Low-fired clay: tiles, pots, jaalis, and hollow blocks. Fired hot enough to survive rain, gently enough to stay porous.

Grasses, palm, and water reed bundled and layered into roofs and even whole halls; harvested from the landscape the building stands in.

A grass that grows structural sections in three to five years: hollow, jointed, and strong for its weight in both tension and compression.

The structural plant material: deodar and teak in the Himalaya and the south, cedar and pine elsewhere. Stores carbon for as long as the building stands.

The woody core of the hemp stalk bound in a lime slurry and cast or sprayed around a frame: a wall that is insulation, moisture buffer, and carbon store at once.

Finely ground fired brick mixed into lime, giving the mortar a gentle chemical set (a pozzolan). India's answer to Roman volcanic ash concrete.

The fine ash captured from coal power station flues, chemically eager to react with lime: a waste stream that behaves like Roman pozzolana.

Burnt rice hulls, nearly pure reactive silica. Every rice-growing district produces mountains of husk; controlled burning turns the residue into a cement enhancer.