"Lime is wonderful stuff", says restoration consultant Bob Bennett. "Absolutely wonderful! I love it." He drops his trowel into a tub of lime putty and scoops some out in a quivering, brilliant white lump. Then he mixes it with three parts sand to make a malleable, cohesive mortar. "It's sticky, too," he says, deftly tossing a bead of mortar onto a 15ft-high beam where defying gravity it remains.

Lime, says Bennett, has been the principal binder in mortars, renders, plasters and washes for thousands of years. The Romans were using it in the 1st century AD and they were certainly not the first. Portland cement, by contrast, has only been around since 1811, although it has managed almost completely to take over from lime. It is a hard, brittle and unyielding material, with little to recommend it save it's slightly lower cost - and it definitely has no place in historic building conservation.

"Lime is a living, breathing material," Bennett enthuses. Living because it can move slowly with a building without cracking; breathing because it respires moisture, avoiding the damp build-up characteristic of so many over-sealed modern houses. It's almost good enough to eat: "biscuits" of mortar are prepared so as to achieve a colour match against materials to be repaired; you "draw out the fat" as you smooth out a coat of lime render; and the putty feels far more like cream cheese than chalk!

Yet chalk, or more commonly limestone (both are forms of calcium carbonate), is where the lime cycle begins and ends. The limestone is burnt in a kiln to drive out the carbon dioxide and moisture, leaving behind quicklime, or calcium oxide. The cooled quicklime, which looks like white gravel, is then "slaked" by being added to water: in a vigorous reaction which sets the water boiling, it forms a putty-like emulsion of lime, or calcium hydroxide, which can be stored indefinitely in air-tight tubs. Once used and exposed to air, the lime absorbs carbon dioxide to harden, like the alchemical snake eating its own tail, back into the calcium carbonate from whence it came.