Arcane: Quicklime, slaked lime and whitewash

On an episode of Sharpe, he used Lime to fight off a French column. The effect seemed to be no worse than if they’d got talcum powder in their eyes.

Nowadays, with H&S issues, pupils are unlikely to meet these, once commonplace chemicals, so let’s go through the basics.

Chalk comes in various forms such as marble, limestone, seashells, eggshells etc. Chemically, it is Calcium Carbonate (not Blackboard chalk, Gypsum, alabaster or talc, which are forms or calcium sulphate).

When heated, the Carbon Dioxide is driven off, leaving Calcium Oxide.

Mostly obtained from Limestone, (In the early days of gas lighting, pieces of limestone were placed in cages to be heated in gas flames. The result was the stones glowed very brightly and could be used in theatre to provide the stage footlights, or Limelight.)

Derived this way, gave it its common name of Quicklime.

The Quick deriving from it being “alive”. I.e. it was very reactive and, when poured over French soldiers, would have reacted first with the water in their flesh, then with the fat and then by searing what’s left.

It’s effect is so severe that it has been used at mass burials to help reduce the possibility of disease from the rotting flesh.

As a lad, I was called on to whitewas our backyard walls, to do which, we placed quicklime in a metal bucket and carefully added water. The reaction is very exothermic and the water could be seen to boil as the water was added. (adding the quicklime to the water would have been as bad as trying to put out a fire in a chip pan by throwing water on it).

The quitened mix would be stirred with a disposable piece of wood and then left to cool.

The slaked lime (Calcium Hydroxide) was still dangerous as it is a strong alkali and a small speck in the eye could blind one. (alkali’s combine with fats in the flesh to form soaps).

The slaked lime couldn’t be left as it would pull carbon dioxide out of the air and turn back to chalk, so it had to be applied to the walls the same day.

The whitewash, applied to the walls with an old yardbrush (but without goggles !) , had a threefold effect. It killed crawlies and moulds that thrived in the brickwork, sealed the brickwork with a weather resistant coating and “painted” it a bright white, to reflect the Sun into the patch of garden.



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