Oyster shells

The Chosen Men burn oyster shells to produce lime. (Sharpe's Siege)

Oyster shells when burnt, produced lime that could be used to whitewash houses, but would also blind upon contact with the eyes.

In the novel, Sharpe's Siege, the American captain, Killick, gave this information to Sharpe.

While in the television adaptation, it was Catherine who told Richard Sharpe to burn the oyster shells that were kept in storage.

He did so and repelled General Calvet's attack by throwing it at his troops. (Sharpe's Siege)