Lime was a mineral. It could be produced by burning oyster shells. While under siege by the French, Catherine told Richard Sharpe to burn the oyster shells stored in the fort. Asking William Frederickson the outcome of this, he was told him it produced lime. Having grown up in a fishing village, William has seen fishermen whitewash their houses with lime. He also mention that an individual would go blind if he got lime in his eyes.
With this in mind, Sharpe had his men burn the shells and collect the lime. When the troops of General Calvet attacked, they threw the lime into the column, repelling the attack and allowing them to kill of soldiers without resistance. (Sharpe's Siege)