The black mask has ten charges that are invoked at random against any combat opponents, not just slayer task monsters. A charge reduces the opponent's Defence level by 3-10. Once the charges are gone, there is no way to recharge it, although its Attack and Strength bonuses remain after being uncharged. The charge is only lost when initiating combat, so fighting a single opponent cannot be used as a way of uncharging it. As of 2 November2010, the charges can be removed with the "remove-charges" option. Note that the removal of charges is permanent and cannot be reversed.
With the release of Smoking Kills, players were allowed to craft a black mask and other slayer items into a slayer helmet, provided they had acquired the ability to make the helmets. Slayer helmets are valued because of their effects and the fact that they are not tradeable. The black mask has been steadily increasing in price due to the update which added the hexcrest and focus sight used in the making of the full slayer helmet which also requires a black mask to make.