A thirling is a physical marker or cut made in the rock of a lead mine to define the limits of a miner's allocated workspace, known as a set or pitch. Because historic lead miners often worked on a tribute system where they were paid for the ore extracted from their specific section, exact boundaries had to be strictly enforced. The mark prevented disputes between adjacent crews over who had the right to mine a particular vein.
Heard on the job
Keep your blasting on this side of the thirling, or the next crew will claim the ore.
Insider tip
Accurate boundary marks prevented underground turf wars between tributers working the same vein.