firesetting

Firesetting was a primary method of breaking hard rock in underground mines before chemical explosives became available. A fire was built directly against the working face of the lode, heating the stone intensely. As the heated rock cooled (often accelerated by pouring water on it), thermal shock caused cracking and spalling. Miners then removed the broken stone with picks and hand tools, advancing the excavation into the ore body.

The technique relied on the principle that rock expands when heated unevenly; rapid cooling creates stress concentrations that exceed the stone's tensile strength. Success depended on the rock type, the fire's intensity, and timing of the quench. Granites and massive sulfides fractured reasonably well; layered or heavily jointed rock was less predictable. A single burn cycle might advance the face by only a foot or two, making progress slow compared to later blasting methods.

Firesetting was brutally inefficient and dangerous. Smoke and heat created lethal underground conditions; carbon monoxide and oxygen depletion killed workers standing near the burn. The method consumed enormous quantities of timber for fuel in mines deep underground, where wood had to be brought down by hand. Water for cooling had to be carried or channeled to the face. Despite these costs, firesetting persisted in European and other mines into the 16th and 17th centuries, especially where suitable rock types warranted the effort.

The term reflects the literal act: setting fire to stone. It appears in mining records from the medieval period onward. Glycerin-based explosives, and later dynamite and blasting powder, rendered firesetting obsolete by the 19th century. The method survives today only in historical archaeological context and in rare situations where conventional explosives cannot be used or are economically unjustifiable.