Fire-setting

photography — Fire-setting in **Georgius Agricola** (1556): **De Re Metallica**. Public Domain.

Fire-setting is probably the oldest technique used to quarry very solid rock. In the absence of stable tools and a drive, crushing rock was very strenuous and a battle of materials. The miner took several irons (chisels) with him on the shift, which all became blunt relatively quickly, after the shift he handed them in at the mine's forge where they were sharpened again. But the number of chisels he could carry when he went in was limited, and they were also expensive. So fire-setting was a very helpful alternative.

Fire-setting consists, as the name suggests, of setting a fire. At the extraction site, i.e. at the rock face that was to be worked, piles of plywood or firewood were piled up and set alight. Usually, the strongly heated rock was then sprayed with water or vinegar, which resulted in a sudden cooling. This process had to be repeated several times, depending on the rock. The method only works with certain types of rock and ore, but was used both to mine ore and to drive tunnels.

Fire-setting was used to loosen or blast the rock so that it could then be easily chipped away using mallets and irons. Most rocks are sensitive to heat because the components expand to different degrees. This causes so-called thermal stresses in the rock, which wear down the rock structure. Rapid cooling with cold water, the thermal shock, further intensifies this effect. As a result, the mining speed could be drastically increased. Disadvantages, however, were the large consumption of wood and problems with weathering. This method was mainly used in wood-rich areas. The process had to be repeated very often because it could only remove thin layers.

It was described at the beginning as the oldest mining technique, because traces in the flint mine Mur-de-Barrez in France show fire-setting already in the Stone Age. The ancient civilisations used it, for example the gold mines of ancient Egypt, according to a description by Diodorus Siculus. The method is mentioned several times in the Bible and was also used by the Romans, as described by Plinius. According to Livius, Hannibal used the method to blast his way across the Alps. In the Middle Ages, it was used all over Europe, in lead mining in the Gurgltal in the Tyrolean Oberland, in the Rammelsberg in the Harz, and in silver and tin mining in the Erzgebirge.