Wind, ventilation and geometry of the shafts of ancient Laurion silver mines (Attica, Greece).

International audienceThe silver mines of Laurion were the most important mining district of ancient Greece during the fifth and fourth centuries BC. These mineral resources highly contributed to the power of Athens. Even today, mining, ore dressing and metallurgical remains cover about 120 km2. We focused on the most obvious traces left by the ancient miners in Laurion, namely the figure of the deep vertical shafts. In the southern part of the district, 4 km SW of Lavrio, fifteen shafts sinked in the Spitharopoussi plateau have been studied. They constitute the whole inventory of shafts in a given area. Among them, twelve are major deep shafts (65.6 m in average, 101.5 m for the deepest). One wonders how miners were able to breathe at such a depth. The location of the shaft heads, on the plateau surface, in roughly horizontal field, has only low level variations. According to the regularity of the geological structure, all these shafts are mainly open in the marbles of the Kamareza unit (Rosenthal and al. in this conference) with a broadly rectangular shaped section (average size : 1.35 m x 1.64 m). Subsequent field surveys and meteorological data, indicate uniform climatic conditions with the same distribution of wind directions. At ground level, against all the odds, the straight sections of shafts are not oriented according to the direction of prevailing winds, for the purpose of ventilation ; their orientation depends on the natural slope. This gives a clue that a particular plan was followed in the arrangement of surface hoisting workshops. These shafts revealed heterogeneous characteristics : different depths, twisted sections, traces of notches on the sides. They also revealed different techniques of ventilation for deep workings that could have been applied at different times. In three cases, an auxiliary shaft was located closed to the main one ; both are connected by one or more galleries (twin shafts or parallel shaft in literature). For the others, did the ancient miners take advantage of continuous blowing winds over the plateau to direct airflow into the depth, using different kinds of deflectors ? This study undertakes a return to the field, with new tracks, particularly on detailed study of surface hoisting workshops and requiring long term monitoring of climatic conditions over and under ground