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36

time of the alpine orogeny. Aa we 11 aa cha silver chla dapoalc contalna •ora chan 60 ochar dlffaranc mlnerals whlch makes lt a paradiaa for tha geologlsc (Claval 1964). Ic raaeablaa nelthar tha formatlons of Sainte-Marie-aux-Mines ln tha Vosges nor thoaa found Łn Saxony. The allvar was dlscovered by chance ln 1767 by a shepherdess. Marle Payan, who collacted a lump whlch was racognlaad to be of silver and was aoon sought by tha aaateur prospactors. But lt was a metallurgist from Saxony. Johann-Gottfriaf Schrelber. who dlscovered the complex network of the ora flalds. The complexlcy of the argentlferous earths caused by alteratlon underllned the inadequacy of the flrst furnaces usad for thalr treatment. He substl-tuted smali shaft furnaces, whlch Immediately allowed an econony of 30% ln fual, than Scottlsh typa furnaces, and finally reverbatory furnaces whlch dld away with all the Intermedlary roaatlng stages and allowed further reductlon ln fuel consumptlon and an lncrease ln sllver recovery. Tha remains of thls process have been found at the most anclent sltes of metallurgy.

But thls region of the Olsans had other rlches, notably about ten kllometres to che south of the Chalanches deposlt, where Schrelber extrac-ted gold from a placer deposlt. Thls was then the only gold minę, apart from the gold-bearlng sands usually worked at thls period. Then there are the deposits at Pontet, near to the Gardette, and finally those of tha Grand Ciot, the minę of Pesey (really Peisey-Nancroix) 1300 m high ln the Tarten-talse mlning dlstrict (see Figurę), known ln antlqulty for 'Salluscius Copper* (Pilny) of the Ceutron dlstrict, although copper was exhausted ln Pliny’s cime. Buc the area was rlch ln argentlferous lead whlch the Romans also worked.

Morę recently the worklngs prospered between 1745 and 1791, and pro-duced ln total 14,670 tonnes of lead and 36,670 kg of silver. The minę, called Macot, ln a valley runnlng parallel to that of Pesey but hlgher, at 2,000 metres, was rediscovered by a local child, Prancols Pelissler, and Schrelber worked lt under the name of La Plagne Minę. Galena ln a gangue of ąuarcz and ba ryt es was washed downstream from the minę. The ore also concained a significant percentage of cerusite (lead carbonate) whlch helped the roasting and reaction process become standard practice. The metallur-glcal processes were carrled on ln an old saltworks bought by Schrelber at Conflans, at the confluence of the Isere and the Arly rlvers. He applled his experience acąuired at Pesey. notably the installatlon of a Scottlsh type furnace and a cupellatlon furnace.

Iron ores

Turning to iron ores, they are roasted when they come from vein deposits, and could contain sulphur, or they may be carbonates (siderites) or again they may be dense magnetltes whlch are difflcult to process. The breakdown of the anhydrous carbonate is an exothermic reaction when there Is an excess of air:

APeCOj + 0^---* 30 calories

The reaction evolves 65 calories per kilo of carbonate or 134 calories per kilo of Iron. Thls corresponds almost exactly to the quantlty of heat needed to raise a kilogram of Iron carbonate to a temperaturę of 400*C whlch ls the dissociation temperaturę of the carbonate. Theoretically with pure iron carbonate, once the furnace is heated and the flrst load of ore ls raised to the dissociation temperaturę by burnlng a little wood or cbarcoal, the reaction should continue indefinltely wlthout appreclable combustlon of fuel. But ln practice lt is not bo, as sonę calories are

necesaary co heat the ore and the gangue, to drlve off molsture from the ore, and to compeneate for heat loeses by radlatlon. conductton, and by convection with the fume.

It can be sald that with ore whlch is not too frlable and with shaft furnaces of helght and capaclty to suit the 3 i ze of the ora fragments and to avold crushlng, the avarage consumption of charcoal need not exceed 10 kg ^te”* of roasted carbonate, and say 7.5 kg te^-1 of crude ore. It ls the same for the roasting of magnetite ores, whlch could theoretlcalły proceed wlth-out appreclable expendlture of heat as the oxldlsatlon reaction ls exother-mlc (

*F*jO^ ♦    —-> ćPe^O^ ♦ 94 calories

Thls reaction evolvea 101 calories kg"* of magnetite and 140 calories kg^-* of iron (Angles d‘Aurlac 1930). For the same reasons as ln the case of carbonates, lt ls necesaary in practice to supply heat. Thls ls all the morę Import ant when one wants to achleve a coaplete reaoval of the sulphide for whlch a hlgher temperaturę is reąułred and whlch te often the Principal obJectlve ln roasting magnetite anyway as lt ls often mlxed with pyrites.

The orlglnal process of roasting ln heaps or stalls was applied to all spheroslderities (Black-bands). whlch provlded thelr own fuel, on account of the simpllclty of the process and the trlfllng cost of lnstallation.

The heaps are in the form of a truncated pyramld with a rec tangu lar base, whose helght must not exceed 2.5 m to facllitate che circulatlon of gases through the mass. The breadth depends above all on the slze of the ore fragments and thelr reslstance to the movement of the gases; lt ls usually 4-18 m depending on the ore. The length ls onły Limited by the tonnage of ore to be roasted and, naturally, by the available space; it can reach 60 m. The length of the process varles between 15 days and several raonths. Roasting ln stalls or in compartments depends on the cost of lnstallation. The flrst furnaces used were discontinuous ln operation: they were emptied after each roasting. whereas with continuous furnaces the roasted ore was removed from the bottom in such a way as to help the gradual descent of materiał charged at the mouth. The simplest furnaces of Sleger-land and those of Erzberg in Styria and of Carlnthla have been described elsewhere as well as those of Dannemora ln Sweden where tne air entry is controlled by holes in the plates of the openlngs for removing the roasted ore. The large cylindrical furnaces of Soamorostro, Italy, 10 m in helght and 4 m in dlameter at the base, are somewhat reminlscenc of those shown on some of the plaąues found at Lascours (see p.31).

It ls approprlate here to correct an error whlch has appeared ln several publicatlons on the early iron lndustry and whlch concerns the dis-covery in 1870 of the remalns of large furnaces in a brlck earth quarry at Lustin, 10 km to the south of Namur in Belgium. The furnaces were found at a depth of 70 cm beneath the foot of an outcrop of calcereous tuffa (Rochersde Frenes) and were apparently 4.3 m by 3.3 m ln dlameter. The two hollows were found about 10 m apart. They were erroneously described by Berchem (1872) as Iow bowl furnace. In fact the dlameter of the broad,

Iow bowl furnaces never attains these exceptional dlnensions. One can only interpret them as furnaces for the roasting of ores mined nearby.

Thls could include the ores found elther in the Civetien or in the Famennlen, or again in the substitution pockets along the length of the contact between the llmestone and the schistes forming a strlng of deposits at varying depth, now empty (Delmer 1913). Since at depth these ores usually change to iron carbonate with a varylng degree of pyrites, lt was necessary to roasc