Minerals and Metals in Ancient India v1.c26.s6.t3: The Retort Residue or the Spent Charge: Results from the British Museum Research Laboratory

Minerals and Metals in Ancient India v1.c26.s6.t3

Language: English

The Retort Residue or the Spent Charge: Results from the British Museum Research Laboratory

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The retorts were found filled with somewhat less that 1 kg of spent charge most of which could be tipped out but some of which was fused to the walls, more towards the centre of the retorts than at the rear or front. The material was retrieved typically in the form of cindery pellets one cm³, most representative of the charge, and a fine dust fraction, which is essentially a mixture of the pellets and material that has spalled off the retort walls. This is evident from the analysis of the different components of the retort residue (Tables 18.2 and 18.3). The analysis of residue from different parts of the retort and also ash heap are provided in Table 18.4.

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X-ray diffraction and polarised light microscopy of polished thin sections (Freestone *et al.*, 1985a: 236) show the pellets to be composed primarily of carbonates (of Ca and Mg) but their state of aggregation and rigidity is imparted by a skeleton of silicate glass, in which are embedded numerous crystals of diopside. Fine grains of metallic iron were also seen. The EPMA or Electron-Probe Micro-analysis data (columns 1 and 2 in Table 18.2) show association of diopside CaMgSi₂O₆ crystal with some Fe, Mn and Zn. The glass is rich in zinc, containing up to fifteen weight per cent ZnO, and also contains higher proportions of Al, Mn, Na, K, Cl and P₂O₅.

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Fig. 18.5 Plot of Partial Oxygen Pressure versus Temperature for Selected Reactions. The Stippled Area represents the likely conditions within the zinc Reduction-Distillation Retorts.

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Zinc is markedly depleted at the closed end or top of each retort (vide Table 18.4), reflecting the movement of zinc vapour during the smelting process. In contrast lead does not appear to have been very mobile during smelting (Tables 18.3 and 18.4). Substantial amount of ZnO (2.18 %) in the retort body or wall (Tables 18.1 and 18.3) is due to the entry of zinc vapour and zinc-rich glass into the pores. The pore volume decreased in the outer and hotter surface of the retort and that is why the zinc content in the outer surface is much less (columns 3 and 4 in Table 18.1).

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Sodium in the spent charge could have been derived from gangue such as albite NaAlSi₃O₈, but high chlorine content (0.53 %, vide column 5 in Table 18.2) is less easy to explain. The fourteenth-century AD text Rasaratnasamuccaya (2.157) recommended the inclusion of salt in the zinc-smelting charge, an idea which was incorporated by William Champion in his AD 1748 British patent. Common salt would have served the zinc distillation process in two ways. Firstly, the soda vapour would help to sinter calcium and magnesium oxides and hence keep the charge within the retorts in a porous open texture. This Analysis of the Spent Charge from Zawar Zinc-Smelting Retorts

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| | (1) | (2) | (3) | (4) | (5) |
| SiO₂ | 54.36 | 50.50 | 20.82 | 40.48 | 42.48 |
| TiO₂ | b.d. | 0.28 | 0.15 | 0.27 | 0.17 |
| Al₂O₃ | 0.45 | 8.97 | 3.61 | 6.42 | 5.42 |
| FeO | 4.42 | 5.72 | 8.30 | 8.97 | 8.12 (Fe₂O₃) |
| MnO | 1.93 | 3.11 | 1.60 | 0.96 | 1.64 (MnO₂) |
| MgO | 14.10 | 0.28 | 10.16 | 5.94 | 11.16 |
| CaO | 21.85 | 6.43 | 24.54 | 15.25 | 24.54 |
| Na₂O | 0.34 | 7.13 | 0.34 | 0.80 | 1.28 |
| K₂O | b.d | 5.07 | 0.51 | 1.06 | 0.31 |
| P₂O₅ | b.d | 0.27 | n.a | n.a | 0.27 |
| Cl | b.d | 0.70 | n.a | n.a | 0.53 |
| S | b.d | 0.43 | n.a | n.a | 0.23 |
| ZnO | 3.44 | 12.45 | 0.61 | 1.22 | 2.28 |
| PbO | — | — | — | — | 1.72 |
| TOTAL | 100.89 | 101.34 | 70.65* | 81.37* | 100.15 |

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Silicate fractions: (1) diopside, from carrier material, EPMA electron-probe micro-analysis; (2) glass, from carrier material, EPMA; (3) example of carrier fragment, AAS atomic absorption spectrometry; (4) fine fraction from within retort, AAS; (5) overall analysis of the spent charge (Hegde, 1989: 92).
* Most of CaO and MgO present as carbonates, accounting for low totals.

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would minimize the chance of zinc vapour getting trapped in any one part of the retort and forcing that part to burst open. Little bit of the glassy phase helped in sintering by acting as skeletal adhesive; 0.7 per cent chlorine in the residue glass, with 7.13 per cent Na₂O (column 2 in Table 18.2) is probably close to saturation. (Freestone *et al.*, 1985a: 237) Secondly, soda vapour would react with the silica and alumina of the clay in the interior surface of the retort wall to produce a thin film of glaze. This glaze would prevent the loss of zinc vapour through the micropores in the thin clay wall of the retort. (Hegde, 1989: 92) The low level of lead oxide (1.72 %, *vide* column 5 in Table 18.2, also 18.3 and 18.4)

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Silver, Lead and Zinc Contents of Zawar Retort Residue Materials

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| | | %Ag | %ZnO | %PbO |
| Coarse fraction of carrier | (a) | 0.023 | 0.61 | 0.36 |
| | (b) | 0.029 | 0.65 | 0.03 |
| | (c) | 0.012 | 0.30 | 0.04 |
| Fine fraction of carrier | | 0.010 | 1.22 | 0.26 |
| Retort Body | | — | 2.19 | 0.03 |

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Table 18.4

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Analysis of Residues from Different Parts of a Zawar Retort and of Ash Heap

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| Sample No. | Pb(%) | Zn (%) | Fe (%) | Ag (ppm) | Cd (ppm) | Comment |
| 1 | 1.17 | 4.50 | 7.7 | 124 | 23 | Retort residue |
| 2 | 0.82 | 5.95 | 7.6 | 29 | 10 | Retort residue |
| 3 | 2.9 | 2.83 | 7.2 | 126 | 12 | " |
| 4. | 1.85 | 5.05 | 9.8 | 92 | 22 | " |
| 5 | 0.17 | 1.15 | 13.1 | 88 | 12 | " |
| 6 | 2.50 | 4.45 | 11.5 | 121 | 12 | " |
| 7 | 1.44 | 3.00 | 3.6 | 90 | 18 | " |
| 8. | 2.24 | 6.42 | 7.2 | 92 | NA | Retort bottom |
| 9 | 2.58 | 0.12 | 8.1 | 119 | NA | Retort centre |
| 10 | 2.40 | ND | 9.4 | 264 | NA | Retort top |
| 11 | 0.50 | 0.12 | 11.0 | 57 | NA | Retort top |
| 12 | 0.85 | 2.62 | 12.2 | 58 | NA | Retort bottom |
| 13 | 0.21 | 1.31 | 1.5 | 10 | 133 | Ash heap |
| 14 | 0.61 | 2.75 | 2.7 | 22 | 214 | Ash heap |
| 15 | 0.66 | 3.34 | 3.8 | 24 | 167 | Ash heap |
| 16 | 0.14 | 1.55 | 1.6 | 12 | 198 | Ash heap |

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in retort residue suggests that much care was taken in beneficiating the zinc ore.

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The numerous heaps of white ash in the Zawar area seem to represent the spent fuel ash removed from the furnaces (after a smelt) contaminated with some residue from the retort, while dismantling and cleaning of the set-up took place. The ash contains lower proportions of Pb, Zn, Fe and Ag compared to the retort residue, but is substantially enriched in terms of cadmium (Table 18.4).

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The R.O.M. in Zawar mine contains 3.5-4.4 per cent zinc and 0.014-0.02 per cent cadmium. Cadmium is reduced from its oxide at 600°C and boils at 767°C. Thus cadmium is likely to be depleted from the retort before zinc and not completely condensed. As oxide it is likely to find its way to the cooler parts of the furnace and absorbed in the spent fuel ash. (Freestone 1985b: 10)

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It may be noted here that we referred to zinc-rich copper ore of Ambaji and Basanthgarh in the previous chapter. The Ambaji ore contains 400 ppm cadmium which exceeds the value for Zawar ore.

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In addition to zinc smelting, Zawar yielded considerable evidence for the smelting of lead, in the form of at least sixteen deposits of slag, distributed along the valley of the river Tiri. We have mentioned these slags in our previous chapter and provided their compositions in Table 17.5. The glassy slag, not associated with zinc-smelting debris, assayed high in zinc content (7-11%) and contained melilite Ca₂ZnSi₂O₇, apart from olivine Mg₂SiO₄ and magnesium-rich clinopyroxene CaMgSi₂O₆. The crystalline slag was associated with zinc retort distillation debris, and contained lower proportions of zinc (less than 3%) presumably because this metal had been extracted or lead smelting was done after manual separation between sphalerite-rich and galena-rich portions of the rock.

Cites (1)

Rasaratnasamuccaya — Vāgbhaṭa (alchemist), Sanskrit