Qasam and the Kashani cobalt

As outlined in previously, Kashan, Iran seems to be the initial production site of Blue on White ceramics. This is due to Kashan's proximity to a source of cobalt and other minerals useful in creating glazes. The Kashani cobalt originates from Qamsar aka Ghamsar which is located in the hills south of Kashan. Ghamsar is located proximal the the boundary between the Sanandaj-Sirjan Zone (1) (associated with the subduction of the Neo-Tethys Ocean) and the Urumieh-Dokhtar Arc. The U-D Arc hosts porphyry copper-gold deposits (2) as well as epithermal lead-zinc deposit (3).

Figure1: Geology of Iran - adapted from Geology of the Sari Gunay Epithermal Gold Deposit (4). The blue star marks the location of Kashan and to the south, Ghamsar (aka Qamsar).

The cobalt in Ghamsar is formed as part of a skarn deposit (5) resulting from the intrusion of a microdiorite into the Qom Formation. For non-geologists, the limestone in the Qom Formation is dissolved by the hot hydro-thermal fluids originating from intrusion of diorite, a volcanic rock. The contact between these two rock types metamorphises and recrystalises becoming the skarn deposit. The mineralology of a skarn is determined by the composition of the hydrothermal fluids and the limestone. The Ghamsar deposit appears to be structurally controlled, with the mineralising fluids travelling through faults. The cobalt is spatially associated with magnetite veins in the faults however the cobalt may precede the magnetite ore. The cobalt (smaltite) likely took the form of a silvery metal and would have been locally processed and washed before transport. Cobalt glaze requires less than 5% cobalt to achieve a rich blue colour. Thus it is a valuable and highly transportable product. Given the comparative weights, the washed cobalt, and other metals, would have been transported to markets in nearby cities.

Summary: Due to the unique geological processes and structures involved, precitipation of cobalt ore is rare and unlikely to be accessible at surface in a form useful for glaze production.

As a side note, the presence of copper-gold in the area would have provided minerals for glaze and lustre while the nearby lead-zinc deposits allowed for the transparent lead glaze that resulted the beautiful underglaze ceramics (6).

Why Kashan rather than Ghamsar?

We can see why ceramics are produced at Kashan instead of Ghamsar, the source of the ore, by consulting a soils map. Figure 2 indicates that the soils around Ghamsar are calcareous lithosols which are calcium rich, and often contain chunks of partly or fully unweathered rock. They are calcareous (calcium rich) as their parent rock is likely the limestones of the Qom Formation. Kashan, on the other hand, has a source of salt marsh soils. Salt marshes are a low energy environment which accumulate sediment over long periods of time. Marsh soils are typically fine grain and well sorted, these appear to be grey and low in organic matter. Small grain clays require less work for the potter to purify and create into fine earthenware. The clay would likely have been dried, ground and sifted before being utilised.

When creating blue glaze, cobalt commonly composes <5% of the overall blend (8). The low concentrations required make transporting the washed cobalt the short distance to the source of clay highly viable.

Figure 2: A map of the soils of Iran indicates which medieval towns would have superior clay. (7)

1: A new tectonic scenario for the Sanandaj–Sirjan Zone (Iran) 2005 A. Ghasemia, C.J. Talbotb Journal of Asian Earth Sciences.

2: Porphyry Copper Deposits of the Urumieh-Dokhtar Magmatic Arc, Iran. 2005. A. Zarasvandi, M. Zentilli, S. Liaghat. PGS Publishing, Linden Park, 2005. pg 441-452

3: The Ay Qalasi deposit: An epithermal Pb-Zn (Ag) mineralisation in the Urumineh-Dokhtar Volcanic Belt of northwestern Iran. DOI: 10.1127/njma/2015/0284

4: Geology of the Sari Gunay Epithermal Gold Deposit 2006 Northwest Iran. by J.P Richards, D. Wilkinson and T. Ulrich. Economic Geology. DOI: 10.2113/ gsecongeo.101.8.1455 . 

5: Ore Mineralization at Qamsar Cobalt Deposit:Skarn and Metasomatism Evidences .  Hadi Mohammaddoost, Majid Ghaderi, Nematollah Rashidnejad-Omran  The 1st International Applied Geological Congress

6: Islamic pottery, a brief history. Metropolitan Museum of Art.

7: Soils of Iran 1961. M.L Dewan, J. Famouri. Food and Agriculture Organisation of the United Nations. 

8: Ceramic Arts Network, accessed 31/12/2018.