Monday, 17 July 2017

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.
Summary: Due to the unique geological processes and structures involved, cobalt ore is unlikely to form and 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 mineralise for glaze and lustre while the presence of lead-zinc provided for the transparent lead glaze that permitted the beautiful underglaze ceramics (6).

Why Kashan though?

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 partly or unweathered rock. They are calcareous 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. So while the washed cobalt can be transported a short distance, it's unlikely the clay would be transported.


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:

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. 

Saturday, 1 July 2017

block printing with rubber stamps

I'm playing with block printing again. I've acquired some sample bags from work. Typically the B sample bag for standards is thrown away. I saved a bunch, washed the orange dust off and decided to experiment a bit. There are block printing tutorials all over the internet. Most of them discuss the use of carved wooden blocks following the Indian tradition. The other half seem to focus on the carving of lino or rubber blocks. here

Not this blog post! Nope.

Today my criteria for a craft activity required not leaving the house and the ability to stay in my PJ's all day. So I had to come up with a project that used supplies already on hand. Rather than teach you how to block print, I'm going to discuss different methods I'm trying over the next couple of posts.

Block printing with rubber stamps.


You will need to:

1 - acquire a friend who sends you rubber stamp when she thinks you'll like them. Alternativly, head to Art Riot or your local scrapbooking store and buy up some rubber stamps sold for embossing card and whatnot.

2 - acquire some fabric. (Or save aome from being thrown out). Wash, dry and iron them.

3 - scrounge up some paint. I'm using left over Pebeo fabric paint from a previous project. You can also blend JoSonya's fabric medium 50x50 with acrylic paint.

4 - grab a foam brush or sponge. You need to apply a thin even coat of paint to the block. Having no sponge, I cut down one of my silk painting brushes. Apparently you can use a paint roller and paint pallet too.



Place your fabric on a towel or wool blanket covered table. Back it with newspaper to prevent bleed through. Dab a thin coat of paint on the block. Press firmly to the fabric and peel off catefully. Repeat many many times. Rinse all tools. Iron as directed by your paint. Done!

Learnings:

Rubber blocks work well and the stamps have well shaped wooden backs to help you press down.

The old Pebeo is a bit sticky and could probably be watered down. (Watered down works better)

Measurements won't necessarily work as you lose or gain space between applications.

The paint may need to be thicker to completely cover the bag number, but as it is, the printed design is light and flexible enough to be good on clothing.

It takes a while to cover a distance with a small stamp. (I listened to two Hello Internet episodes).

My bags aren't square so I get uneven prints where the paper won't quite fit.

Frequent stamp washing helps avoid accidental paint build up and a toothbrush makes a good scrubbing tool. (Must buy new toothbrush).

Rubber blocks backed with wood have an overhang making lining up the patterns difficult if you aren't looking directly down at the printing.

Next I'll try a clear plastic stamp kit I found at Kmart to see if that fixes my alignment issues.