Physics for Cultural Heritage : Need of the future generations
- Part 2 -

Dr Md Raheijuddin Sheikh *

Iron Smelting found in Wollega, Ethiopia

Slag is a term applied to the silicate complex formed in the bloomery process when iron ore is reduced in a smelting furnace. The main component of slag is the compound fayalite (Fe2SiO4). Slag may also contain gangue minerals from the ore, impurities derived from the fuel, and in some cases wustite, silica, and various reaction products formed in the smelting process.

Cinder is drossy solid material that collects on the top of molten slag. When removed, it resembles a mass of material infusible at the working temperature of the furnace, embedded in partially fused material. Thus cinder never reached a molten nor free-flowing condition in the furnace.

The physical structure of the slag from Tumu Hills, Manipur reveals the presence of both slag and cinder portions. Only a few slags show the presence of cinders while most of the slag does not exhibit any physical outlook which supports the presence of cinders. Depending on the conditions of furnace construction and the way of slag solidification either outside or inside the furnace, two classes of smelting slag can be discerned: tap slag (as in most European bloomery furnaces) and non-tap slag (as in prehistoric Southern African smelting furnaces). Two sub-types can be recognized.

1. Flow-type slag. Solidified from molten or semi-molten condition. Lava-like rippled appearance. Black, dense, smooth surface

2. Furnace-bottom slag, resembling a ‘flat cake’. Formed at furnace bottom, containing higher amounts of impurities from ore, fuel, and bloom. Irregular ‘coral-reef’ appearance often spongy and porous. Coarser distribution of matrix and grain than in flow-type slag

Most of the slag found in the area under consideration is of type 2 yet there are also some other slag of type 1. Solidified molten or semi molten condition are some peculiar characteristics of the slag of type 1 found in Tumu Hills. Irregular coral reef appearance often spongy and porous is the characteristics of the slag of type 2 as abundantly found in Tumu Hills, Manipur.

The high FeO-Si02 system in its primary melting phase can probably be represented reasonably well by the quasi- ternary Si02-FeO-anorthite phase diagram. The two predominant compounds in the slag systems are FeO and Si02, or in the primary melting phase wustite and fayalite, both of them distinct with low melting points when taken separately. One would thus expect slag of low melting point, generally below 1350°C.

XRF analysis shows high content of silica and iron oxide present in the slag. X-ray diffraction revealed the presence of fayalite (Fe SiO3) as major phase. Wustite and metallic iron were also found to be present in small amount in the slag. The phosphorous content has been found to be more compared to the conventional iron produced in India. The presence of Fe2O3 and also Fe3O4 were also recorded. In ancient times, high amount of P (0.05 to 0.5 wt %) has been reported to be present in the iron implements.

This has been attributed to iron production using Indian bloomer furnaces where limestone was not used. However, our studies revealed the phosphorous amount to be higher in Kakching slag, which indirectly hints about the use of limestone during the production of iron as the dephosphorization efficiency of fayalite/ wustite/ phosphate slag is, generally, lower that of iron bearing slag.

The XRF analysis indicates the presence of Al, Mg, Mn, S and P to be quite high in the slag. Fayalite can absorb oxides of manganese, magnesium and aluminium present in the gangue.

Slag is a by-product produced during smelting of iron. It contains Silica, Calcium Oxide and other impurities in the form of Silicate matter. As we know, iron and its alloys are the most used material in the world and they have made their mark in the development of human civilization which is popularly known as Iron Age.

The beginning of Iron Age varies from region to region. As for example, it is believed to have begun in the Middle East and South Eastern Europe as early as 1200 BCE whereas in China as late as not until 600 BCE. It continued form nearly 2000 years and helped in formation of powerful empires. In a recent publication, Rakesh Tewari reported the date and origins of iron-working in the Central Ganga plain and eastern Vindhyas. But in North Eastern India, we do not have the chronology of iron smelting.

The date of Tumu Ching, Manipur, gives simply a glimpse into the ancient iron industry in Manipur, whose beginning and end are yet to be known. However, this work clearly demonstrates that ancient metallurgists of Manipur were at work, smelting iron during the period 300- 500 CE. This is found to be almost contemporary with the TL dating age of three iron smelters located at the Birimi site in the Northern Region of Ghana.

The TL measurement of a piece of the sample collected from Tumu Ching, Manipur reveals that iron had been smelted at this site around 300 to 500 CE. The large quantity of slag found at the site suggests that iron ores must have been richly deposited in the adjoining areas and iron smelting must have been done at large scales. Proper investigation must have led to the revival of these technology at large scales. The dating of this ancient archaeometallurgical slag really contributes tremendously to the archaeological asset of Manipur.

Our TL dates of Tumu Ching, located practically on the border of the present Indo – Mayanmar border lies between 3rd and 4th century A.D. This work is just the beginning of establishment the chronology of the beginning and end of ancient iron smelting in the Indo – Mayanmar border where in ancient time must have been dominated by Tribes moving freely in this zone of North Eastern India.

Thus, the tribes present in the area of Kakching were using charcoal from the native sources for this iron smelting. The optically stimulated luminescence (OSL) and TL dating carried out on this sample gave the age to be 1600 ± 80 years. Like carbon dating, TL dating has been widely accepted by the geologists, mineralogists and archaeologists throughout the globe.

Accordingly, the dating of iron bearing slag was found to be around 400 AD. During this time the King Ura Konthouba was ruling Manipur and it is reported that the King used to give presents in various forms of swords to other Chieftains like the king of Manipur at Kangla, the kings of Tripura or Takhel Kings and even to the king of Kabaw (Myanmar).

The beginning of Iron Age varies from region to region. As for example, it is believed to have begun in the Middle East and South Eastern Europe as early as 1200 BCE whereas in China as late as not until 600 BCE. It continued form nearly 2000 years and helped in formation of powerful empires. In a recent publication, Rakesh Tewari reported the date and origins of iron-working in the Central Ganga plain and eastern Vindhyas. But in North Eastern India, we do not have the chronology of iron smelting.

The date of Tumu Ching, Manipur, gives simply a glimpse into the ancient iron industry in Manipur, whose beginning and end are yet to be known. However, this work clearly demonstrates that ancient metallurgists of Manipur were at work, smelting iron during the period 300- 500 CE. This is found to be almost contemporary with the TL dating age of three iron smelters located at the Birimi site in the Northern Region of Ghana.

An important aspect influencing the results of archaeo-metallurgical investigations is the statistical validity of the sampling methods and the accuracy of the testing methods used. These factors have to be taken into account in the planning, undertaking, and interpreting of archaeo-metallurgical work.

Concluded....