Iron is manufactured at the tatara by putting iron sand and charcoal together in the furnace, heating it, and reducing the iron sand. At this point, the impurities contained in the iron sand are melted at high temperature and drained out as slag. In tatara iron manufacturing, this slag is commonly called tessai. About half of the iron sand will be reduced and turned into iron through smelting. The remainder will react at a high temperature (1,200°C or higher) with the kamado (a term for the clay in the furnace walls) to create a fused silicate mass and melt out into iron slag. Put another way, in the tatara the iron sand is smelted as it eats away the kamado. The walls of the furnace are thinned out as a result, and work is brought to an end when the walls can no longer sustain themselves.

Chemical analysis of the iron slag reveals it to be composed of SiO2 (silicate), Al2O3 (alumina), FeO, Fe2O3 (oxided iron), and TiO2 (titanium dioxide). The chemical composition of a typical piece of tatara iron slag is shown below.

Figure: Chemical composition (mass %) of typical tatara tessai (smelting slag)

• There are differences in the amount of TiO2, arising from the differences in the amount of titanium contained in the source iron sand.
• A disparity in T. Fe (total iron volumes) results from the differences between the kera-oshi (from Yasukuni) and zuku-oshi (Sagaya) methods, which is why the iron yield is slightly better for zuku-oshi.
• SiO2, CaO, MgO, and Al2O3 are the main components in manufactured slag. Comparing their sum total, i.e. the volume of slag, the kera-oshi process (Yasukuni) contains 34.68%, compared to 40.98% in zuku-oshi slag (Sagaya). This also shows one of the characteristic differences.
• Given that oxidized iron is reduced in the sequence Fe2O3=>FeO=>Fe, it is possible to measure the state of the reduction process by comparing the ratio of FeO/Fe2O3. The ratio is 16 in the case of masa sand, and 65 in that of akome. From this we see that the reduction ratio is higher with akome, that is to say, the zuku-oshi method.

The characteristics of the tessai will differ also at different stages in tatara operations. The chart below offers an example from the zuku-oshi process. Note that the volume of such components contained in iron sand as TiO2 and V2O5 increases from the komori to sagari phases, while total iron volume is reduced. Also, note that reducibility is higher in the agari phase than in the sagari phase, as can be seen in the ratio of FeO/Fe2O3.

Figure: Changes (mass %) in the composition of iron slag with the zuku-oshi method (Sagaya tatara)

The presence of iron slag together with the remains of ironworking furnaces in the sites of iron-manufacturing ruins is considered important, because iron slag provides valuable information regarding iron-making technology. For example:

• Was the source of iron iron sand or iron ore? If it was iron sand, was it masa or akome?
• Did the iron slag result from the smelting process (smelting slag), or from when the smith worked the iron (smithing slag)? If it's from smithing, is it from the okaji (refining and smithing) or kokaji (forging and smithing adjustment) process?
• The level of refining technology (temperature, reduction rate, zuku-oshi, kera-oshi, etc.)
• Estimate of the period of operation based on the kind of charcoal used and carbon-14 analysis of the charcoal embedded in the slag

Other things might also be learned from the iron slag if research proceeds further. Manufactured iron objects often corrode or get reused to make new goods, with the result being that older objects are no longer extant. However, iron slag—often called the “waste of gold”—gets discarded nearby, so researchers may conjecture that the remains of some sort of iron-manufacturing site may be nearby a location in which iron slag is found. Moreover, in its oxidized form iron slag is extremely stable, so it will retain its shape and makeup for extremely long periods of time, making it a precious source of data.

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