Archaeometallurgy – Slag Analysis – Ferrous Metallurgy - Forming of Slag

Forming of Slag

Natural iron minerals are mixtures of iron and unwanted impurities, or gangue. These impurities are usually removed by “slagging” during the smelting process. Slag was removed by liquation, that is, the solid gangue was converted into liquid slag. Therefore, the smelting process must be operated at or above the temperature at which the slag is fluid enough to be removed from the ores.

Smelting could be conducted in various types of furnaces, and the condition within the furnace may differ, thus affect the morphology, chemical composition and the microstructure of slag. Take bloomery and blast furnace, which were two common methods for smelting iron, for example. In the bloomery process, a solid state of iron was produced. This is because the bloomery process was conducted at a temperature higher than that which the pure iron oxide could be reduced to iron metal, but lower than the melting point of iron metal; therefore, solid iron metal was obtained. Blast furnaces were used to produce liquid iron. Generally speaking, the difference between bloomery and blast furnace production is that the blast furnace was operated at higher temperature and a more reducing condition than the bloomery. Because of the higher reducing environment, which was achieved by increasing the fuel to ore ratio, more carbon reacted with iron ore, and thus resulted in the production of cast iron rather than the plain iron. Besides, the more reducing condition within the blast furnace also generated less iron-rich slag.

Many other factors also influence the composition and morphology of slag during the smelting process, in which the charcoal was exclusively added to the furnace, reacted with oxygen, and generateed carbon monoxide, which was responsible for reducing the iron ore into iron metal. The liquefied slag was separated from the ore, and was removed through the tapping arch of the furnace wall. This is called tapped slag. However, the methods of removing slag are somewhat different. Some slag may be left inside the furnace rather than being tapped, therefore resulting in various morphologies of slag, which serves as a useful indicator to investigate the smelting process and furnace type. Smelting, the flux, the charcoal ash and the furnace lining may also contribute to the chemical composition of slag, which could be useful to infer the yield of production.

In addition to the smelting process, slag may also form while smithing and refining. The product of the bloomery process is heterogeneous blooms of entrapped slag. Therefore, smithing is necessary to cut up and remove the trapped slag by reheating, softening the slag and then squeezing it out. On the other hand, the refining process is conducted to refine the cast iron produced in the blast furnace process. By re-melting the cast iron in an open hearth, the carbon is oxidized and removed from the iron. Liquid slag is formed and removed in this process.

To sum up, different furnace types, working conditions and processes result in various types of slags. Through the investigation of slag, either in the macro-scale or the micro-scale analysis, many archaeological questions about metallurgy may be answered. In the following section, the analysis of slag will be addressed in macro-analysis, micro-analysis and yield evaluation respectively.

Read more about this topic:  Archaeometallurgy – Slag Analysis – Ferrous Metallurgy

Famous quotes containing the words forming of, slag and/or forming:

    Opinions are formed in a process of open discussion and public debate, and where no opportunity for the forming of opinions exists, there may be moods—moods of the masses and moods of individuals, the latter no less fickle and unreliable than the former—but no opinion.
    Hannah Arendt (1906–1975)

    On their slag heap, these children
    Wear skins peeped through by bones and spectacles of steel
    With mended glass, like bottle bits in slag.
    Stephen Spender (1909–1995)

    Young people of high school age can actually feel themselves changing. Progress is almost tangible. It’s exciting. It stimulates more progress. Nevertheless, growth is not constant and smooth. Erik Erikson quotes an aphorism to describe the formless forming of it. “I ain’t what I ought to be. I ain’t what I’m going to be, but I’m not what I was.”
    Stella Chess (20th century)