bath is agitated. Often, after quenching, an iron or steel alloy will be excessively hard and brittle due to an overabundance of martensite. In these cases, another heat treatment technique known as tempering is performed on the quenched material to increase the toughness of iron-based alloys. Tempering is usually performed after hardening, to reduce some of the excess hardness, and is done by heating the metal to some temperature below the critical point for a certain period of time, then allowing it to cool in still air. History There is evidence of the use of quenching processes by blacksmiths stretching back into the middle of the Iron Age, but little detailed information exists related to the development of these techniques and the procedures employed by early smiths. Although early ironworkers must have swiftly noticed that processes of cooling could affect the strength and brittleness of iron, and it can be claimed that heat-treatment of steel was known in the Old World from the late second millennium BC, it is hard to identify deliberate uses of quenching archaeologically. Moreover, it appears that, at least in Europe, 'quenching and tempering separately do not seem to have become common until the 15th century'; it is therefore helpful to distinguish between 'full quenching' of steel, where the quenching is so rapid that only martensite forms, and 'slack quenching', where the quenching is slower or interrupted, which also allows pearlite to form and results in a less brittle product. The earliest examples of quenched steel may come from ancient Mesopotamia, with a relatively secure example of a fourth-century BC quench-hardened chisel from Al Mina in Turkey. Book 9, lines 389-94 of Homer's Odyssey is widely cited as an early, possibly the first, written reference to quenching: as when a man who works as a blacksmith plunges a screaming great axe blade or adze into cold water, treating it for temper, since this is the way steel is made strong, even so Cyclops' eye sizzled about the beam of the olive. However, it is not beyond doubt that the passage describes deliberate quench-hardening, rather than simply cooling. Likewise, there is a prospect that the Mahabharata refers to the oil-quenching of iron arrowheads, but the evidence is problematic. Pliny the Elder addressed the topic of quenchants, distinguishing the water of different rivers. Chapters 18-21 of the twelfth-century De diversis artis by Theophilus Presbyter mentions quenching, recommending amongst