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As previously noted, the absolute dating of slag is an often desirable outcome of the application of scientific techniques. The more traditional methods of dating, such as typological sequencing,

cannot be easily applied to slags and there is no current method of producing an absolute date for de-facto slag body material. However, slag can often collect and encapsulate material from its surroundings during the time of its formation, and it is these encapsulated exogenous materials that can be subjected to absolute dating methods.

The slag specimens examined in this study have had no scientific absolute dating methods applied to them, with any dates provided coming from association with typologically datable artefact collected alongside the slag specimens. However, there are a number of specimens in both the Tell Dhiban sets and the Armenian Garden set that exhibit the necessary prerequisites for the application of absolute dating methods.

There are two methods of absolute dating that can be successfully applied to slags, these are radiocarbon methods and thermoluminescence methods. Both of these methods will be examined in more detail below.

1.1.3.1 Radiocarbon Method

Charcoal is the most common fuel utilised in both bloomery furnaces and smithing hearths, and charcoal inclusions are commonly observed trapped within the internal structure of smelting and smithing slags (Fig.1.). This charcoal is approximately 95% carbon by mass and is often sealed within the slags structure, reducing the possibility of contamination over time by external sources.

These charcoal inclusions can often be good candidates for the application of radiocarbon dating.

Carbon has two naturally occurring stable isotopes these are ¹²C and ¹³C which make up 98.93% and 1.07% respectively of all carbon atoms in nature. Carbon also has a number of short-lived radioisotopes ranging from ⁸C to ²²C, and one long-lived radioisotope ¹⁴C or carbon-14 which exhibits a half-life of 5,730 years (Taylor & Aitken 1997). Carbon-14 is found in trace amounts in

nature and is formed via the action of cosmic rays on the Earth’s atmosphere. When cosmic rays interact with the Earth’s atmosphere they can generate neutrons, which can then be captured by the stable nuclei of nitrogen-14 (¹⁴N). This results in the transmutation of ¹⁴N to ¹⁴C by the following mechanism.

The newly formed carbon is rapidly oxidised to carbon dioxide which enters the carbon cycle. This radioactive carbon dioxide is then taken up by plants through photosynthesis where it enters the food chain and is incorporated into all living organisms in the biosphere. The quantity of ¹⁴C in the atmosphere, hydrosphere and biosphere is fairly constant, and the causes of variation are well understood (Bowman & Leese 1995). When an organism dies, the uptake of carbon-14 into that organism ceases, and its quantity will subsequently decrease at a slow rate through radioactive decay. As radioactive decay occurs at a known constant rate, irrespective of chemical environment or temperature, it can be used as a clock, with the difference in the predicted initial quantity of ¹⁴C compared to the current measured quantity indicating how long that clock has been running.

Radiocarbon dating can be used to date carbon-containing materials with an age range of approximately ten times the half-life, which is around 60,000 years (Aitken 1990).

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Fig.1. Two examples of charcoal inclusions in slag specimens from the Tell Dhiban set.

n+147 N →14 6 C + p

The images in Fig.1. show charcoal fuel fragments trapped within slag specimens from the Tell Dhiban set. There are a number of specimens in the Tell Dhiban sets that display encapsulated charcoal, which could potentially be used for radiocarbon dating. It was revealed in personal communications with Dr. Bruce Routledge that a small number of these specimens may be selected for radiocarbon analysis in the future to confirm or refute date estimates. Within the Armenian Garden collection held at Manchester Museum is a quantity of individual charcoal fuel fragments, that were collected from the same contexts as the slag specimens, these charcoal fragments may also be potential candidates for radiocarbon dating.

1.1.3.2 Thermoluminescence Method

Slags will often contain primary minerals incorporated from the local environment (Fig.2.), along with the secondary minerals which form artificially from the pyrometallurgical process. Primary minerals are termed relics (Haustein & Krbetschek 2002), and one of these relic minerals, quartz, can potentially be used to date the slag.

Quartz is a crystalline variant of silicon dioxide (SiO2), which exhibits a continuous SiO4 silicon-oxygen tetrahedra framework. Quartz can occur as many different varieties, and is very common in the Earth’s crust, it is actually the second most abundant mineral after feldspar (Morgan & Anders 1980).

When quartz is irradiated by either non-ionising radiation or ionising radiation, free electrons within the crystalline structure can be promoted to an excited state, lattice defects in the crystalline structure can then trap some of these electrons in their excited state. Over time these excited electrons accumulate in these electron traps within the quartz, due to the effect of natural background radiation. If the quartz is then heated, phonon excitation of the crystal lattice can

release these trapped excited-state electrons causing them to relax back to their to ground-state with the simultaneous emission of a photon of electromagnetic radiation. This type of emission is termed thermoluminescence, and is the basis of the thermoluminescence dating method.

When quartz crystals are incorporated into the body of a slag, the heat that they experience resets the accumulated excitation state, effectively resetting the electron traps. Then, over time, the dose of radiation received by the quartz, known as the palaeodose, begins to cause the accumulation of excited electrons within the crystal lattice once again.

Before thermoluminescence dating is conducted the radiation dose rate must be determined. This is accomplished through the use of alpha particle spectroscopy and gamma ray spectroscopy which can be used to assess the types and quantities of radionuclide present in the slag. Once the radiation dose rate is known, the accumulation rate of excited electrons can be calculated.

When the quartz crystals from a slag sample are heated or exposed to an intense monochromatic light source in the laboratory, the excited electrons relax to their ground state and light is emitted that is proportional to the accumulated palaeodose absorbed by quartz. The intensity and duration of this emitted light allows the amount of time that has passed since the electron traps were reset to be determined.

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Fig.2. White silicon-dioxide (quartz) inclusions within the body of two slag samples.

The images in Fig.2. show sub-millimetre sized quartz inclusions (white spots) trapped within the slag bodies of specimens from the Tell Dhiban sets (left image) and the Armenian Garden set (right image). These types of mineral relics are examples of potential candidates suitable for thermoluminescence dating.

Due to the complex and specialist nature of the absolute dating techniques examined above, and the associated high financial and time costs involved, routine application of these methods is neither practical nor cost effective.