Visual Examination During Sample Preparation

During the sampling process it was observed that some of the specimens, particularly those from Tell Dhiban, exhibited a distinct and clearly different outer cortex that was found only in close proximity to an exposed outer surface (Fig.20.). The cortex appears as a thin layer, rarely exceeding 1mm in thickness, with a distinct and typically abrupt boundary separating it from the bulk slag material beneath. It has a light yellow-white colouration with a dull to waxy lustre and displays a strong mechanical adherence to the surface of the slag. The layer was also relatively hard and resisted mechanical abrasion and was not soluble in water or ethanol. This outer cortex is most likely formed by the accretion of local environmental particulates during burial, and subsequent post-depositional processes cause consolidation and hardening of this material into a relatively dense surface concretion.

Fig.20. A prepared specimen taken from Tell Dhiban set one (BQ43.10.71A) displaying the light yellow-white outer cortex, visible along the top edge.

Along with this outer cortex, it was also noted that some of the specimens displayed a near surface variation layer in close proximity to the exposed surfaces of the slag, this variation layer on occasion also followed crack boundaries to deep within the slags body (Fig.21.). This near surface variation layer in the material is observable as a slight variance in fabric colour and texture when compared to the bulk body slag. The variation layer extends no more than 3mm, with 1.5mm being

Unlike the outer cortex there is no definitive boundary between this layer and the bulk of the slag body, instead a diffuse threshold is typical. These layers were most likely formed through post-depositional processes which resulted in both chemical and physical alteration of the exposed slag surfaces. These processes can be variable and complex, depending on many factors both environmental and compositional. In the case of some particular specimens, these alterations were extensive, and followed cracks and vesicles deeper within the internal material, often causing further cracking and structural weakening. In other cases alterations were superficial or even non-existent. Whilst the sample selection process was under way a conscious effort to avoid obviously altered regions was exercised. However, in some specific cases the altered material was deliberately sampled for further examination and analysis.

Fig.21. A prepared specimen taken from Tell Dhiban set one (BQ43.10.71A) displaying the near surface variation, visible along the top edge, also visible in this image is the yellow-white surface concretion.

Through visual examination alone it was noted that all of the specimens displayed up to six distinctly identifiable components, based on both colouration and micro-morphological appearance.

For each case the colour and predominant micro-morphological attributes have been noted.

I) Firstly the black component; this material exhibits a spectrum of bulk structural features, with the micromorphology ranging from completely amorphous to granular, however, it typically displays a combination of both of these aspects. It is effectively opaque, displaying a vitreous and often greasy, submetallic lustre. This component represents the largest proportion of material in almost

every sample.

II) The second component is dark-grey in colouration, and can exhibits a range of lightnesses or tones. This component appears morphologically very similar to the black component, however, it appears to exhibits either a second interspersed phase or a distinct microcrystalline structure which is most likely responsible for producing this observable variation. This secondary phase or microcrystalline structure also gives the material a dull, almost waxy lustre and a fine grained arenaceous appearance. This dark-grey component can be found in localised regions throughout many of the specimens, the distribution of these regions appears to be random and does not seem to be diagnostic or representative of any process and is otherwise unremarkable.

III) The third component exhibits a dark-orange to brown colouration and a range of micromorphologies, it can display either a distinct or diffuse boundary and can be observed within or surrounded by either of the aforementioned black and grey components.

These three components together make up the bulk of the internal material of all of the slag specimens.

IV) The fourth component is the altered material, which includes both the surface concretions and the near surface variation layer as previously described.

V) The fifth component is in the form of metallic iron inclusions, which can exhibit a number of shapes and sizes, from microscopic polymorphic inclusions, possibly formed through the disproportionation of wüstite or from localised reduction, to larger millimetre size prills that may have been deposited as metallic fragments chipped or spalled from the bulk metallic workpiece.

Metallic iron, in the form of both larger millimetre sized prills and microscopic particulate inclusions, is present in the majority of the samples prepared for this study.

VI) The sixth and final component is that of a light-yellow to light-orange material that can be found occupying vesicles and cracks within the body of the slag. These deposits appear amorphous with a dull, waxy lustre and a soft, friable texture.

Whether these components identified by optical microscopy display any chemical difference remains to be seen, however, with the nature of slags being very similar to glasses, a small variation in trace elements can make a profound change in appearance, so these differences may be otherwise unremarkable.

A. BR43.32.68 B. BQ43.10.71A.C C. BQ43.10.71A.D

Fig.22. Examples of some of the features observed during visual examination. A. displays the dark-orange to brown colouration (left) and black (right). B. displays grey (left) to black (right) and a range of inclusions and vesicles. C. displays a range of grey tones along with surface concretions (bottom) and a large iron prill (top-right).