Optical Microscopy

The optical microscope should be the workhorse of metallurgy in archaeology for the range of compositional and structural data that it can provide. Should be, because compositional analysis using drilled rather than cut samples, or surface analyses using X-ray spectrometric methods still dominate some projects and much that is relevant to archaeology is therefore lost. Indeed, it has been demonstrated that in some circumstances optical metallography can lead to important archaeology that could not have been reached by any other methods.

Versatile though they are, optical methods have a spatial resolution limited by the wavelength of light and, as the power of the objective lenses is increased, a decreasing depth of field and field of view. Current developments in microscopy deal with this by highly accurately controlled motorized stages. When a sequence of images is obtained by stepping the stage along the Z-axis, software can add the in-focus parts of each image to produce a fully focussed image of all parts of an object or a sample surface. The most-fully featured versions of these systems permit a full set of photogrammetric measurements, such as contour maps, depth profiles, etc.; even just knowing whether a feature is a hole or standing proud of the surface. Scanning the stage along the X - and Y-axes enables mosaics of large samples to be produced, reviving the traditional technique of macroscopy, albeit at the cost of very large image files. With lenses optimized for digital cameras just coming into service the possibilities of quantitative analysis of metallographic images have never been better, and a position should be reached where microstructures are routinely recorded quantitatively, with the data treated statistically in the same way as compositional analyses. First steps in this direction have revealed its power and have given new insights into the history of a number of metalwork assemblages.

It is important always to examine a sample before etching: etching by definition damages the polished surface and can obscure or remove structural features such as corrosion products, inclusions, or gas or shrinkage porosity, or other manufacturing defects. With an optical microscope several forms of illumination are available, and at this stage it is plane-polarized light and dark field illumination, in addition to normal bright field incident illumination, that aids the characterization of these structures. Further identification of inclusions may be made using infrared and Raman microscopy. Many details of the original microstructure and alloy can be preserved in more or less fully corroded samples (Figure 1) and can be recovered using these techniques following careful sample preparation. Also, process variables, such as the overall amount of reduction a piece of wrought metal has received can be measured from the elongation of nonmetallic inclusions such as sulphides, calibrated by measurement of experimental test samples.