In practice many of the methods outlined above provide an element of visual examination as well as analytical data. The most common direct observation methods are:
The use of binocular, metallographic and polarising light microscopes in archaeological science is extremely common. Microscopy can provide a great deal of information about objects from finding details of their construction to looking at the preservation of surfaces. Advances in image processing mean that greater focal depth can be achieved in the examination and visualisation of surface detail.
The metallography of cross sections from archaeological objects can provide much information about the manufacture of metal objects, including working, annealing, temperature of melting etc (Scott 1991). Polarizing microscopes which have an adjustable polarising filter have been extensively used in petrographic thin-section analysis of archaeological ceramics, while fluorescence microscopes allow the visualisation of pigments excited by wavelengths outwith the visible spectrum.
In scanning electron microscopy a beam of electrons is rastered across the sample causing scattering and electron and x-ray interactions which are used to generate a visual image in a number of different ways. The detection of secondary electrons depends strongly on the topographical structure of the sample and thus can be used to examine surface detail. Backscattered electrons also reveal compositional differences since their number, and hence the brightness of the image, depends on the atomic number of the area of the sample they interact with. Various other detectors can be used, eg to measure absorbed current, and produce images of different quality dependent upon the conductivity and fragility of the sample.
Used at: NMS, Glasgow Earth Sciences, SUERC, Edinburgh Geosciences
Figure 16: INSERT: SEM image + caption
Conventional radiography produces two-dimensional images of the object, the intensity of the image depending on the amount to which incident X-rays penetrate or are absorbed by the different thicknesses and densities of the material(s) being x-rayed. X-radiography is extensively used to image archaeological materials of all kinds, particularly archaeological iron where very often the shape of the object is hidden by encrustations of corrosion and even when the iron itself is lost the form remains visible within the corrosion mass. Currently the change is from conventional film-bases systems to ones using various forms of digital recording. Medium and low-energy cabinet systems exist, while for large items or the technological investigation of artefacts made from dense metals such as silver or bronze higher X-ray energies are required with appropriate radiation enclosures. Gamma radiography is undertaken industrially using radioactive sources and may have applications for particularly large or inaccessible objects, while neutron radiography is a specialist alternative with advantages for some very X-ray dense materials.
Used at: NMS, AOC Archaeology
CT Image of 17th century pocket watch from shipwreck, revealing maker's signature. Copyright NMS
This technique uses X-rays to produce detailed 3-D images of objects. By rotating the X-ray source and detectors a cross-sectional “slice” of the object can be obtained while the object is moved through the array to generate a complete series of slices which can then be combined to create a virtual 3D image. Medical CT equipment has been suited to the imaging of mummies, including the production of facial reconstructions, while micro CT, using similar technology but on a smaller scale that allows more penetrating X-rays and higher spatial resolution, has been applied to complex objects.
Used at: Glasgow Western Infirmary, Edinburgh Royal Infirmary, Natural History Museum.
See also the ScARF Case Study: Data treatment and Interpretation