Scientific techniques

One of the most important questions asked about any ancient object is just how old it really is.

There are a number of scientific techniques which can be used to date antiquities, but probably the best known and most frequently used is radiocarbon or 14C dating.

Radiocarbon dating can only be applied to organisms that were once alive and is a means of determining how long ago they died.

Radiocarbon dating is possible because of the existence in nature of a tiny amount of 14C, or radiocarbon, a radioactive isotope of carbon.

This isotope is produced in the upper atmosphere by the action of cosmic rays on 14N. This 14C combines with oxygen to produce carbon dioxide (CO2) and is taken in by plants during photosynthesis.

From plants this 14C is absorbed into the tissues of every living thing via the food chain.

Since it is radioactive, it is unstable and decays away at a known rate. While any plant or animal is alive the 14C lost by radioactive decay is constantly replaced through the food chain, but when that organism dies, no more 14C is taken in, and the amount present in the tissues goes down.

By measuring how much 14C remains in ancient organic materials, it is possible to calculate how long ago they died.

To do this requires extensive chemical processing to convert the carbon in the ancient objects to a form in which the very low level radioactivity (way below background levels) can be measured.

Most radiocarbon dating is carried out on bone or charcoal, as these are the organics that most frequently survive from the past, but many other materials can also be dated using this technique.

The British Museum no longer operates a Radiocarbon Laboratory. For further information about dating please see the links below.

Further reading

S Bowman, Radiocarbon Dating (British Museum Press, 1990).

For details on the science behind Radiocarbon dating visit Radiocarbon-WEB

The use of optical lenses to enlarge the image of the object allows a wealth of information to be obtained.

From the single lens magnifying glass through to more complex optical microscopes, visual examination enables materials and surface treatments to be identified and characterised.

The unprepared surfaces of objects can show details of colour, surface pattern and texture, tool marks, joins, repairs, wear, surface coatings, manufacture, corrosion attack and inscriptions.

Finer details, especially of metals, are observed by the examination of polished sections, using a reflected light microscope.

The preparation of these polished sections may involve polishing a small area of the object itself (taper section) or removing a fragment, mounting it in a resin block and then polishing and etching the surface of the block and sample together.

Stones and ceramics often reveal more information when light is passed though a very thin section, rather than being reflected from the surface.

Thin sections of ceramics and stones are made by mounting a small fragment of the material on a glass slide and then grinding and polishing it until it is only 0.03 mm thick – thin enough to allow light to pass through.

These thin sections are examined using a polarising microscope, allowing the observation of textural characteristics, and the identification of individual mineral grains from their optical properties.

Thin sections of organic materials, particularly wood, are made in a similar way and examined microscopically to recognise characteristic cell anatomy. This enables the plant or tree to be identified.

Radiographic techniques, similar to those used in hospitals and by dentists, are used in the study of archaeological artefacts.

As in medical investigations, radiography reveals internal details that would otherwise be invisible.

This renders it especially useful in museums because it is a non-destructive method of discovery that does not require the removal of any sample.

Radiography is a versatile tool.

It's used to detect flaws in heavy metal castings but can also be used to provide images of watermarks in thin sheets of paper.

It may reveal objects concealed in a soil block excavated from an archaeological site and can help curators identify heavily corroded artefacts.

It can also help scientists find out how objects were made, and it is very useful to conservators revealing cracks, corrosion and sometimes previous restorations.

How does it work?

When radiation (X-rays, but also gamma-rays, neutrons or electrons for example) passes into an object, some of it is absorbed, some is scattered and some may pass through the object.

This produces an image on a photographic film or fluorescent screen placed behind the object.

The amount of radiation that passes through will depend on several factors, including the energy of the radiation and the composition and thickness of the object.

As a result, it is possible to see the internal structure and identify breaks, joins, inlays of different materials and enclosed objects - amulets in the wrappings of mummies, just as a chest X-ray may reveal a broken rib.

Because the penetration of the radiation depends on its energy and on the thickness and type of material being irradiated, the radiation source has to be suitable for the subject.

An example is paper. Thin and not very dense, a watermark in the paper, perhaps obscured by ink, can be imaged using the low energy electron radiation from a Carbon-14 isotope source.

But this radiation would be far too weak to examine a bronze statue. For this, high-energy X-rays (250 kilovolts, for example) from an X-ray set or gamma rays from a Cobalt-60 isotope source would be needed.

Further Reading

J. Lang and A. Middleton (eds.), Radiography of Cultural Material, 2nd edition (Butterworth-Heinemann, Oxford, 2005).

R A Quinn and C C Sigl (eds.), Radiography in Modern Industry (Eastman Kodak Company, Rochester, New York, 1980).

R Halmshaw, Industrial Radiology (Chapman and Hall, London, 1995).

Raman spectroscopy is a technique that enables us to identify not only the materials used in the construction of an object, but also those used in decorating the surface.

They can also be used to identify corrosion products, e.g. rust, on the surface of the object.

The spectroscopy works by shining a laser beam onto the surface of the object.

Most of this light is reflected off unchanged however a small proportion interacts with the molecules in the material and is scattered.

The scattered portion of light, known as the Raman effect, is collected to produce a spectrum.

Each material has a unique spectrum associated with it and therefore each one acts as a fingerprint with which to identify materials.

However, the Raman effect is very small and until recently its use as an analytical tool was limited by a lack of suitable equipment.

Recent advances, particularly the development of lasers, mean that Raman spectroscopy is now widely used to analyse a wide range of materials.

These spectra can be used to identify a variety of materials from plastics to precious gemstones.

Raman spectroscopy is totally non-destructive and by using a microscope, spectra can be recorded from very small samples.

By using a special attachment on our laser Raman microprobe we can even analyse tiny areas directly on the surface of objects.

Further reading:

Hanni, H.A., Schubiger, B., Kiefert, L. and Haberli, S. Raman Investigations on Two Historical Objects from Basle Cathedral: The Reliquary Cross and Dorothy Monstrance, Gems & Gemmology, 34 (1998, pp. 102–125).

Edwards, H.G.M. and Chalmers, J.M. Raman Spectroscopy in Archaeology and Art History,Royal Society of Chemistry, London, (2005).

The scanning electron microscope (SEM) is used to study the materials, technology and manufacturing history of museum objects in great detail at magnifications over several thousand times higher than with light microscopes.

The SEM has a greater depth of focus and high resolution so that sharp images and the finest of details are revealed.

The object is viewed by scanning a beam of electrons onto the surface and collecting the various signals that are produced, which are used to form the image and to analyse the surface.

The resulting images are often grey in colour to emphasise the more interesting features.

Electrons provide a magnified image of the object's surface while X-rays are produced at the same time by the interaction of the electrons on the atomic structure of the material.

The x-ray spectrum composes the material being examined.

The spectrum is quite similar to that produced in X-ray fluorescence analysis, and the technique is called energy dispersive X-ray analysis (EDXA).

The distribution of the chemical elements across the sample surface can be mapped and compared to the electron image.

This provides greater understanding of the physical and chemical properties of the materials, the methods of manufacture and the condition of the object for conservation purposes.

We have two new SEMs in Scientific Research: a 'Variable Pressure' microscope with a large specimen chamber into which can be put whole objects such as pottery or gold torcs for direct examination.

The new technology VP SEM allows it to be used to examine any antiquity material directly, whether it is non-conducting such as glass, ceramics, paper, shell, charcoal and corrosion products etc. or conducting metals.

The second SEM is an ultra-high resolution microscope that is used to examine samples of materials and objects at much greater magnification in much greater detail in order to more fully understand the microstructure of the materials.

To examine the internal structure of a material, a polished cross-section may be prepared.

The microstructure and composition are locked into the material of an object as a record of the ancient manufacturing processes, which can de unravelled by such study.

Further reading

Joseph Goldstein et.al, Scanning Electron Microscopy & X-ray Microanalysis, (2nd edition, Plenum Press, 1981).

V.D. Scott & G. Love, Quantitative Electron Probe Microanalysis, (John Wiley & Sons, 1983).

X-ray powder diffraction is a technique which uses a finely collimated beam of X-rays to characterise the crystallographic structure of natural and manufactured materials.

The technique has been put to a number of uses in materials research.

However, in the cultural heritage field its most valuable application is in identifying materials such as corrosion on metals, inlaid decoration on jewellery and pigments on wall paintings.

An advantage of the method is that the sample size required can be extremely small – a few mm2 of grains of powder – and with some types of equipment, analysis can be performed directly onto the surface of small, flattish items.

All minerals and compounds have unique crystallographic structures so XRD can be used to precisely identify a mineral or compound by comparing its diffraction data against a database of known minerals and compounds.

X-ray fluorescence analysis and other methods of chemical analysis will provide information on elemental composition but XRD identifies the exact mineral, or mixture of minerals.

For example, white inlays on Anglo Saxon garnet brooches from Kent were sometimes made of shell and sometimes of chalk paste, which must originally have looked different from each other.

These materials are chemically the same (calcium carbonate) but XRD is very easily able to distinguish between chalk (the mineral calcite) and shell (usually aragonite).

Examples of the application of powder XRD in arts and cultural heritage: 

• identification of minerals, pigments, rocks, gemstones
• corrosion products on metals, inclusions and phases in ceramics, opacifiers in glass and enamel, niello
• salt growths on glass, stone and ceramics
• old conservation materials such as fillers and colourants

Crystallographic structure can be calculated using Bragg's Law

nλ = 2d sin θ

where n is 1, λ (lamda) is the (known) wavelength of the incident X-ray beam, d is the interatomic spacing (in ångstroms), θ (theta) is the angle between the incident X-ray beam and the lattice plane.

The identification of the material is made by matching its d spaces with the ICDD (International Centre for Diffraction Data) database.

Further Reading:

R. Jenkins and R.L. Snyder, Introduction to X-ray powder diffractometry (Wiley, New York, 1996).

Energy dispersive X-ray fluorescence (ED-XRF)

X-ray fluorescence is a technique of chemical analysis.

It has been called 'the curator's dream instrument' because measurements are non-destructive and usually the whole object can be analysed, rather than a sample removed from one.

The technique involves aiming an X-ray beam at the surface of an object; this beam is about 2 mm in diameter.

The interaction of X-rays with an object causes secondary (fluorescent) X-rays to be generated.

Each element present in the object produces X-rays with different energies.

These X-rays can be detected and displayed as a spectrum of intensity against energy: the positions of the peaks identify which elements are present and the peak heights identify how much of each element is present.

ED-XRF is accurate and fast (a result can be obtained in a few minutes), but it is not sensitive enough to measure low concentrations such as trace elements (i.e. those present at levels below about 0.1%).

However, it will quickly determine the alloy composition of a metal artefact and it can also be used to analyse some non-metallic materials such as ceramics and glass.

One limitation of the technique is that only a thin layer, less than 0.1mm, is actually analysed. This can sometimes give misleading results on corroded or plated metals unless the surface is cleaned.

Further Reading

Van Eriken, R.E, Markowicz, A.A, Handbook of x-ray spectrometry: Methods and techniques, (Marcel Decker Inc., New York, 1992).

Cowell, M.R, Coin analysis by energy dispersive X-ray fluorescence spectrometry (Ed's W.A. Oddy and M.R. Cowell), Metallurgy in Numismatics Vol. 4, (Royal Numismatic Society, London, 1998, pp. 448–60).