- Non-destructive techniques
- a sample does not need to be removed from the artefact for analysis.
- Destructive techniques
- a small sample is removed from the artefact for analysis.
Conservators use X-radiography (X-rays) on artefacts ranging from coins to mummies and paintings. The X-rays penetrate materials at different rates. It is the density, rather than the thickness, of the object that determines the strength of the X-ray used and the quality of the image produced. Just as in medical use, X-rays reveal the structure beneath the surface of an object and this can provide the conservator with useful information, such as:
- metal structure: indicating the technology used in manufacture; different metals used in construction; details of decoration hidden by corrosion
- mummies: revealing breaks and cracks which indicate fragile areas; the presence of amulets; different burial practices; bone structure indicating gender, age, or illnesses
- ceramics: structure and technology used in manufacture; contents, such as cremation remains in funerary urns
- paintings: structure; preparatory drawing; underpainting; presence of lead pigments; previous repairs
a sheet of paper used
This is mostly used for recording the watermarks in paper, especially where the mark is obscured by printing or drawings. The paper is 'sandwiched' between a sheet of plastic that has been impregnated with a radioactive form of carbon and a sheet of film that is sensitive to the radiation given off. It is left in darkness for several hours. Radiation passes from the carbon impregnated plastic, through the paper, to the sensitive film. More radiation can pass through the area of the watermark because the paper is thinner at that point, and so an image of the mark is made on the film.
Proton Induced X-ray Emissions (PIXE)
When a focused beam of protons (positively charged particles) is aimed at an object the atoms near the surface emit X-rays. These X-rays are detected and displayed on a graph as a series of peaks. The peaks each represent particular chemical bond energies, enabling a conservation scientist to identify the chemical structure of the sample.
Fourier Transform Infrared Spectroscopy (FT-IR)
FT-IR is a method of analysing the composition of organic materials based on the fact that every chemical bond has a characteristic energy level. In FT-IR an infrared laser beam is focused on a small sample from the object, which then absorbs energy. The energy that has not been absorbed is detected and displayed on a graph (spectrum) as a series of peaks. These peaks each represent particular chemical bond energies, enabling a conservation scientist to identify the chemical structure of the sample.
X-ray Fluorescence (XRF)
XRF is also based on characteristic energy levels. Here, it is the energy produced when an X-ray beam directed at the object causes the electrons (negatively charged particles) in an atom to jump to a higher energy level. As the electrons return to their original state they release energy characteristic to that element. This is detected and is used to determine the elements present. XRF is mainly used for the identification of metallic elements, such as the quantities of silver, copper, and lead in a coin.
Scanning Electron Microscopy
algae on a sculpture.
Optical microscopes use lenses to focus light to produce a clear magnified image. Similarly, SEMs use electromagnets to focus a beam of electrons that is directed at a sample. The focused electrons are detected and displayed on a screen. SEM is useful to conservation as it provides a greater depth of focus and higher magnification than the optical microscope and in addition analytical equipment can be attached for the identification of chemical elements.