Collector’s Guide to Scientific Lab Art Tests
An experienced art appraiser can advise which tests are suitable and financially practical for your art. We understand this list is intimidating, but are glad to explain it to you in plain English.
Carbon 14-Radiocarbon Dating – Extremely useful for items over 200 years old. Ideal for antiquities. Cannot give a precise date, but a range of years. Excellent for textiles, wood, leather, ivory, paper. Does not work on metal or ceramics.
Dendrochronology – also known as “tree-ring dating”. By measuring the widths of individual growth rings in a sequence visible in the end grain of a wood panel, it is possible to match the ring pattern against a master chronology and give a date for the last ring measured. There is also a test measuring the deterioration of wood cells. These shrink at a known rate and are compared to previously dated wood.
Infrared photography, infrared reflectography – infrared radiation is too long in wavelength for the eye to see, so imaging techniques must be used to make it visible. This is frequently used to “see through” paint layers that are opaque to the human eye. Infrared radiation passes through the paint until either it reaches something that absorbs it or it is reflected back to the camera. Carbon black is highly absorbing, so if an artist began a painting by drawing the design in black on a white ground, an infrared image can often make this underdrawing visible. Infrared photography uses special film that can then be printed like normal photographs. However, the narrow range of wavelengths used in infrared photography limits the pigments that is possible to penetrate. Infrared reflectography is capable of recording a wider range of wavelengths, allowing for more even penetration of different colors.
X-ray – also known as X-radiographs. X-ray images are made by placing X-ray sensitive film on the surface of the painting and transmitting X-rays through the painting from underneath. Larger paintings require several sheets of film; the resulting image is called a X-ray mosaic. Painting materials are more or less transparent to X-rays. Denser materials, such as lead-containing pigments or iron tacks used to pin canvas to stretcher, block the penetration of X-rays and appear white in the X-ray. X-ray reveals loses to a painting, and changes that may have occurred at different stages in the painting’s development. X-ray can be difficult to interpret, because the image shows all of the layers of the work superimposed.
HPLC – high performance liquid chromatography is one of several techniques employed for separating mixtures, allowing more precise characterization of organic materials in complex paint samples. HPLC uses liquid to carry a chemically modified sample through a column of porous material, causing the molecular components of the sample to separate according to their chemical and physical properties. When molecules emerge from the chromatograph, a mass spectrometer is used to provide information on molecular structure of each component.
EDX, SEM-EDX – a scanning electron microscope (SEM) is capable of revealing details of objects at magnifications up to 100,000x far greater than is possible with the optical (light) microscope. The SEM uses a focused beam of electrons to scan the surface of a sample, forming a very detailed image of its three-dimensional structure. SEM instrument usually have analytical devices attached to them. The most useful for paint analysis is an energy-dispersive X-ray (EDX) spectrometer. This works on the principle the electron beam which falls on the sample generates small amounts of X-rays, which are characteristic of various elements in the sample These results must be combined with optical microscopy to determine which pigments are present.
The above tests can be used for paintings, drawings and sculpture. Other age tests are also available (Carbon 14 and Thermoluminescence)