Glossary Girl in the Spotlight

Craquelure

A network of cracks that develops in the paint and ground layers as they age.

Cross-section

A minuscule sample from the painting embedded in a block of transparent resin that is polished in a transverse section to reveal its layer structure. When examined using optical or light microscopy (generally at magnifications between 100-500x), different paint layers and pigments can be visually distinguished by colour, form, size, fluorescence and other visual properties. Ground, varnish and overpaint layers can also be examined. Chemical analyses can be carried out on certain layers.

Cusping

A wavy distortion of the weave along the edges of a canvas caused by it being stretched and attached to a strainer or stretcher using cords or tacks.

Dead colouring

The stage of the painting process in which light and dark tonal values are introduced and modelling is usually initiated. It is mostly monochrome, in brownish or subdued colours.

Dryer

An additive (often a pigment containing lead) that speeds up the drying of a paint or binding medium.

Fourier Transform Infrared spectroscopy (FTIR)

A technique that identifies chemical functional groups present in a sample by their characteristic infrared absorption patterns, which are then compared with reference spectra. The sample is exposed to infrared radiation so that the material’s radiation absorption pattern can be detected. In addition to organic compounds, some inorganic compounds that absorb infrared light can also be identified with this technique.

Focused Ion Beam / Transmission Electron Microscopy (FIB-TEM)

FIB-TEM can magnify up to one million times. In this study it was used to examine cross-sections at extremely high magnifications and differentiate the black pigments. The sample is prepared with the Focused Ion Beam (FIB) then the elements are mapped and analysed with transmission electron microscopy (TEM). 

Gas chromatography-mass spectrometry (GC-MS)

A method used primarily to analyse binding media: for example, oil in oil paint. The different components of a paint sample are separated using a gas chromatograph, then the molecular mass of each component is detected.

Glaze

A layer of translucent paint applied over another paint layer to modify its colour.

Ground

Preparatory layer(s) that are applied to cover the entire support. These layers give the support an even surface, and to modify its absorbency and texture. Its colour may also contribute to the tonal value of the painting.

Infrared

The infrared part of the electromagnetic spectrum ranges from 700 nm to 1 mm. The near-infrared range (780-3000 nm) can be especially useful for revealing materials beneath the surface of paintings. Depending on the material and wavelength used, infrared radiation is absorbed or reflected to a greater or lesser extent. For example, black carbon-containing material absorbs infrared radiation, whereas the white ground reflects radiation. The reflected light is made visible on film or computer screen. Infrared can also provide information about painting technique, changes made during painting, damages and retouchings.

Lake

Pigment derived from animal or vegetable dyes that are precipitated onto solid particles. When mixed with oil, it often produces translucent paints (red, yellow) that are suitable for glazes.

Lead isotope analysis

This method can be useful in determining the source of lead white pigment. The ratio of the four stable isotopes naturally present in lead ore (from which the pigment lead white is made) is indicative of its geological origin. By identifying the isotopic composition of the lead white it is possible to trace its provenance.

Lead soaps

A deterioration product caused by the interaction with lead-containing pigments with an oily binding medium.

Lining

An additional piece of canvas glued to the back of the original canvas (as part of a restoration treatment) to provide extra support.

Microscopy

Several different types of microscopy are used in the technical examination of a painting.
Stereo- or binocular microscopy is used to examine the surface of a painting. High-resolution digital microscopy (using the Hirox hybrid digital microscope) can magnify the surface up to 7000 times to show cracks, brushstrokes, and pigment particles.
Optical or light microscopy is used to examine paint cross-sections and dispersions under high magnification (up to 1000x), in order to reveal the paint build-up and to identify pigments. Paint cross-sections are examined in reflected light, under varying light conditions: bright field (BF), dark field (DF) and ultraviolet (UV).

Muller

A glass or stone tool with a flat base used to grind pigments or paint on a (stone) slab.

Retouching

Paint applied by a conservator or restorer to integrate damages in the original paint.

Sample

A tiny fragment removed from the edge of a damage or paint loss. The sample can be embedded in resin and polished to examine as a cross-section. It can also be examined using analytical techniques.

Pigment

A coloured powder that is mixed with a binding medium to create paint. 

Raking light

Casting light across the surface of a painting at a low angle to make its texture more visible.

Scanning electron microscopy / Energy Dispersive X-radiography (SEM-EDX)

A beam of electrons is used to scan the surface of a paint sample. The electrons scattered by the paint sample are collected and used to generate a black-and-white video image at high magnifications (up to 100,000x). Depending on the type of detector, this technique provides topographical information or compositional information. These images are especially helpful in the investigation of paint cross-sections. Pigments, which are often rich in heavy atoms like lead, contrast strongly with the paint matrix, which is rich in binding medium and contains mainly low atomic weight atoms. 
By coupling SEM with an Energy Dispersive X-ray micro-analysis system (EDX), scattered X-rays can be measured to determine the elemental composition of a particle, layer or small area of the sample. 

Scumble

Semi-opaque paint layer applied so thinly that it becomes translucent, affecting the colour it covers. It is usually a light paint applied over a dark layer.

Strainer

A framework onto which a canvas is mounted, which has fixed corners.

Stretcher

A framework onto which a canvas is mounted, which has corners that can be expanded with wedges or wooden keys to adjust the tension of the canvas. Stretchers were invented in the eighteenth century; before this, a strainer was used.

Support

The surface onto which the ground and paint layers are applied: in this case, canvas. In the 17th century, some artworks were painted on wood panels or copper plates.

Tacking margins

The edges of the canvas that have been wrapped around the stretcher or strainer, and tacked to the wood.

Ultra High Performance Liquid Chromatography (UHPLC)

This form of column chromatography can be used to separate, identify and quantify compounds found in mixtures (i.e. paint, which is a mixture of binding medium, and different pigments). In this study, UHPLC was used to identify organic dyes and pigments. 

Ultramarine disease

A degradation phenomenon that can cause a paint layer that contains ultramarine to turn lighter or greyish in colour, develop a wrinkled appearance and/or lose details.

Ultraviolet (UV) fluorescence

Exposing the paint surface to ultraviolet rays (UV) in the range 10-400 nm causes some materials to give off different coloured light (fluorescence). Some dyes and pigments fluoresce in this range, which can help in identification. UV is helpful in identifying later additions to a painting, including retouchings and varnish. Newly applied paint absorbs ultraviolet and appears dark, while aged varnish fluoresces. Paint cross-sections are also studied in UV.

Varnish

Varnish is a applied to a painting to saturate the colours and protect the paint. During modern restoration treatments, an isolating varnish is usually applied before retouching begins (to separate the original from restoration materials). A final varnish is applied at the end of treatment. Natural resin varnishes fluoresce greenish in UV. 

Wet-in-wet

Painting one colour next to or on top of another before the first layer is dry. Colours will be mixed whilepainting.

X-ray

A technique in which X-rays are passed through the whole painting. The varying absorption of the radiation by different materials creates a superimposed image on a film or screen. Areas that absorb more X-rays – depending on the atomic weight and thickness of the materials – appear lighter in the X-radiograph, while areas that contain radio-transparent materials appear darker. Besides being an important tool in assessing the condition of a painting, radiographs can also provide valuable information about the painting technique, changes made to the original design, and the construction of the support (canvas) and ground layers.