Glossary of Conservation Terms

aqueous lining: A lining process that uses an aqueous, or waterborne, adhesive such as glue, paste, or a glue-paste mixture. See also: lining.1

commercially primed: Supports, such as canvas, prepared with sizing and ground layers in a factory or workshop of a color merchant instead of being primed in the artist’s studio.2

cross section / cross-sectional analysis: For this type of analysis, a microscopic sample—including any combination of support, preparatory layers, paint, and surface layers—is taken, usually from the edge of an artwork or an area of loss, and mounted in a transparent resin. The resin block, once hardened, is ground and polished, exposing all the constituent layers of the sample in one plane. Cross sections can be observed and documented with a microscope in reflected light and ultraviolet illumination; they can also be subjected to analysis with complementary techniques such as scanning electron microscopy coupled with energy-dispersive spectroscopy.3

cusping: The pattern created along the edges of a stretched textile where the weave distorts in reaction to stretching and attachment to a secondary support, such as a stretcher. Also called scalloping. Primary cusping results from stretching before preparation—either by an artist or color merchant stretching an individual canvas before priming or as an effect of larger commercial or industrial preparation of a canvas. Secondary cusping is related to the stretching of a pre-primed canvas on a stretcher or strainer, usually by either a color merchant or the artist.4

drying cracks: A condition of painted surfaces caused by uneven rates of drying between layers. Drying cracks can occur in the paint or varnish layers and tend to be wide with rounded edges. Also called traction cracks and shrinkage cracks.5

extender: Inert materials, such as calcium carbonate (chalk), calcium sulfate (gypsum), and barium sulfate (barite), added to paint and ink, to give bulk and dimensionality or adjust the working properties. Kaolin (hydrated aluminum silicate or China clay) is often used as an extender in chalks, crayons, and pencils.6

false-color infrared: Unlike a true-color image, a false-color image assigns a color to regions of the electromagnetic spectrum not visible to the eye. By convention, a false-color infrared image displays information related to the infrared range (c. 800 nm to 1000 nm) as red, while information on the visible range—red and green—are displayed as green and blue, respectively. By assigning a color to the infrared spectrum, features of interest, such as retouching or painting materials that appear identical in the visible range, but are different in the infrared range, are more easily visualized.7

furnish: The materials from which paper is manufactured. This term can be used to describe the type of plant fiber used in the raw stock (such as wood pulp and rag), as well as the general composition or combination of materials.8

foldover: The crease made in a canvas where it is folded over stretcher or strainer bars. The original foldover corresponds to the first stretching of the canvas. If a canvas is mounted onto a new stretcher of slightly different dimensions, a new foldover is created. Also called turnover.9

Fourier transform infrared microspectroscopy (FTIR): A technique that allows characterization of a variety of materials, such as pigments, fillers, binders, and varnishes. A few microscopic particles of a sample are typically compressed into a thin film, placed under a microscope, and produce a graph—called a spectrum—that shows peaks at the discrete frequencies at which the sample absorbs the incoming infrared radiation. By interpreting and comparing this spectrum with a reference database it is possible to identify an unknown material. Using portable instruments, the technique can also be used noninvasively by collecting the infrared radiation that is reflected off the surface of the work of art examined, without the need to remove a sample.10

ground: An opaque layer applied to a canvas, usually over a sizing layer, to prepare the canvas to receive paint. Grounds in the nineteenth century were commercially available in white and a variety of pale colors, and in both absorbent and nonabsorbent consistencies. Also called priming or preparatory layer(s).11

Hyperspectral imaging (HSI): Hyperspectral cameras acquire simultaneously a number of images from c. 400 nm to 900 nm. As used in this study, each pixel of the hyperspectral image contains spectral information related to the percentage reflectance of the surface. When compared with known standards, reflectance spectra can aid in the noninvasive identification of painting materials, such as some iron oxides and organic colorants.12

ICA spring stretcher: The Intermuseum Conservation Association (ICA) spring stretcher, designed by Richard Buck in 1950, is the most common type of spring-tension stretcher. It is constructed with spring-loaded corners intended to exert continuous, even tension on the canvas as it changes in response to fluctuations in humidity. The stretcher was available, under the name Superior Spring-Stretcher, until 2001.13

impasto: Thick, textural application of paint.14

impressed watermark: A type of watermark found in machine-made paper where a design fabricated in relief, often in rubber, is affixed to a rolling cylinder (called a dandy roll) and rolled over the moving web of damp pulp, pressing the design into it. The impressed design is then visible in transmitted light similar to a watermark.15

infrared reflectogram: See infrared reflectography.16

infrared reflectography (IRR): This technique exploits the varied transmission, absorption, and reflection properties of infrared radiation by artists’ materials in the 900–1700 nm range, as used in this study. The degree of penetration depends on the thickness of the paint, the pigments used, and the wavelength of the infrared radiation. Many paints appear partially or completely transparent, while others, such as carbon-based blacks, absorb the infrared radiation and appear dark. An infrared-sensitive camera captures the radiation reflecting from the artwork. The resulting image is known as an infrared reflectogram. Infrared reflectography is used to distinguish pigments, inscriptions, underdrawings (particularly those done in carbon-rich materials such as charcoal and pencil over a white or light-colored ground), and changes in a composition not visible to the naked eye. 17

inpainting: Painting done by a conservator to restore areas of loss or damage in an original paint layer. Inpainting is limited to the area of loss and is carried out in a medium that remains readily distinguishable from the original, does not discolor, and can be easily removed in the future. Also called retouching.18

keying out: The act of tapping the keys to expand the stretcher and increase tension on a canvas support. See also: keys.19

keys: Small wedges, often roughly triangular in shape, inserted into the joints of a stretcher to allow the stretcher to be expanded and tension on the canvas increased. This process is called keying out or tapping out.20

lake pigment: A translucent pigment made by precipitating a dye onto an inert substrate. Lake pigments can be made from naturally sourced or synthetic dyes; common examples include alizarin, madder, and carmine (cochineal).21

lay-in / laying in: The act of establishing either the major elements of a work in solid paint or the initial paint layers. The lay-in is a form of underpainting, also referred to as the ébauche or block-in. 22

lining: The act of structurally reinforcing the original canvas or paper support with a similar or like material to provide strength or reinforcement to the artwork. A painting can be lined with a second textile or canvas with an adhesive, typically aqueous materials, wax-resin mixtures, and synthetic resins. 23

luminescence: As used in this study, the term luminescence is used to indicate the emission of visible light when exposed to ultraviolet radiation.24

overpaint: Paint or restoration material applied over the artist’s original paint. Most often applied by another artist or a restorer at a later date, overpaint was used to cover damages and make aesthetic changes to the artwork. It is often excessive and is generally avoided in favor of localized inpainting.25

palette knife: A spatula-like tool used for mixing and applying paint and ground layers. Palette knives usually consist of a handle and a thin, flexible, blunt-tipped metal blade. 26

photomicrograph: An image taken through a microscope.27

pigment: Colored particles used in artistic media such as oil paint, pastel, chalks, and watercolor.28

plain weave: Weave pattern where the weft threads are woven over every other warp thread. Also called tabby weave.29

polarized light microscopy (PLM): This technique uses a light microscope with two polarizing filters (one above and the other below the specimen) to characterize the optical and morphological properties of a specific material. A minute sample is mounted on a glass slide, dispersed in a transparent medium with a known refractive index, and examined in transmitted light. The microscopist studies the color, shape, size, sample homogeneity, refractive index, and other optical phenomena of pigments and fibers and how they interact with light. These observations, when compared to a database of reference materials, allow an experienced analyst to determine the individual pigment or fiber material present in a sample.30

pre-primed: Indicates a canvas that was primed before being cut to size and mounted on a stretcher. The presence of ground extending to the edges of the canvas is indicative of pre-priming. Canvases were sometimes pre-primed by an artist or assistant, but, by the second half of the nineteenth century, more commonly by commercial manufacturers or color merchants who sold pre-primed canvas ready-stretched or in rolls or by the meter for artists to stretch themselves.31

raking light: Illumination from an oblique angle used to highlight the topographical features of an artwork.32

reflected-infrared imaging: Reflected-infrared imaging, as used in this study, captures the percentage reflectance of the surface of an artwork using a modified commercial camera. The technique is similar to IRR, but works in a different spectral range (c. 800–1000 nm). The materials used in an artwork may transmit, absorb, or reflect infrared radiation depending on their composition, thickness, and the wavelength of the infrared radiation used. Reflected-infrared images can be used to distinguish pigments and to visualize inscriptions, underdrawings (particularly those done in carbon-rich materials such as charcoal and pencil over a white or light-colored ground), and compositional changes not visible to the naked eye.33

retouching: This term is commonly used interchangeably with inpainting; however, it can also refer to overpaint or repaint, generally in reference to a conservator’s intervention. Retouching may also include toning of original paint that has been damaged or abraded.34

scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM-EDS): This technique utilizes electrons, rather than light, to create an image of a microscopic sample. A beam of electrons is focused on the sample’s surface. Secondary electrons allow scientists to obtain a topographic image at very high magnification (up to 100,000×); backscattered electrons highlight the distribution of the elements, recording them in different shades of gray according to their atomic weight; and X-ray fluorescence emission allows analysts to determine which elements are present and to infer inorganic (mineral and synthetic) pigments and fillers. The EDS spectrum consists of a graph containing a series of peaks, each one occurring at a precise energy characteristic of a particular element. It is important to note that this technique gives information about what elements are present in a sample but not about their relationships to one another; it also does not indicate the organic compounds (such as binding medium) present. For these reasons, it is best complemented with polarized light microscopy or a molecular fingerprinting technique such as FTIR. 35

scanning macro X-ray fluorescence spectroscopy (macro-XRF or MA-XRF): This XRF technique creates an image of the distribution of specific atomic elements contained in the paint layers of an artwork. An XRF spectrometer is scanned across the area of interest (which may be the entire artwork or only portions of it) and collects XRF spectra for individual spots in successive rows, which are then combined in a single image mapping the distribution of each element in the area examined. These images can be used to formulate hypotheses on the distributions of specific pigments and extenders above or below the surface of the artworks scanned.36

scumble: An opaque paint layer applied so thinly it appears as a translucent veil over the preexisting layers. Sometimes a scumble is applied roughly or with a stiff-bristle brush so that it retains an uneven texture and thickness.37

sizing / size: Material additive or layer, commonly gelatin or glue, added to a support, such as wood, canvas, or paper, to seal it and reduce its ability to absorb paint and other media. In the preparation of canvas, hardboards, and panels, the size is applied before the ground layers. For paper, this material is incorporated in the pulp or applied to the surface of the formed paper during its manufacture to increase its resistance to water penetration (introduced by ink or watercolor). The process of applying the size layer is called sizing.38

standard format / standard size: The range of ready-made commercial supports—canvas, panel, paper, and card—available in an array of fixed sizes that are constant regardless of manufacturer. Standard-size supports are numbered from 1 to 120 and are available in three shapes: portrait (figure), landscape (paysage), and seascape (marine). These three formats have their longer dimension in common, with the short side becoming progressively shorter, so that the portrait format is the closest to being a square, the seascape the narrowest, and the landscape between the two. Some suppliers offered additional variations on select landscape and marine formats, further classifying them as haute (tall) and basse (short). The dimensions of these supports range from 12 to 130 by 22 to 195 cm.39

stereomicroscopic examination: Examination of an object with a stereomicroscope, which utilizes reflected light and is designed with two oculars, one corresponding to each eye, to give a three-dimensional visualization of the object at a magnified scale. 40

stretcher: The wooden framework with expandable joints over which a canvas painting is stretched and attached. It is the secondary or auxiliary support for a canvas painting. Also called a chassis.41

substrate: An inert substance, such as chalk, onto which dyes are precipitated. In the present study, the term is used with regard to lake pigments.42

support: The physical surface upon which an artwork is executed. This can include textiles, wood, other types of board, metal, and paper.43

thread count: The number of warp and weft elements in a textile per linear unit of measurement. This can be estimated by localized, manual counting or determined by specially developed automated software. Researchers from the Thread Count Automation Project—including Rick Johnson of Cornell University; Don Johnson of Rice University; and Robert G. Erdmann, Rijksmuseum, Amsterdam, University of Amsterdam, and Radboud University, Nijmegen—use a program that precisely counts the threads in a canvas and maps variations in the weave density from an X-ray. The results can provide information about weaving and priming processes and can be used to identify canvases cut from the same fabric.44

thread-angle map: Heat map for the warp and weft threads generated by Thread Count Automation Project software that displays the angles of corresponding threads across the painting. These maps give indications about the weaving and preparation processes and mainly show cusping, both primary and secondary. In addition, weft-angle maps can reveal weaving features such as warp-thread repairs.45

transmitted-infrared imaging: This technique is similar to reflected-infrared imaging, except that the light source is placed behind the painting or drawing so that the light is transmitted through it. It has proved especially useful in discovering artists’ changes that are not visible in X-ray, IRR, or reflected-infrared images, as well as original canvas stamps obscured from view as a result of lining. As used in this study, transmitted-infrared images capture the percentage transmittance of a surface in the c. 800–1000 nm range.46

transmitted-light imaging: Imaging a work of art, usually with a canvas or paper support, illuminated from behind so that the light passes through the artwork.47

underdrawing: Compositional sketch drawn usually directly on top of the ground layer in painting or directly onto the paper support in watercolor. Underdrawings can vary in media and level of detail, from simple contours to drawings that are volumetric and heavily detailed.48

ultraviolet (UV): Invisible to the naked eye, ultraviolet radiation has energies beyond the visible light portion of the electromagnetic spectrum. Materials at the surface of an artwork may exhibit luminescence in characteristic colors in response to ultraviolet radiation, depending on their chemical composition. For example, zinc oxide typically has a greenish-yellow luminescence when viewed under ultraviolet, while certain red lakes often display a pinkish-orange luminescence. Ultraviolet-induced luminescence is visible to the naked eye and can be documented photographically. Ultraviolet imaging is used to help verify or differentiate surface coatings, pigment composition, and reworkings or restorations and can aid in their identification when coupled with other means of point analysis. Ultraviolet radiation is also routinely used to study the luminescence from constituent layers of cross sections under a microscope.
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ultraviolet radiation: See ultraviolet.50

varnish: Natural or synthetic resin-based surface coatings on a work of art. Spirit varnishes are made with resins dissolved in a volatile solvent that harden to form a film as the solvents evaporate. These coatings are usually transparent and are intended to protect the surface of a painting from grime and pollution, saturate the colors, and produce a consistent surface gloss.51

warp thread: Fixed longitudinal threads on the loom during the weaving process.52

wash: Thin paint layer produced by diluting the paint with water or solvent, depending on the binding media. In oil paintings washes tend to appear more matte and are more liquid than glazes. For works of art on paper, the term refers to dilute ink, watercolor, or other paint applied by brush.53

watermark: Traditionally formed in paper by attaching wire designs (such as coats of arms, crests, or letters) to the paper-mold screen; fewer fibers collect over the wire designs during the sheet formation, resulting in thinner areas visible in transmitted illumination. Watermarks can be studied to learn more about the paper’s date and manufacture. Machine-made papers sometimes utilize an impressed watermark, which can have a similar appearance.54

weave: Woven pattern in textiles created by overlapping warp and weft threads. Common weave patterns include plain, or tabby, weave; twill; and basket weave.55

weft thread: In the weaving process, the weft threads are laterally woven through the fixed warp threads.56

wet-in-wet: The application of wet paint onto a still-wet paint layer; the two paints may be blended or mixed together on the canvas. The presence of wet-in-wet paint application can be used to identify parts of a picture that were painted within a short time frame. Also called wet-into-wet.57

wet-on-wet: The application of wet paint onto a still-wet paint layer without mixing or blending the two paints.58

wet-over-dry: The application of wet paint onto a paint layer that has already dried or hardened. This generally indicates some passage of time between the two applications. Also called wet-on-dry.59

wove paper: A type of handmade or machine-made paper formed on a mold screen of finely woven wires lacking the pattern of laid and chain lines. When the paper mold is dipped into water containing a fine dispersion of paper fibers, the fibers settle onto the wire screen as the water drains off; the formed sheet bears the wove wire screen pattern that is most clearly seen when light is transmitted through the paper sheet.60

X-radiography (X-ray): X-radiography involves exposing a painting to X-ray radiation and imaging the transmitted X-rays on film or digitally. Materials vary in their ability to absorb or transmit the radiation, depending on their thickness, density, and chemical composition. X-radiography can reveal changes in composition, artist’s brushwork, damages, and details in the support not visible to the naked eye. Because all of the layers of the painting are superimposed on one plane, X-ray images (commonly called X-rays) require careful interpretation.61

X-ray fluorescence spectroscopy (XRF): This noninvasive means of analysis utilizes a focused beam of X-rays to excite the atoms that constitute the artwork and measures the emitted energy. It provides a characteristic fingerprint of the elements contained in the sampled volume, thus allowing one to formulate hypotheses on the artwork’s inorganic components (pigments, extenders, metallic elements, and the like, but not binding media or organic pigments). In the case of paintings and works on paper, the sampled volume often includes all painting layers and may also include information about the support.
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