A

• Abrasion: Physical wear on a painting’s surface caused by friction, scraping, or improper cleaning. Abrasion appears as loss of paint or varnish in affected areas, often exposing underlying layers. It is usually the result of rough handling or over-cleaning that grinds away the upper paint or coating layers.
• Accretion: An accidental deposit of foreign material on the artwork’s surface that was not part of the original paint process. Common accretions include dried liquids, fly specks, or dirt/grime buildup that can disfigure the painting until removed.
• Acrylic: A class of synthetic polymer resins widely used as binders in modern paints, varnishes, and adhesives. Artists’ acrylic paints are water-borne emulsions of acrylic polymer and pigment, which dry quickly to form durable films; in sheet form the resin is known by trade names like Plexiglas or Perspex. In conservation, acrylic resins (e.g. Paraloid B-72) are also used as consolidants and varnishes due to their stability.
• Aging (Age cracks): Natural deterioration of paint and ground layers over time, often manifesting as a network of fine cracks (craquelure). Aging can cause embrittlement of oil binders (leading to yellowing and cracking) and increased transparency of paint, revealing underdrawings or changes (pentimenti) beneath. All older paintings develop age-related cracks from environmental, mechanical, and chemical stresses accumulated over decades.
• Alligatoring (Alligator Cracks): A pattern of wide, disfiguring drying cracks resembling alligator skin. Alligatoring occurs when a fast-drying upper paint or varnish layer shrinks over a still-soft slower-drying layer beneath. The result is a characteristic branching crackle with polygonal islands of paint – a form of traction crack – which does not usually penetrate all the way to the support.
• Auxiliary Support: The secondary support framework over which a canvas is stretched, typically a wooden stretcher (with adjustable keys) or strainer (fixed corners). The auxiliary support holds the primary support (canvas) taut; it may also refer to any additional backing or panel attached to reinforce the painting. Proper function of the auxiliary support is crucial to prevent canvas slackening, stretcher marks, or mechanical stress on the paint layer.

B

• Backing Board: A rigid board (e.g. archival foamboard, corrugated plastic, or Masonite) attached to the reverse of a stretcher or frame to protect a painting’s back side. Backing boards shield the canvas from dust, impacts, and environmental fluctuations, and they provide a surface for labels (so they are not applied to the canvas itself).
• Batten: A wooden strip attached to the reverse of a panel painting to provide additional structural support. Historical panel paintings often have multiple battens (sometimes with sliding joints) to prevent warping; a system of crossing battens is called a cradle. Conservators must monitor battens or cradles, as they can sometimes cause stress or splitting in the panel if improperly designed or if environmental changes occur.
• Beva 371: A synthetic thermoplastic adhesive (a mixture of ethylene-vinyl acetate resin, paraffin, and microcrystalline wax) commonly used in painting conservation. BEVA 371 is used for lining canvases, consolidating flaking paint, and other treatments because it is stable, reversible with heat or solvents, and has a long working time. It was introduced in the 1970s and remains a versatile adhesive for structural repairs in paintings.
• Binder (Binding Medium): The film-forming component of paint that binds pigment particles together and adheres them to the support. For example, linseed oil is the binder in oil paint, egg yolk is the binder in tempera, and gum arabic in watercolor. The binder’s properties (drying rate, flexibility, refractive index) strongly influence paint behavior, and its deterioration (e.g. oxidation of oil, or hydrolysis of glue) can lead to issues like cracking, blanching, or chalking.
• Blanching: A whitish, opaque discoloration (milky haze) in a paint or varnish layer. Blanching often appears in localized spots and can result from moisture or solvent action that disrupts the refraction of light in the paint film. For instance, high humidity or improper cleaning can alter an oil binding medium or degrade a natural resin varnish, producing this pale bloom on the surface. (See Bloom.)
• Bleeding: Unintended spreading or seeping of one paint into adjacent areas, causing color migration. Bleeding is commonly seen in works on paper or fresco secco, where a liquid paint or ink runs along fibers or cracks. It can result from water damage or solvent action that mobilizes a pigment or dye, creating halos or blurred outlines. (Note: “Bleeding” may also describe intentional wet-into-wet effects by the artist, but in conservation it usually refers to unwanted movement of color.).
• Blister: A small, raised bubble or air pocket in a paint or varnish layer. Blisters occur when a portion of the paint/ground lifts from the layer below, trapping air or vapor. Heat exposure can cause blistering (e.g. a “burn blister” from intense heat) or solvent interaction under a varnish. Blisters are unstable: they may eventually break, leading to flaking if the top of the blister detaches. Treatment involves relieving the pressure and re-adhering the layers before they rupture.
• Bloom: A bluish-white or cloudy haze on the surface of varnish or paint. Bloom is typically caused by high humidity or temperature changes that lead to moisture condensing within a natural resin varnish, or by certain atmospheric pollutants. It differs from blanching in that bloom is often more superficial and associated with moisture. Historically, bloom on varnished paintings (especially those with shellac or dammar varnish) was reduced by gentle warming or solvent vapor, but modern practice is to remove or replace severely bloomed varnish.
• Bole: A fine, colored clay (often red or yellow earth) used as a preparatory ground for water gilding on panels and frames. In panel paintings (especially tempera icons or gilded backgrounds), bole mixed with a binder (like glue) is applied over gesso; gold leaf is then laid on the smooth bole layer. The warm tone of red bole imparts depth and warmth to the gold. Conservators encounter bole when consolidating flaking gilding or during cleaning – it is water-sensitive and must be handled carefully to avoid loss.
• Buckling: A distortion of a painting’s surface into gentle waves or ridges, often accompanied by cracks or cleavage in paint layers. Buckling typically results from the support shrinking or swelling (e.g. a canvas slackening with humidity or a panel warping) which compresses the paint/ground, causing it to lift in ridges (sometimes called cupping when edges lift). Active buckling can lead to paint loss if not stabilized. Preventive measures include humidity control and ensuring the support is properly tensioned and backed.

C

• Canvas: A woven fabric support for paintings, usually made of linen, cotton, or hemp. Canvas is typically coated with a size (glue) and ground before painting. Linen has historically been preferred for oil paintings due to its strength and durability. Over time, canvas can deteriorate – becoming brittle, torn, or slack – requiring conservation interventions like lining, strip-lining (adding new fabric to tacking edges), or careful re-stretching.
• Chalking: The powdery degradation of a paint layer when the binding medium has deteriorated or was insufficient. Chalking is characterized by a fine, dusty pigment layer coming off the surface (e.g. one can rub the surface and get pigment on the finger). It occurs in frescoes or outdoor murals where binder (lime or medium) leaches out, or in oils where UV exposure breaks down the binder. Chalking results in color loss and a matte appearance. Consolidation or re-binding of the surface may be required to prevent further powdering.
• Cleavage: Loss of adhesion between layers in a painting, causing separation of the paint or ground from the layer beneath. Cleavage can be incipient (just beginning, with curling but not detached), blind (separated but without flaking off yet), or lifting (actively raised, about to flake). It often appears as bubbles or tent-like lifts (see Tenting) and sounds hollow if gently tapped. Conservators treat cleavage by consolidation – injecting or applying an adhesive under the lifting areas and re-adhering them under pressure.
• Cockling: An undulating, warped distortion in a paper or fabric support, often caused by moisture and drying cycles. In works on paper (drawings, watercolors), cockling appears as wavy buckles across the sheet; in canvas paintings it can refer to minor rippling not caused by sharp creases. Cockling does not involve planar creases but indicates dimensional changes. Preventive conservation (proper mounting, humidity control) and gentle humidification and flattening treatments are used to reduce cockling. (Also see Buckling for more pronounced distortions.)
• Compensation (for Loss): Any method of replacing or visually infilling missing areas of original material in a painting. This can include fills (to rebuild lost ground or canvas areas) and inpainting (retouching paint losses). The goal of compensation is to restore structural integrity or visual coherence, while remaining discernible under close examination or UV light. Common compensation techniques are toned fill material for losses in ground or support, and reversible conservation paints for loss integration in the image.
• Consolidation: The process of strengthening powdery, flaking, or delaminating materials by impregnating them with an adhesive. For paintings, consolidation often refers to securing lifting paint or ground back onto the canvas or panel using adhesives like dilute acrylic resin, gelatin, or BEVA applied by injection or through tissue. Heat or solvent may be used to reactivate adhesives (as in wax-resin linings or BEVA treatments). Proper consolidation stabilizes the painting without leaving a gloss or tide line on the surface.
• Craquelure (Cracking): The network of fine cracks that develops in paint, ground, or varnish as a painting ages. Craquelure patterns may be age cracks (from natural drying and desiccation, often penetrating ground and paint), drying cracks (traction cracks from a too-fast drying layer over a slow-drying layer, often with an “alligator” pattern), or mechanical cracks (from impact or flexing, often radiating in a cobweb pattern). Craquelure is a normal phenomenon in old paintings and can be stable; however, if cracks widen or cupping develops, flaking can occur. Conservators distinguish crack types to infer causes and appropriate treatments (for example, restretching a canvas can exacerbate certain mechanical cracks, so caution is needed).
• Cupping: A form of paint deformation where the edges of small paint flakes curl upward, creating a concave “cup” shape of the paint fragment. Cupping usually follows extensive craquelure and occurs as the paint/ground shrinks or loses adhesion – the edges lift while the center remains attached. It is often a precursor to flaking, as cupped paint is vulnerable to detachment. Treatment involves consolidation to flatten and secure the cups back to the surface. Cupping is commonly seen in old oil paintings with heavy craquelure or in wall paintings as plaster loses adherence.

D

• Deformation: Any change in the original flat plane or shape of a painting’s support, such as bulges, depressions, or warps. Canvas deformations might be bulges from impact, sags from loose tension, or stretcher bar creases (lines caused by the canvas resting on the stretcher edges). Panel deformations include warping or splitting. Even paper can deform as cockling or waviness. Deformations can lead to paint stress and cracking. Conservation typically addresses deformations by adjusting tension (keying out a stretcher), humidification and flattening, or adding supports (cradle, backing board) to regain planar orientation.
• Delamination: A term often used interchangeably with cleavage, referring to separation of layers – e.g. a ground layer detaching from a canvas, or plaster layers separating in a fresco. In wall paintings, delamination of plaster (intonaco from arriccio) is a serious structural issue requiring grouting or injection to re-adhere layers. In easel paintings, one might say the canvas delaminated if an attached lining peeled off, or a multi-layered support (like laminate board) came apart. Prompt consolidation is needed to halt delamination once detected. (See Cleavage and Lifting.)
• Dent: A shallow concave impression on a painting’s surface caused by an impact or pressure, without a full puncture. On canvas, a dent may occur if something presses the back of the canvas (e.g. a stretcher cross-bar or a blunt hit), creating a local depression visible from the front. Dents disturb the paint and ground, often causing concentric mechanical cracks around the impact area. Minor dents can sometimes be reduced by gentle humidification and backing support; severe dents might require restretching or reinforcement of the canvas.
• Diagonal Cracks: Cracks that run at an angle, often roughly 45 degrees to the horizontal/vertical axes of a painting. They commonly appear at canvas corners due to stresses from keying out (over-tightening the stretcher) or from a painting being dropped on a corner. Diagonal cracks are usually mechanical in nature. They can be accompanied by corner draws (distortions radiating from corners). Restoration involves stabilizing the crack and alleviating the stress (for instance, ensuring the stretcher is not over-expanded and using backing or loose-lining for support).
• Drying Cracks (Traction Cracks): Cracks that form in a paint or ground layer during the drying and curing process, rather than from long-term aging. They occur when the top layer dries and shrinks over a slower-drying underlayer, leading to a curved, often wide crack pattern confined to the paint layer. These are the same as alligatoring in severe cases. Unlike age craquelure, drying cracks can appear early in a painting’s life if improper technique was used (e.g. oil paint applied over insufficiently dried underpaint). Conservators typically leave stable drying cracks as-is, as they are part of the original condition, but must ensure they do not threaten paint adhesion.
• Drying Oils: Oils that chemically polymerize and harden upon exposure to air, traditionally used as binders in oil painting. Common drying oils are linseed, walnut, and poppyseed oil. They form a solid paint film by oxidizing and cross-linking. Understanding drying oils is important in conservation because their long-term oxidation leads to embrittlement and yellowing of oil paints. Knowledge of the specific oil can inform treatments (for instance, linseed oil films tend to yellow more than poppy oil, and each responds differently to solvents or cleaning). (This term relates to technical analysis; e.g., GC-MS can identify the oil type in a paint sample.)

E

• Efflorescence: A powdery or crystalline white deposit on the surface of a material like plaster, stone, or paint, caused by soluble salts migrating and crystallizing. In wall paintings or frescoes, moisture dissolves salts within the wall, then evaporates at the surface, leaving salt crystals as a crust or fuzzy bloom. Efflorescence is often hygroscopic – it attracts moisture – leading to cyclic crystallization that can break apart the plaster or paint (sometimes causing spalling). Treatment may involve controlled desalination (salt extraction) and environmental controls to prevent further salt activity.
• Encrustation: A hard crust or build-up on a painting’s surface. This can refer to heavy grime accumulation or materials like wax or resin that solidified on the surface. In wall paintings, mineral encrustations (from salts or calcite) can form a tough layer. Cleaning encrustations requires care to avoid damaging the original surface beneath. In paintings on canvas, old coatings can cross-link into encrustations (e.g. dark, insoluble varnish blisters that solidify). Often a combination of mechanical and chemical cleaning is used to reduce encrusted layers. (See also Accretion for any extraneous surface material.)
• Exfoliation (Spalling): The flaking or peeling away of surface layers in scales or sheets, usually due to weathering, salts, or freeze-thaw cycles. On a wall painting or mural, exfoliation might describe plaster coming off in shallow flakes; on stone architectural sculpture or ceramics, it describes surface layers splitting and falling off. Salt crystals growing beneath the surface push the layers outward, causing them to spall off. In paintings, one might use this term for large-scale delamination of ground/paint (e.g. if gesso is lifting off a panel in sheets). Conservation involves removing harmful salts or stabilizing environmental conditions to halt the cycle, and carefully re-adhering any loose scales.

F

• Fading: The loss or change of color in pigments due to light exposure or chemical reactions. Many organic pigments (dyes, lake pigments) and some early synthetic colors are fugitive, meaning they fade significantly with light, especially UV. Fading can also occur from reactions (e.g. verdigris copper green can brown over time, or a bright red organic pigment might bleach out). Conservators document fading by comparing covered vs. exposed areas and may use UV filters on lighting or exhibit such works under low light to slow further fading. Reversing fading is not possible, so preventive care is critical.
• Fill: A material used to compensate for losses in a painting’s structure, such as missing areas of gesso ground, lost corners of a canvas, or small losses in paint. Fill materials range from traditional gesso or chalk/glue putty to modern acrylic fillers or gels. A good fill replicates the texture and profile of the original surface without overstressing it. After filling and smoothing to match the surrounding surface, the area can be retouched (inpainted). For example, a wormhole in a wooden panel might be filled with a stable insert and gesso putty, or a small paint loss on canvas filled with a reversible acrylic spackle.
• Flaking: The detachment of small fragments (flakes) of paint or ground from the layer beneath, often dropping out and leaving lacunae. Flaking is the active stage of cleavage: portions of paint/ground curl or lift and then fall away if not secured. Causes include loss of adhesion (from age, damp, salts) or physical shock. Flakes can range from tiny specks to larger chips. In condition reporting, flaking is noted as active (if flakes are loose and in danger) or stable (if lifted but not worsening). Treatment requires consolidation – applying an adhesive under the flake and gently laying it down – to save the original material. Untreated flaking leads to losses that must then be filled and inpainted, so early intervention is preferred.
• Foxing: Brownish or rust-colored spots and specks that appear on paper, often in 18th–19th century works on paper (including watercolors). Foxing is believed to result from mold growth and/or metal impurities in the paper that catalyze discoloration under humid, stagnant conditions. While foxing stains are usually only an aesthetic issue (not structurally weakening the paper significantly), they are disfiguring. Conservation treatment for foxing may involve careful washing, chelation, or bleaching, but often complete removal is difficult; preventive storage in dry, ventilated conditions can inhibit further foxing.
• Frame Abrasion: A specific type of abrasion where the edges of a painting are worn by contact with its frame rabbet. It shows up as rubbed, sometimes discolored lines along the perimeter of the image. This happens if a canvas was frame too tightly or without proper spacers, so that expansion/contraction causes rubbing. The term may also include liner marks or rabbet burns. To prevent frame abrasion, conservators ensure there is clearance or use protective rabbet padding. In retouching, frame abrasions are often compensated to restore the original appearance at the edges.
• Fresco: A mural painting technique where pigments are applied to freshly laid wet lime plaster so that the paint becomes an integral part of the wall as the plaster sets. In buon fresco (true fresco), the artist paints on the final wet plaster coat (intonaco), and water is the vehicle that carries the pigment into the plaster, binding as it carbonates. This results in a very stable, long-lasting image. Fresco secco, by contrast, involves painting on dry plaster (usually with a tempera or limewash medium); secco is less durable since the paint sits on the surface. Fresco paintings deteriorate by flaking of plaster (if the wall is damp or salts are present), so conservation includes securing loose plaster, controlling moisture, and desalination.
• Fugitive Pigment: A pigment that is not lightfast and tends to fade or alter when exposed to light or other environmental factors. Examples include carmine lake (fades to brown), some aniline dyes, or early magenta pigments. These pigments were sometimes used by artists despite their poor stability (e.g. Turner’s paintings have faded skies due to fugitive reds). In conservation, identifying fugitive pigments (through spectroscopy or historical research) informs how a painting is displayed (low light, shorter exposure) and guides any retouching choices (modern lightfast substitutes might be used for compensation).

G

• Gesso: A traditional ground layer made of chalk (calcium carbonate or gypsum) mixed with animal glue, used to prepare rigid supports like wood panels for tempera or oil painting. Gesso creates a smooth, white, absorbent surface. In conservation, one encounters gesso as the brittle layer that can crack or flake off from panel movement or water damage. Losses in gesso are often filled with similar material for coherence. Gesso grosso (coarse first layer) and gesso sottile (smoother second layer) are terms in traditional panel preparation. Modern acrylic gesso (actually a polymer-based ground) is used on canvases; it’s more flexible but serves a similar priming purpose.
• Grime (Surface Dirt): A disfiguring layer of dirt, soot, nicotine, dust, and other pollutants that accumulates on a painting’s surface over time. Grime is usually a gray or brown film that can dull colors and alter the intended tonal balance of the work. It often sits on top of varnish (if present) or directly on paint in unvarnished works. Removal of grime (surface cleaning) is a common conservation task, done carefully with dry methods (soft brushes, smoke sponges) or aqueous solutions if safe for the surface. Unlike accretions, grime is generally a thin, even layer of particulate soiling.
• Ground: The preparatory layer applied to a support to provide a suitable surface for painting. A ground can be a chalk or lead white in oil (for canvas), true gesso on panel, or plaster (arriccio and intonaco for fresco). It is usually opaque white or toned and ensures good adhesion of paint and an even absorption. Deterioration of the ground (becoming brittle, cracking, or separating from the support) is a common cause of paint cleavage. For instance, an oil ground on canvas can become brittle and show flaking if the canvas is slack. Conservation treatments may include re-adhering loose ground to the support (lining or local injection) and filling areas where ground is lost.

H

• Half-Lap Joint: A type of wooden joint often used in historical auxiliary supports (like old strainers or stretcher cross-bars), where two pieces are halved in thickness at their meeting and overlapped flush. While not a “deterioration” term, it appears in technical descriptions of stretchers. Mentioned here for completeness, as understanding stretcher construction (joints like half-lap, mortise-and-tenon) helps conservators during treatments like stretcher repair or replacement. Loose or broken joints in a stretcher can cause canvas slackening; conservators will stabilize or replace such joints to ensure proper support.
• Humidity (Relative Humidity): The amount of moisture in the air, expressed as a percentage. While not a damage term per se, RH is critical in painting conservation: fluctuations in RH cause hygroscopic materials (canvas, wood, glue, gesso) to swell and shrink, leading to cracking, flaking, and support warping. High RH (> ~65%) can promote mold and efflorescence; low RH (< ~40%) can embrittle organic materials. Museums strive to keep RH stable (usually around 50% ±5%) to minimize physical deterioration. Conservators use humidification in treatments deliberately (e.g. to relax canvas or paper fibers), but uncontrolled humidity swings are a major agent of deterioration for paintings.
• Hydrolysis: A chemical deterioration process where water (or humidity) breaks down chemical bonds, often in organic materials. In paintings, hydrolysis can affect oil binders (breaking ester bonds, contributing to soap formation or making the oil more brittle) and natural adhesives like glue (depolymerizing collagen, causing loss of strength). It also affects cellulose in paper, contributing to acidic degradation. Signs of hydrolysis in oil paintings might be increased acidity in the paint film or weakened canvas sizing. Preventive measures include controlling humidity and acidity; sometimes conservators deacidify paper or use neutral sizing to combat hydrolysis byproducts.

I

• Impasto: Thickly applied paint that stands in relief from the surface, often created with a palette knife or stiff brush. While an artistic technique, impasto has conservation implications: thick peaks of paint can be more vulnerable to cracking or cleavage (the peaks drying faster and differently than underlying layers). Impastos are also prone to abrasion (they stick out and can be knocked off). Conservators pay special attention to impasto when handling or lining a painting – soft, cushioned supports and avoidance of pressure on raised paint are crucial. If an impasto flake breaks off, it can sometimes be recovered and reattached under magnification.
• Inpainting (Retouching): The application of new paint in areas where the original paint is lost or abraded, as a restoration step. Inpainting is done with stable, reversible materials and is confined strictly to areas of loss (lacunae) to reintegrate the image without covering original paint. Modern ethical standards require inpainting to be detectable under UV light or other means, and not to deceive future caretakers. The term retouching was formerly used broadly, but now overpainting refers to improper retouch that covers original paint, whereas proper inpainting limits itself to losses. Inpainting techniques include color-matching with watercolors, acrylics, or conservation pigments bound in resin.
• Intonaco: In true fresco technique, the final smooth layer of fine lime plaster onto which the artist paints. Intonaco is applied in patches (jornate) that the painter can complete in one session before it dries. Deterioration of the intonaco (cracks, detachment from the underlying rough plaster arriccio) directly affects the fresco image. Conservation of wall paintings often involves reattaching loose intonaco via injections of lime-based grout and filling lacunae with compatible mortar. Understanding the structure (intonaco over arriccio) helps conservators locate delamination between layers and treat accordingly.
• Iron Gall Ink: (Though more relevant to works on paper, it may appear on some artworks like drawings or underdrawings on panel.) It’s a common historic ink made from tannin (gall nuts) and iron sulfate, which, unfortunately, can burn through paper over time due to its acidity. If a painting (say, a map or an inscription on a canvas) has iron gall ink, it can fade to brown and the lines may eat into the substrate. Conservation of iron gall ink documents involves careful washing and adding alkaline buffers to slow degradation. (In easel painting context, underdrawings in ink are usually carbon-based, not iron gall, but this term might appear in a broad glossary including works on paper.)
• Infrared Reflectography (IRR): An imaging technique using infrared radiation to “see through” paint layers and reveal underlying drawings or changes in a painting. Many pigments, especially those lacking carbon, are transparent to certain IR wavelengths, whereas carbon-based underdrawings (charcoal, graphite, ink) absorb IR and become visible. IR reflectography is performed with a specialized IR-sensitive camera; it can uncover the artist’s preparatory sketches, pentimenti (earlier composition changes), or even hidden paintings beneath the visible one. It also helps detect later additions or retouchings if those use materials with different IR absorption. IRR is a standard examination tool in conservation, and its findings guide restoration (e.g. distinguishing original lines from later overpaint).
• Inpainting, Invisible: A goal (often not 100% attainable) in retouching where the new paint applied to losses matches the old paint so precisely in color, tone, and gloss that it is not noticeable to the naked eye under normal viewing. However, it should remain discernible under UV or magnification. Techniques like tratteggio or pointillism retouching are used to achieve visual integration while signaling to a trained eye up close that the area is restored. Invisible inpainting is an ideal in cosmetic reintegration, but ethical practice balances that with honesty of restoration.
• Intumescent Varnish: Not commonly referenced in painting conservation, more in fire protection, but if ever mentioned, it would describe a type of coating that swells when heated, forming an insulating char. In context of paintings, the term might arise if discussing historic attempts at fire-proofing canvases or if a piece has a particular protective topcoat. Typically not in a glossary of this nature; likely omitted unless specifically relevant. (Included here only to clarify if encountered, but usually not applicable).

L

• Lining: The process of attaching a new secondary canvas (or other support) to the back of a painting’s original canvas to reinforce and stabilize it. A lined painting has effectively a double-layered canvas, often bonded with an adhesive (historically hot animal glue-paste or wax-resin, modern methods use BEVA or heat-seal films). Lining was once a common treatment for aged, brittle canvases with flaking paint, as it can flatten deformations and secure flaking by pressing it in contact with a new support. However, invasive linings can alter a painting’s texture and tension, so minimal intervention is now favored. Variants include loose lining (an unattached safety backing canvas) and strip lining (adding strips to tacking edges). A painting that has been lined may be described as “re-lined” if it had an earlier lining replaced.
• Loss: Any area of original material (paint, ground, or support) that is missing due to damage or deterioration. Losses can range from tiny paint losses (small flakes) to larger areas where canvas or plaster is gone. Documentation of losses is crucial in condition reports (often mapped). Loss compensation involves fills and inpainting to restore legibility. In wall paintings, a lacuna usually refers to a loss area. Determining the cause of loss (flaking, impact, vandalism, pest damage) helps in planning stabilization and preventive measures.
• Mat Burn (Mat Stain): A darkened band or line on the margins of a paper artwork (such as a watercolor) caused by contact with an acidic mat or backing board. The lignin in poor-quality mat board can leach acids into the paper, “burning” it to a brown or yellow tone resembling a scorch. This discoloration typically follows the window opening of the mat. Removal involves careful deacidification and washing, but severe mat burn can be irreversible or require conservation bleaching. Preventively, archival (acid-free, buffered) matting materials are used to avoid mat burn. (Though primarily a term in paper conservation, it’s included here since watercolors are within scope.)
• Mechanical Cracks: Cracks in paint resulting from physical stresses or impacts rather than aging or drying processes. They often have sharp, irregular edges and may align with areas of stress such as along the stretcher bars or around a site of impact. Examples include concentric cracks (cobweb patterns from a blow to the canvas) or stress cracks radiating from a dent. Mechanical cracks are frequently accompanied by deformation of the support (e.g. a dent or bulge where the blow occurred). Conservation focuses on stabilizing any loose paint around these cracks and addressing the cause (for instance, reinforcing a weak canvas or improving framing to dampen shocks).
• Metal Soaps (Saponification): Degradation products formed by the reaction of heavy-metal pigments (like lead white, lead-tin yellow, or zinc white) with fatty acids from the oil binder. This process – saponification – creates metal carboxylates (soaps) that can migrate and aggregate in the paint. Visible symptoms include translucent areas where paint has become more transparent, a hazy appearance, or protruding whitish lumps or blisters on the surface as the soaps form and crystallize. For example, prominent lead soap protrusions up to 0.2 mm can develop and even rupture the paint surface. Zinc soaps are notorious for causing delamination of paint from the ground in 20th-century oil paintings. While consolidation can secure detached paint, the chemical process is ongoing and not fully preventable; research is active on how to slow or mitigate metal soap formation.
• Microscopy: The examination of a painting’s material at high magnification, either with optical microscopes or electron microscopes, to reveal fine details and structure. Optical microscopy (using visible light) is employed for surface examination (e.g. identifying pigment particle morphology or observing craquelure in cross-section) and for paint cross-sections, which are tiny samples showing the stratigraphy of layers. Under magnifications of 50×–200× (and up to 1000× with polarizing scopes), a cross-section can show ground, paint, and varnish layers distinctly. Polarized Light Microscopy (PLM) is used for identifying pigments by their optical properties. Scanning Electron Microscopy (SEM) uses electrons for much higher magnification and depth of field, useful for studying pigment particles or layer interfaces. SEM is often paired with EDS (Energy Dispersive X-ray Spectroscopy) to map elemental composition of microscopic areas. In sum, microscopy is an invaluable set of techniques to analyze deterioration (like pinpointing whether a white bloom is mold vs. efflorescence crystals) and to inform restoration (like choosing compatible materials after seeing layer structure).
• Mold (Mould): Fungi that can grow on paintings under high humidity or damp conditions, appearing as fuzzy patches, dark spots, or a fine diffuse bloom on surfaces. On canvas paintings, mold can feed on the organic components (size layer, glue paste, even oil binder to some extent), leading to pigmentation (black or colored spots) known as mildew. On paper (watercolors, etc.), molds cause foxing-like stains or tide marks and weaken the paper. Mold infestation requires immediate environmental correction (lower humidity, increase airflow) and careful cleaning with appropriate biocides or mechanical removal of spores. Mold staining can be permanent, and the priority is to sterilize and stabilize rather than fully remove all traces. Preventive measure: 50% RH or lower and good ventilation to discourage mold growth.
• Mortise-and-Tenon Joint: A wood joinery technique commonly used in high-quality traditional strainers or frames, where a projecting “tenon” on one piece fits into a cavity “mortise” on another. In the context of conservation, understanding such joints matters for disassembling or repairing a frame or auxiliary support. A mortise-and-tenon stretcher can often be disassembled for transport or treatment. If joints loosen over time, the stretcher might become unstable, affecting canvas tension. Conservation might involve shimming the tenon or adding reversible adhesives to tighten the joint (or replacing it with a more stable support system).

O

• Overpainting (Overpaint): Paint that is applied on top of the original paint layer by someone other than the original artist, usually during an old restoration or alteration. Overpaint often covers more area than necessary, obscuring original work – for example, large repainted sections to mask damage or “improve” the image. It is considered disfiguring when it mismatches the original or has aged differently. Identification of overpaint is done via close visual exam, UV light (overpaint can fluoresce differently or appear dark if on top of an older varnish), or even IR/X-ray if the overpaint hides earlier features. A major part of paintings conservation is overpaint removal, carefully removing these later additions to reveal the artist’s original intent, and then properly inpainting only the actual loss areas. The term “retouching” in older sources can mean overpainting (hence now avoided or clarified).
• Oxidation: A chemical reaction in which a substance combines with oxygen, often altering its appearance or properties. In paintings, common oxidation processes include the drying of oils (which is essentially oxidative polymerization), leading to darker, harder paint films over centuries. Pigments like lead-tin yellow can oxidize to dark brownish lead oxides; bronze or copper-containing pigments may green or brown as they oxidize (verdigris can turn brown-black). Varnishes made of natural resins also oxidize, contributing to yellowing. Oxidation is accelerated by heat and light. While some oxidation (like oil drying) is normal and necessary, ongoing oxidation can cause embrittlement of paint and loss of gloss. Conservators combat unwanted oxidation by controlling environment (reducing light and heat) and sometimes chemically stabilizing materials (antioxidants in modern consolidants or using stable synthetic replacements for components prone to oxidize).

P

• Panel: A rigid support for a painting, typically wood (like oak, poplar, mahogany) or modern composites (masonite, plywood, ACM). Many early paintings (and icons) are on wood panels. Panels can suffer splits along the grain, warp with humidity, or develop insect damage (wormholes). Conservation of panel paintings involves controlling humidity to prevent warping, using cradle or batten systems carefully, or even panel joins repair and inserts for missing wood. The term “panel” also extends to metal plates used as supports (copper panel paintings were popular in the 17th century). Each panel type has specific deterioration: wood gets wormholes and splits, metal can corrode. Treatments include backing boards, edge strip supports, or climate-controlled enclosures for highly sensitive panels.
• Pentimento: An Italian term meaning “repentance,” referring to an alteration in a painting where evidence of an earlier composition or drawing becomes visible through the top paint layer. This often happens because oil paint becomes more transparent with age, revealing an underlying form that the artist had painted over or changed. Pentimenti are not damage, but rather intrinsic to the history of the artwork – they can inform art historians about the artist’s process. Conservators take care not to mistakenly cover or remove pentimenti during restoration. For example, if an X-ray or IRR shows a pentimento (like a moved arm or shifted building), the conservator ensures any cleaning or inpainting doesn’t obscure this feature. Pentimenti can also guide authentication or dating of paintings.
• Pigment: Finely ground, colored particles that give paint its color; pigments are mixed with a binder to form paint. Pigments can be natural (earths, minerals) or synthetic. Each has particular properties (lightfastness, opacity, chemical reactivity). In terms of deterioration, some pigments interact with binders or environment: e.g. lead white can form soaps, copper pigments can catalyze degradation of oil, and cadmium yellows can oxidize or darken if incorrectly formulated. Pigment analysis via XRF, Raman, or FTIR helps identify pigments and any degradation products (such as noting that a blackened white might indicate a dark lead sulfide, which could be chemically treated or left as part of age). The choice of pigment also affects cleaning – some pigments are sensitive to solvents or high pH, so conservators tailor cleaning methods accordingly.
• Polarized Light Microscopy (PLM): An analytical technique where a sample (often a tiny pigment particle) is examined under a microscope with polarized light. Pigments have characteristic optical properties (refractive index, birefringence, crystal shape) that can be observed by rotating polarized filters. PLM is a classical method for pigment identification in conservation labs, especially for historic pigments. It requires expertise, but is non-destructive to the sample. For example, PLM can distinguish ultramarine (with its isotropic particles) from azurite (birefringent blue crystals). Though beyond the scope of a short definition, it’s good to note that PLM is one microscopy method in the conservator’s toolkit for understanding the material composition of paintings.
• Puncture: A small hole in a painting’s support, usually a pierced canvas or paper. Punctures are often caused by an impact with a sharp object (corner of furniture, falling stick, etc.). They typically have radiating cracks or tears and sometimes a distortion (push-through) around them. The first step in treating a puncture is to re-align and re-weave any loose fibers or edges, then mend the tear (lining patch or thread-by-thread repair for canvas, or Japanese paper for a tear in paper artwork). Finally, the loss in ground/paint is filled and inpainted. The result should restore structural integrity and make the damage invisible at normal viewing. Preventatively, backing boards can help stop objects from reaching the canvas back and causing punctures.

R

• Rabbet: The ledge on the interior of a frame into which the painting fits (it “rabbets” the painting). Over time, contact with the rabbet can cause abrasions (see Frame Abrasion) or indentations. Conservators often pad the rabbet with felt or other material to cushion and create a small gap between the frame and the painting. This prevents damage and also helps secure the painting without strain. In documentation, one might note “minor paint loss along edges from rabbet wear.” When reframing, using an archival tape or felt liner on the rabbet is a standard preventive conservation measure.
• Relative Humidity (RH): (See Humidity). Maintaining a stable RH is crucial for painting preservation. Fluctuations in RH can cause cyclic expansion/contraction of materials, contributing to craquelure and delamination. Sudden high RH can induce bloom on varnish or mold growth; very low RH can cause organic materials to become brittle (canvas fibers, glue). Museums aim for an RH around 50% for mixed collections. For panel paintings, even tighter control (around 50% with minimal fluctuation) is often recommended to avoid splitting or warping. Microclimate enclosures or backing boards are used to buffer RH changes for sensitive paintings.
• Restoration: The act of intervening on an artwork to stabilize it and improve its legibility or appearance. Restoration in painting conservation typically involves both stabilization (conservation treatments like consolidation, tear mending, cleaning to remove harmful varnish or grime) and aesthetic integration (such as inpainting losses, reintegrating discolored areas). The term “restoration” is often used in contrast to “conservation,” with the latter emphasizing minimal intervention and preventive care. Nonetheless, professional practice today integrates both – a conservator-restorer will restore a painting with reversible techniques and document all changes. An example of restoration is cleaning a yellowed varnish to recover original brightness, then revarnishing and retouching losses. Ethical restoration follows the principles of reversibility, minimal intervention, and not falsifying the artwork’s intent.
• Raking Light: Light directed across a painting’s surface at a low angle, used to reveal surface texture, distortions, and craquelure. While not a deterioration term, raking light examination shows deformations like tenting, cupping, bulges, or previous restorations (e.g. a lined canvas might show weave impressions or flattening in raking light). Conservators use raking light to document condition and after treatments to ensure surface evenness (for example, confirming that fills are level with original paint). Severe planar deformations are highly visible in raking light; thus this simple technique is fundamental for condition reporting.
• Relining: See Lining. Relining specifically refers to lining a painting that has been lined before. Early relining techniques (like wax-resin linings) can themselves age poorly (wax linings can darken or make the canvas stiff). A conservator may choose to de-line (remove an old lining) and then reline afresh using modern adhesives if the old lining is causing issues (such as excess stiffness causing new cracks). Relining is considered only when absolutely necessary due to its invasive nature – many paintings have survived without lining by using more conservative measures like strip-lining or backing boards.
• Retouching: In common usage, synonymous with inpainting, though historically it had broader meaning. Retouching is the application of paint to integrate losses or damages. Good retouching is limited to losses and does not cover original paint. Excessive retouching can become overpaint. The term “retouching” can also apply to minor color adjustments like dotting in tiny paint losses under magnification (as opposed to filling large lacunae). In documentation, conservators specify the materials used for retouching (e.g. “retouched losses with Gamblin Conservation Colors”) and that retouching is reversible and discernible under UV.

S

• Scumble (Scumbling): An artist’s technique of applying a thin, opaque or semi-opaque layer of lighter paint over a darker underlayer, often to soften or modulate color. While not directly a conservation term, scumbles can pose a challenge in cleaning: they are delicate, thin layers that might be more solvent-sensitive than robust glazes. Recognizing scumbled passages (often matte and light-valued) helps conservators adjust cleaning methods to avoid thinning these original effects. Deterioration-wise, scumbles, being lean and often with added fillers like chalk, can be prone to chalking or flaking if the adhesion was poor. They also can be visually impacted by dirt or discolored varnish filling in their interstices, so cleaning can dramatically improve their appearance if done carefully.
• Skinning (Over-cleaning): The abrasion of the original paint layer due to harsh or inexpert cleaning, resulting in the topmost paint being partially “skinned” off. Skinning often exposes the lighter ground or underpaint on the peaks of brushstrokes or canvas weave, giving a washed-out appearance to the image. It usually happens from using excessive solvents or abrasive methods to remove varnish or dirt, thereby taking original paint with it. A skinned painting may show a patchy, threadbare look in dark areas (since dark glazes are often the first to be worn off). Conservation cannot fully undo skinned areas, but inpainting can restore some visual unity. The best approach is prevention: gentle cleaning tests, adequate magnification, and stopping as soon as original paint is at risk.
• Spalling: (See Exfoliation.) Commonly used in wall painting and architectural contexts, spalling refers to chunks or flakes of a material (plaster, stone, brick) popping off due to internal pressures (like salt crystallization or freeze-thaw). In a fresco, spalling plaster leaves pit-like losses with granular surfaces. Each cycle of moisture can cause more spalls. Stabilization involves environmental control (reducing moisture, salts) and sometimes facing fragile areas with tissue and adhesive until deeper intervention is done. The term “spall” can be a noun (a spall = a flake that has come off) or verb (plaster is spalling).
• Spectroscopy: A broad category of scientific techniques that analyze how materials interact with electromagnetic radiation (light, X-rays, etc.) to identify composition. In painting conservation, common spectroscopies include Infrared (IR) spectroscopy – particularly Fourier Transform Infrared (FTIR) – which identifies organic binders and resins by their vibrational absorption peaks; Raman spectroscopy, which identifies pigments (especially inorganic) via inelastic light scattering “fingerprints”; and UV-Visible spectroscopy (or colorimetry) for analyzing pigments and color changes. By examining the spectra (plots of intensity vs. wavelength), conservators can detect functional groups or elemental signatures. For instance, FTIR might detect the carbonyl peaks of an aged oil or the resin components of a varnish, while Raman might confirm the presence of ultramarine vs. azurite by characteristic peaks. Spectroscopic analysis is typically done on tiny samples or even in situ with portable devices (like portable XRF or FTIR). These methods are non-destructive or micro-destructive and provide crucial information for making treatment decisions (e.g. identifying a varnish as natural or synthetic guides the solvent choice for cleaning).
• Stretcher: A wooden frame with expandable corners (usually held by keys or wedges) over which a canvas is stretched. Stretchers came into use in the 18th century, replacing fixed strainers, to allow re-tensioning of a canvas by tapping in the keys (called keying out). Problems associated with stretchers include stretcher bar marks – lines of deformation or cracking in the paint that align with the bars, often due to the canvas slackening and then resting on the bar edges. Also, old stretchers may warp or their joints may loosen, causing uneven tension. Part of preventive conservation is to periodically check stretcher keys (ensuring they are not so loose that the canvas sags, nor so tight that they risk tearing the tacking margins). If a stretcher is in poor condition, a conservator might replace it or add a strip-lining to give more material for stretching. Modern innovations like tensioning systems (keyless stretchers or spring-stretchers) also address some limitations of traditional keyed stretchers.
• Stretcher Bar Crease (Stretcher Mark): A faint line or ridge in the paint and canvas that corresponds to the location of a stretcher bar. It occurs from the canvas pressing or bending over the bar, usually when the canvas slackens and the bar edge imprints the paint film. Over time, the paint along that line may crack in a straight line following the bar. To mitigate this, conservators use beveled-edge stretchers (so the canvas mostly contacts at the outer edge, not the flat of the bar) and/or insert a padding between canvas and bar (like Ethafoam strip). If a stretcher mark is pronounced, a restorer might choose to re-stretch the canvas with slight offset to shift the line out of plane, and then retouch the crack. Preventative backing boards help by keeping canvases more taut and buffered from environmental changes, thus reducing the chance of developing stretcher creases.
• Surface Cleaning: The process of removing dirt and grime from a painting’s surface without affecting any original layers. This is often the first step in treatment, using dry methods (soft bristle brushes, vulcanized rubber sponges) or aqueous solutions (custom pH-adjusted detergents, gels) on robust surfaces. Surface cleaning can dramatically improve appearance by restoring brightness and contrast that were muted by gray grime. It must be done carefully to avoid removing delicate glazes or affecting a sensitive unvarnished surface. A successful surface cleaning leaves the original varnish or paint intact for subsequent steps (like varnish removal, if needed). The term distinguishes from deeper cleaning like varnish removal.

T

• Tear: A rip or separation in a canvas or paper support with irregular, ragged edges where the material has pulled apart. Tears in canvas often occur from physical trauma (impact or excessive stress) and can be vertical, horizontal, or angled. A long tear might have accompanying flaking of paint along its edges. Paper tears often result from mishandling. Conservation repair of tears involves aligning the edges and applying a tear strip or patch on the reverse with a strong, stable adhesive (for canvas often a fine polyester or linen fabric with BEVA 371, for paper Japanese tissue with wheat starch paste). The goal is a mend that restores structural stability and minimizes the visual scar. Tear mends on canvas are sometimes supported further by a lining or thread-by-thread reweaving for important paintings. After mending, any losses along the tear line are filled and inpainted to make the damage invisible.
• Tenting: A form of paint lifting where the paint (and sometimes ground) is forced upward in peaks that resemble tiny tents or ridges. This is usually caused by compression forces – for example, when a canvas or panel shrinkage compacts the paint layers, which then buckle upwards since they cannot compress further. Tenting often occurs in drying oil films that have become embrittled; the stress causes the paint to break and lift along crack lines, forming tent-like clefts. It is a serious condition because the tents can easily break off, resulting in loss. The remedy is to relax the compression if possible (e.g. correct the environmental cause or support issue) and consolidate the lifted “tents” back down. Humidification can sometimes gently lower tented paint before consolidation, but this must be done with extreme caution. Tenting is frequently seen in paintings that underwent past treatments like wax-lining improperly (the cooling wax caused shrinkage) or in panel paintings where wood movement compressed the paint. Early detection and consolidation of tenting is critical to prevent flaking.
• Tideline: A discolored line or tide-shaped stain that forms at the edge of a liquid that has penetrated a painting or paper, then receded. In works on paper, tidelines from water damage or cleaning solvents leave brownish perimeters (from mobilizing impurities or degradation products that get concentrated at the drying front). On canvas, if water leaks behind a painting, it can leave a tideline of surfactants or dirt on the front. Tidelines often indicate that a liquid has carried solubles (like acids, dirt, sizing) and deposited them at the drying edge. Removing tidelines can be challenging; in paper, careful washing or localized chelation is used, while on canvas, surface cleaning or solvent packs might reduce it if in the varnish. Preventing tidelines during treatment is important: conservators avoid letting cleaning solvents evaporate in a way that would cause new tidemarks, often by using blotters to draw liquids out.
• Transfer (Painting Transfer): A historic restoration process (mostly 19th–early 20th century) where a painting was removed from its original canvas or panel and transferred to a new support. This drastic method was used for paintings on wood panels that were rotting or on canvas that was extremely deteriorated. It involved adhering a new canvas to the front of the painting (face-side), then removing the original support from the back, and finally lining the exposed paint layer onto a new support. Many Renaissance panel paintings in the 1800s were transferred to canvas to “solve” wood problems – but transfers often resulted in loss of original glazes, flattening of impasto, or even distortion of the image. Today, transfers are rarely done (only if absolutely no alternative), as the preference is to preserve the original support whenever possible. Knowing if a painting was transferred is important for conservation, as the current support is not original and the paint layer might be fragile from the process. Signs include the absence of original canvas on the reverse or tool marks on the back of paint visible under raking light (from planing off the panel).
• Ultraviolet (UV) Examination: Using long-wave UV light (around 365 nm) in a dark room to examine a painting’s surface, taking advantage of the phenomenon of fluorescence. Natural resin varnishes (like dammar or shellac) typically fluoresce a greenish or bluish haze under UV, whereas areas of restoration (overpaint or modern synthetic varnishes) often appear as dark patches since they either fluoresce differently or quench the fluorescence. UV can reveal varnish presence, uneven cleaning (cleaned spots show as dark against fluorescing varnish), and retouches (which usually show as blackish spots if they are applied on top of an older varnish). It also helps spot residues of glue or mold and can differentiate pigment types (for example, zinc white vs. lead white: zinc white absorbs UV and looks dark, while lead white reflects and stays light). UV examination is a quick, non-destructive tool routinely used in conservation to map previous restorations and condition. Conservators document UV findings with UV photographs to guide varnish removal and retouching steps.
• Ultraviolet (UV) Varnish Removal: Not an official term, but refers to the practice of using UV light during cleaning to monitor progress. As one removes a yellowed natural varnish, viewing the painting under UV helps ensure all residual varnish is gone (remaining bits will still fluoresce). This prevents uneven cleaning. It’s more a technique than a term for a condition. It underscores how UV is not only for examination but also a live feedback tool during treatment.
• Underpainting: Preliminary layers of paint the artist places to establish composition, tones, or forms, meant to be built upon by subsequent layers. Underdrawing (in dry media) and underpainting (in paint) are often hidden by the final surface, but due to increased transparency with age or thin passages, they sometimes become visible (related to pentimenti). Conservation-wise, underpainting becomes important when cleaning or varnish removal might increase its visibility – the conservator must ensure the final presentation balances the artist’s intended final look. Also, analytical imaging like IRR is essentially revealing the underpainting or underdrawing. If underpainting is exposed due to abrasion (as in a skinned area where upper layers were lost, showing the brown underpaint), a conservator might inpaint carefully to simulate the intended surface.
• Underdrawing: The artist’s preliminary drawing on the ground (often in charcoal, pencil, or ink) laid down before painting. Invisible to the naked eye in a finished painting (except where pentimenti show), underdrawings are commonly revealed by infrared reflectography, since carbon-based media absorb IR and show the drawn lines. The presence and style of underdrawing can inform attribution and technique (e.g. whether the artist sketched freehand or used pounced cartoons). Underdrawings have no direct deterioration issues unless the material (like iron-gall ink) causes support problems, but their visibility can change: an oil paint becoming transparent may reveal underdrawing that was originally hidden. Conservators take care not to accidentally dissolve fragile underdrawing when doing treatments (fortunately, most are embedded under ground or paint).
• Varnish: A transparent protective (and often aesthetic) coating applied to the surface of a painting after it’s finished. Traditional varnishes are natural resins (dammar, mastic, shellac) which impart saturation and gloss, but yellow and degrade over time. Modern varnishes can be synthetics (acrylic or ketone resins) chosen for better stability. Deterioration of varnish includes yellowing, cracking (craquelure in the varnish itself, sometimes called crazing when very fine), and blooming or blanching in high humidity. Removing a discolored varnish is one of the most common restoration tasks, done with carefully tested solvent mixtures that dissolve the varnish without harming the underlying paint. Some paintings (especially modern ones) are unvarnished by design; conservators must respect that and not varnish them unless necessary for saturation or protection and in line with artist’s intent. If a painting is left unvarnished, its surface is more vulnerable to dirt and abrasion, so display and cleaning must be gentler.
• Warping: Distortion of a rigid support (panel, stretcher, etc.) out of flat plane. A wooden panel can warp into a curve or twist due to changes in humidity (wood swells across the grain but not along it). Warping puts stress on the paint and ground, often causing cracks or cleavage particularly along the grain lines or where the panel is restrained. A severely warped panel may even cause paint to flake as the curvature pulls layers apart. Conservation of a warped panel might involve gradual moisture conditioning and mechanical restraint or adding a support cradle, though care is taken not to introduce new stresses. For canvas, the term warping may refer to a twisted stretcher or even distortion of the canvas weave (like tension garland patterns). In any case, warping is addressed by environmental control and structural corrections to the support rather than attempting to force the painting flat in one go (which could cause worse damage).
• Watercolor: A painting medium in which pigments are bound in a water-soluble binder (typically gum arabic) and applied with water on a support, usually paper. Watercolor paintings are known for their transparency and delicacy. Deterioration issues in watercolors include fading of fugitive pigments (as many aniline dye-based colors were historically used), foxing and discoloration of the paper support, and sensitivity to moisture (the paint can readily bleed or move if wetted). Thus, watercolors are usually displayed under glass with controlled lighting and humidity. In a conservation glossary context, watercolor is included as one of the major media; its conservation involves minimally invasive treatments like light dry cleaning of paper, flattening cockled paper, washing or deacidifying if safe, and inpainting losses on paper with reversible colors. Unlike oils, watercolors have no varnish traditionally (though some 19th-century ones were varnished, essentially turning them into something closer to an opaque painting). Removing such varnish from a watercolor is risky since the paint can solubilize – often conservators leave a discolored varnish if its removal endangers the watercolor.
• Wear: General abrasion or erosion from handling, cleaning, or just age. “Wear” might be used to describe minor cumulative loss: e.g. “frame wear on edges” (paint lightly rubbed on edges from past framings), or “wear in the gilding” (high points of gilded surface showing base red bole). It indicates the artwork has been subject to use or touch that gently degraded it. In textiles or objects, one talks of wear patterns; in paintings, it’s less often used formally, but you might encounter phrases like “overall surface wear.” Conservators typically address wear by cosmetic inpainting if it’s visually disfiguring (like worn-through canvas weave that catches the eye) or by preventive measures to stop further wear (like adding protective glazing to avoid future touching).
• X-radiography (X-ray): Radiographic imaging using X-rays to examine a painting’s internal structure. X-rays penetrate materials based on density and thickness: higher-density components (e.g. lead white paint, nails, canvas tacking edges, or a wooden panel) absorb more X-rays and appear light on the X-ray film/image, whereas lower-density areas appear darker. X-radiography is a fundamental tool revealing features such as: the artist’s layering and pentimenti (e.g. seeing if a composition lies beneath another, as X-rays can show lead-based underpainting), changes in the painting (a hidden figure or different positioning, since earlier paint may have different opacity), structural damages (like cracks in a panel, or repairs, and lost canvas pieces), as well as the presence of old linings, nails, or other inserts. It can also differentiate pigments to some degree – for instance, heavy metals like lead, mercury (in vermilion), and barium (in some whites) show up bright. In conservation, X-rays help map out where significant features are before treatment; for example, if an old canvas has many tack holes from re-stretching or evidence of a different painting underneath, this will influence the approach. The X-ray image (radiograph) itself becomes part of the artwork’s documentation.
• X-ray Fluorescence (XRF): A non-destructive analytical technique that identifies elements present in the artwork’s materials by detecting the characteristic secondary X-rays emitted when the object is irradiated with a primary X-ray beam. In practice, a handheld or desktop XRF device is pointed at spots on the painting, and it returns spectra showing peaks for elements like lead (Pb), mercury (Hg), copper (Cu), etc. XRF is widely used to identify pigments: for example, a detection of Pb and Sn suggests lead-tin yellow; Hg suggests vermilion; Zn suggests zinc white, etc.. It’s especially useful to identify heavy elements associated with pigments or contaminants (like arsenic from emerald green, or iron from earths). XRF can also reveal elemental composition of corrosion or efflorescence products (e.g. if a crystalline bloom contains calcium, it might be calcium oxalate). It does not directly tell you the compound (that’s where techniques like XRD or Raman help), but knowing elements is a big clue. Being non-invasive, XRF is a go-to examination in situ for historic paintings, to guide deeper analysis or confirm suspicions (like seeing if a supposed 18th-century painting has titanium – which would likely be a 20th-century pigment presence, indicating possible forgery or overpaint). Limitations: XRF’s sampling depth might be a few tens of microns, so it reads a combination of surface and somewhat subsurface layers, and cannot easily differentiate if an element is in a top glaze or underlying paint unless done in a stepwise approach (sometimes through comparisions and known layer structures). Nonetheless, XRF has become almost standard in museum laboratories for material identification.
• Yellowing: The discoloration of originally white, bright, or color-balanced areas to a yellow tone, usually due to aging of the binding medium or varnish. Oil paint naturally yellows as the linseed oil oxidizes – noticeable in light-colored areas or where lead white with oil becomes warmer in tone. Varnishes (dammar, mastic) significantly yellow and brown with time, altering the overall tonality of a painting, often muting blues and making the painting appear uniformly warmer. Even acrylic resins can slightly yellow over decades (though some modern formulations try to minimize this). Conservation addresses yellowing mostly by cleaning: removing or reducing a yellowed varnish to restore the original color relationships. For oil paint that has yellowed: interestingly, aged linseed oil can bleach somewhat in light (so some paintings left long in dark storage get a yellow cast that partially reverses with exposure to light – a phenomenon conservators might exploit by temporarily exposing a painting to sunlight after cleaning to reduce residual oil yellowing). However, one must be cautious with light exposure to avoid fading pigments. If the oil binder in the actual paint is very yellow and it’s objectionable (for instance, a 19th-century sky gone greenish because the white and blue have yellowed), some treatments like controlled peroxide bleaching have been attempted in the past but are risky and not standard. Usually, removal of yellowed varnish suffices, and any remaining tonal imbalance is accepted as part of aging. Preventive measures: storing paintings in moderate light (not darkness) if they have no varnish, to avoid “dark yellowing” of oil that can occur and to allow natural bleaching.
• Zinc Oxide: A white pigment introduced in the 19th century (Chinese white in watercolors, zinc white in oils) prized for its brightness. However, in oil paint, zinc oxide is now known to cause serious conservation issues: it forms zinc soaps that lead to embrittlement and delamination of paint layers. Many mid-20th century oil paintings with high zinc content (e.g. in priming or mixed with titanium white) have developed widespread cracking and cleavage — sometimes entire paint layers separate from the ground because zinc soaps form an intermediate layer with poor adhesion. Conservators must handle paintings containing zinc white with extreme care, often avoiding lining with heat (as heat can exacerbate zinc soap formation) and focusing on gentle environmental control. If flaking due to zinc is present, consolidation is done, but the underlying issue remains chemically active. Research is ongoing into consolidants or strategies to stabilize zinc-heavy paint. The presence of zinc can be identified by XRF or FTIR, and it’s an example where knowing the chemistry informs the approach: e.g., a conservator might avoid water-based treatments on zinc paints because zinc ions can migrate. In summary, zinc oxide is a cautionary tale in art materials: great optical properties, but a “time bomb” in oil paint — part of a conservator’s glossary for its role in deterioration.

(This glossary provides concise definitions of technical terms related to the deterioration and conservation of paintings across media, aiming to aid professionals in understanding condition reports, treatment proposals, and conservation literature. All terms are organized alphabetically and cover both the symptoms of aging or damage and the methods/tools used to examine and restore paintings.)