Glossary


Acid
A substance able to donate protons or accept an electron pair in reactions.

Acid free
Any material having a pH level of seven or higher.

Acidic
In paper, an unstable state whereby the molecular structure of the paper breaks down, causing discoloration and weakening of the sheet.

Adhesive or tape staining
Yellowing and/or darkening caused by animal based adhesives. 

The image below has adhesive or tape staining on the bottom margin. 

Example of adhesive or tape staining

Alkaline buffer
An additive used in paper-making processes and conservation treatments that will raise the pH level.

à la poupée 
An early method of printing in color. A full color image is created with only one plate and avoids the difficulties of registration and color theory required for multiple plate or stone printing. À la poupée inking utilizes a cloth daubing pouch whose shape resembles a poupée (“doll” in French). The copper plate is inked in full color; in essence, the plate is hand colored. The print is then made in full color with one pass through the press. The artist would usually add final touches to each print with watercolor, hence the term “finished by hand.” An engraving colored in this manner reveals the various colors within the actual engraved lines or stippled dots. This differs from a hand-colored engraving where the engraved lines reveal one color of ink (usually black) underneath the broader areas of transparent watercolor applied after the print is made.

Alum
Aluminum sulfate; employed to precipitate rosin sizing in paper.

Aqueous treatment 
Any restoration procedure that utilizes water.

Archival
A material with good aging properties due to a neutral or slightly alkaline pH. 

Aquatint
An etching process that creates tonal areas on the plate. It is usually used in conjunction with line engraving or etching. The best examples of this technique are seen in the early nineteenth-century English works of Havell, Brookshaw and Thornton. A porous resin ground is applied to the plate. This ground is then made to contract either by a heating process or suspension in a solvent. The contraction of the resin particles causes an irregular linear pattern to form around each particle. The plate is then etched with acid, which affects only the linear patterned parts of the exposed copper. The word aquatint is derived from aquafortis, which translates to “strong water.” The term refers to nitric acid, which is used to etch the copper plate.

Blind stamp or embossing
Indentations in paper, usually text or symbols, that do not contain color. Our Oppenheimer Editions prints are blind embossed.

Blister
Small swellings that occur when a paint layer becomes too warm.

Bloom 
A fine cloudy discoloration which can form beneath the surface of varnish.

Cellulose
A substance constituting the chief part of the cell walls of plant material, trees and paper.

Chine-collé or chine collé
A technique used in the printmaking process, often etching or lithograph, where the image is printed on a thin sheet of paper that is mounted to a heavier sheet of paper. The heavier sheet of paper often has a plate mark. 

The image below is an example of a chine-collé etching on paper.

Example of Chine Collé etching on paper

Cockling
A waviness that occurs in paper due to exposure to moisture. 

Below is an example of cockling in a vellum document.

Example of cockling in a vellum document

Cold suction vacuum table
A device used for localized chemical and aqueous treatments to paper.

Condition report
A written documentation of the condition of a work of art itemizing the details of the art like the medium, and any accompanying labels or framing materials. A condition report most importantly documents damage of a work of art, and will notate any recommended conservation or restoration procedures. Artwork should be thoroughly documented with photographs before and after restoration. Condition reports in additional to being a conservation and restoration guide for the conservator, may also necessary when selling or transporting artwork.

Conservation (art conservation)
The preservation of works of art, including drawings, paintings, photographs, fiber art, and objects. Conservation prevents the further deterioration and/or stabilizes damage to works of art. Conservation techniques should always be reversible, and not permanently change a work of art. 

Consolidation 
Conservation treatments using a variety of adhesives to re-adhere loose paint or broken parts of the support and to prevent recurrence of delaminations.

Covalent bond
A chemical bond formed by sharing of one or more electrons between atoms; the type of molecular structure that forms cellulose in paper fibers.

Craquelure
A network of fine cracks in a paint surface. 

Below is an example of craquelure in an oil painting.

Example of craquelure in an oil painting

Crazing 
In varnish, the cracking of the layer into such a fine network that the layer becomes opaque.

Chromolithography or Chromolithograph
A natural extension of one-color lithography, chromo (color) lithography involves using separate stones for each color. Before the use of chromolithography, prints were usually colored by hand. This new process was an ambitious development utilizing multiple stones. The difficulty of aligning the paper perfectly (registering) for each color was the primary challenge in the development of this medium. Chromolithography was developed during the second half of the nineteenth century. Its best-known early proponent was Julius Bien, who used this technique when printing the 1858-1860 Bien Edition of The Birds of America.

Cupping
Deformation of a paint film where the edges of cracks turn upward to create cup-like formations in response to environmental changes in temperature or humidity.

Deacidify
To chemically stabilize acidic paper; may be either an aqueous or non-aqueous treatment.

Delamination
The separation of a material into layers.

Double elephant folio
A bookbinding and printing term used to refer to very large prints and books. Double elephant folio size is the largest and can range up to 50" on the longest side. Audubon's Havell and Bien Editions are double elephant sizes. The 1981 publication of The Birds of America by the Audubon Society is often referred to as the baby elephant folio. 

Joel Oppenheimer with Audubon Bien Edition double elephant folio.

Joel Oppenheimer with Audubon Bien Edition double elephant folio

Dry peel
A restoration technique that is manual (non-aqueous and non-chemical) and employed to reduce or remove a mount material.

Dry treatment
Restoration treatments that do not involve water or chemicals.

Drypoint
Etching that involves the use of a sharp tool “burin” to physically incise lines into the copper plate. This differs from traditional etching which relies on a chemical process to etch the lines into the plate. The characteristic soft lines of a drypoint etching are created by the jagged edge of the lines cut by the burin, this rough edge is called the burr.

Embrittlement
A fragile condition that results from dehydration and prolonged acidic degradation.

Emulsion
A photosensitive (light sensitive) material which consists of a coating of silver halide or other photo-active grains in a gelatin layer, on photographic metal plates (for a daguerreotype), glass plates, film, fabric, paper or other surfaces.

Engaged face mat
An artwork is engaged when the facemat covers part of the artwork.

Etching 
In etching the lines in the copper plate are chemically etched into the copper by either immersing the plate into acid or otherwise applying acid to the copper surface. The plate is first coated with a solid ground of resin or wax. This ground is then scratched away with a needle that selectively exposes the copper in those areas. The ground resists the acid, allowing it to “bite” into only the areas that have been drawn on with the needle. The depth of the line is dictated by the duration of exposure of the plate to the acid. Although the technique of making the plate differs from engraving, the method of printing an etching is identical to that of printing an engraving, and both show the characteristic plate mark in the paper. The quality of the line created, however, is quite different and distinguishable to the trained eye.

Expansion
The result of change in the dimension of a sheet of paper due to excess humidity; more pronounced across the grain.

Fiber fill
Utilizing paper pulp to complete losses in a sheet of paper.

Finished corner or closed corner
A frame which is finished by hand after joining the moulding to create a seamless look. 

Fillet
A spacer device placed between the glazing and the mount in a frame which prohibits the glazing material from coming in contact with the artwork.

Flaking
Extreme cracking that causes the paint and/or ground layer to dislodge from its support, often through a combination of cleavage and cracking.

Flattening
A restoration procedure involving controlled humidification and controlled drying under pressure.

Floated face mat or with fillets
An artwork can be floated with a facemat or with fillets. With either of these options, the entire artwork is visible.  

Foxing
A type of stain in paper resulting from mold growth.

The dot-like pattern in the image below is an example of foxing.

Example of foxing on paper

Fragmenting
When a sheet of paper has portions broken off or detached.

French mat
A mat with hand painted lines and color. 

Example of a French mat

Fugitive pigment
A pigment that becomes unstable when exposed to extreme environmental conditions.

Giclée
A generic term used to describe a variety of sophisticated large-scale ink-jet printing devices. Giclée is a French word meaning “spray” or “spurt”, which refers to how the ink or dye is applied to paper or fabric. A giclée printer is capable of producing millions of colors using continuous-tone technology. The extra-fine image resolution possible in this printing process permits retention of a high degree of fine detail from the original image, rendering deeply saturated colors having a broad range of tonal values.

Gelatin
One of many sizings that may be used to make paper less liable to bleed; a glutinous material obtained from animal tissues through continued boiling.

Handling crimps
Small creases that only cause minor disruption in the paper fibers. Often caused by the handling or gripping the artwork.

Heat-set tissue or dry-mount tissue
Any one of a variety of heat-reactive tissues used as an adhesive material.

Hinge
In framing, a folding or hanging device used to attach artwork to a mount allowing limited movement to occur.

Hydration
In restoration, the process of introducing moisture to an embrittled, dehydrated sheet.

Hygroexpansivity
Expansion or contraction of paper due to changing conditions of humidity.

Inpainting
Replacing missing pigment strictly within the borders of the loss.

Interrupted fiber
In a fold, when the paper fiber has ruptured or torn and the surface is no longer continuous.

Leaf casting
A process employing cold suction to complete voids with pulp.

Lifting
Layer separation whereby a painting’s surface is raised.

Lignin
A complex polymer; the chief non-carbohydrate constituent in wood that, in a living tree, binds to cellulose fibers and strengthens the cell walls. In processed wood pulp paper, the element that becomes acidic and breaks down the cellular structure.

Line engraving
Copper plate engraving was introduced in the fifteenth century. A thin copper plate is polished smooth. The lines are manually incised into the plate with a sharp, steel-bladed tool called a burin. The tip of the burin is V-shaped, thus allowing the thickness of the engraved line to be controlled by the depth of the incision in the plate. The plate is inked, and the ink is then wiped away. The ink remaining in the recesses of the engraved lines is transferred to paper under extreme pressure in a printing press. The resulting image is an engraving.

Lithograph or lithography
This technique is completely different from the other printing processes described here in that it does not rely on designs cut in relief or incised in the printing element to retain the ink. Lithography was discovered by a fluke occurrence observed and developed by a German printer named Senefelder. In 1797, Senefelder, out of paper, scribed a laundry list with a grease pencil on a limestone slab in his studio. Attempting to clean the stone, he found that the greasy area resisted the washing. The lithographic technique relies on this principle. First the drawing is made with a grease-based crayon. The stone is then wetted. The limestone absorbs the moisture and the crayon drawing repels it. Ink is then applied to the stone and the reverse occurs. The wet stone repels the ink, and the crayon drawing accepts it. A print is then made by pressing paper against the stone. This is done with a printing press designed for this purpose. The ease with which an artist can translate his talent into lithography stimulated a proliferation of art prints in the latter half of the nineteenth century. Stone lithography, as described here, is not to be confused with commercial offset lithography, a photomechanical process that does not involve the artist’s hand.

Mat burn or reverse mat burn
Discoloration under a facemat often caused by acidity. Reverse mat burn is discoloration of the area not covered by a facemat and is often the cause of acidity or exposure to UV light.

Below is an example of reverse mat burn.

Example of reverse mat burn

Mezzotint
An engraving technique that, like line engraving, is manual and does not involve a chemical process. In contrast to line engraving, the effect of the mezzotint is purely tonal. The copper plate is worked with a tool called a rocker. The rocker is a crescent-shaped instrument with many sharp points or teeth on the curved face or crescent. A rocking motion inflicts scores of points upon the plate until it is completely covered with them. If the plate were inked and printed in this state, it would yield an image of solid black. However, the copper plate is then slowly burnished by the artist, thereby smoothing areas of the plate. The smooth areas do not retain ink and become the lighter or white parts of the image. This is also known as a subtractive method.

Mulberry tissue
A long-fibered, unsized, acid-free Japanese paper which is used as a support mechanism for weakened paper or tears.

Octavo
Refers to a specific size of a book page. Refers to folding of a single sheet of paper eight times resulting in an approximately 6 1/2" x 10" page. Audubon's miniature edition is referred to the Octavo Edition.

Offset lithograph or lithography
Offset lithography is generally considered a commercial photomechanical four-color (CMYK) process.

Paper inlay
A solid piece of paper grafted to a sheet to complete a void.

Patina
The natural changes in appearance of a material due to aging.

pH value (power of hydrogen)
A method of measuring acidity or alkalinity, numerically equivalent to 7 for neutral solutions, increasing with increased alkalinity and decreasing with increased acidity. The pH scale commonly in use ranges from 0-14.

Pigment
A substance that provides color; usually suspended in a binding agent.

Plate mark
A depression produced in the printing process caused by the edge of the printing plate.

Below is an example of a plate mark on a Miro aquatint etching.

Example of a plate mark on an etching

Preparator
A person who performs or supervises duties involving the handling of art objects.

Pressure-sensitive tape
A tape that is adhered by light pressure. Pressure-sensitive tapes degrade over time causing the adhesive to seep into the paper and cause adhesive staining.

Printer's crease
A crease caused in the printing process and generally considered original to the artwork.

Pulp
In paper making, the fibrous substance resulting from the pulping process.

Rabbet
An L-shaped cutout that forms the lip of a frame. The part that holds the picture and other framing materials.

Rag board or cotton rag paper
Paper or board made of 100% cotton fibers is generally considered archival and will not age as fast as wood pulp papers or boards

Raking light
Illumination from the side at a low angle, creating long shadows; used by conservators to reveal flaws in the surface of a work of art.

Recto
The front or face of a single sheet of paper or painting, or the right-hand page of an open book.

Reline
To adhere a piece of cloth, usually linen, to the back of an original textile or painting as a strengthening measure.

Restoration (art restoration)
A process by which a professional restorer of artwork with expert training reverses damage caused by age, environment, and/or accident.  The restoration process aims to return a work of art to it's original state by utilizing reversible techniques.

Reversibility
A material possessing the property that allows the it to be completely removed without damaging the original.

Resin
In painting conservation, a natural or synthetic substance which when dissolved or suspended in a medium becomes varnish.

Serigraph, silkscreen or screenprint
A printmaking process in which the image is created by forcing ink through a series of mesh screen that is like a stencil. Serigraphs most commonly have a very matte surface.

Shrinkage cracks
These occur when quick-drying layers are applied over soft, slow-drying paint. They appear in glazed areas that are thick with medium and sparse in pigment.

Size or sizing
A substance added to paper to create a degree of water resistance.

Strip line
A procedure used to support the margins of a painting when stretching it on stretcher bars.

Stipple engraving
Stippling a plate can be achieved with either an engraving or an etching process. A needle is used to individually incise minute depressions of varying depth into the surface of the copper plate. The forms and shapes are made up of thousands of tiny dots rather than lines. This process was developed in England but perfected in France in the early nineteenth century. Stippled engravings were particularly well suited for the à la poupée coloring method. The subtlety of tone and seamless transition of color achieved made this the preferred technique of the great French botanical artists like Redoute.

Substrate
The primary layer of material; can relate to a mount substance or the base material upon which a work of art is executed.

Surface clean
Any one of various dry, non- abrasive methods used to remove surface dirt, soot or grime from a sheet of paper or painting.

Below is a severe example of surface dirt on an engraving.

Example of severe surface dirt on an engraving on paper

Thymol vapor treatment or thymolize
Thymol is a white crystalline aromatic compound derived from thyme oil or made synthetically that is employed in a gaseous state as a fungicide to kill mold.

The image below is an extreme example of mold on a print. The mold was neutralized by the thymol treatment. 

Example of mold on a serigraph print

Tide mark
A stain; a deposit which occurs at the point where water entered a sheet of paper and stopped.

Ultraviolet light
Primarily invisible light, ranging from the X-ray region, about four nanometers wavelength, to just beyond violet in the visible spectrum, about 380 nanometers. UV light causes deterioration and fading.

Van der Waals forces
Weak, short-range electrostatic attractive forces between uncharged molecules, arising from the interaction of permanent or transient electric dipole moments. One of the phenomena that bonds fibers together to form a sheet of paper.

Varnish
Any one of the various transparent coatings, synthetic or organic, used as a final layer on the surface of a work of art.

Verso
The back or underside of a single sheet of paper or painting, or the left hand page of an open book.

Void
A gap or area missing from the original paper or canvas.

Watermark
Text or symbols in paper that are visible on a light table. Watermarks are applied in the paper making process by creating thickness or density variations in the paper.

Wet peel
A method of reducing or removing a mount material involving aqueous treatment, but avoiding immersion in a water bath.

Woodcut or wood block print
The earliest method used for making natural history prints. The first woodcuts or wood block prints appeared in Europe in the fifteenth century. The face of a hardwood plank is carved or scooped out with specially designed knives. The woodcut differs from other printing methods in that the artist must create the image by working the negative space. The untouched areas remaining in high relief receive the ink and form the printed image.

Wood engraving 
The same technique as that of the woodcut. In the early sixteenth century, artists discovered that utilizing the end grain of a very hard wood, rather than the plank side, would result in a finer, crisper line. The method used in wood engraving does not resemble metal engraving in any way and should not be confused with it.

Wood pulp mount or paper
Wood fibers commonly used in the paper making process. Wood pulp papers and boards become acidic over time.

Acid
A substance able to donate protons or accept an electron pair in reactions.

Acid free
Any material having a pH level of seven or higher.

Acidic
In paper, an unstable state whereby the molecular structure of the paper breaks down, causing discoloration and weakening of the sheet.

Adhesive or tape staining
Yellowing and/or darkening caused by animal based adhesives. 


Alkaline buffer
An additive used in paper-making processes and conservation treatments that will raise the pH level.

à la poupée 
An early method of printing in color. A full color image is created with only one plate and avoids the difficulties of registration and color theory required for multiple plate or stone printing. À la poupée inking utilizes a cloth daubing pouch whose shape resembles a poupée (“doll” in French). The copper plate is inked in full color; in essence, the plate is hand colored. The print is then made in full color with one pass through the press. The artist would usually add final touches to each print with watercolor, hence the term “finished by hand.” An engraving colored in this manner reveals the various colors within the actual engraved lines or stippled dots. This differs from a hand-colored engraving where the engraved lines reveal one color of ink (usually black) underneath the broader areas of transparent watercolor applied after the print is made.

Alum
Aluminum sulfate; employed to precipitate rosin sizing in paper.

Aqueous treatment 
Any restoration procedure that utilizes water.

Archival
A material with good aging properties due to a neutral or slightly alkaline pH. 

Aquatint
An etching process that creates tonal areas on the plate. It is usually used in conjunction with line engraving or etching. The best examples of this technique are seen in the early nineteenth-century English works of Havell, Brookshaw and Thornton. A porous resin ground is applied to the plate. This ground is then made to contract either by a heating process or suspension in a solvent. The contraction of the resin particles causes an irregular linear pattern to form around each particle. The plate is then etched with acid, which affects only the linear patterned parts of the exposed copper. The word aquatint is derived from aquafortis, which translates to “strong water.” The term refers to nitric acid, which is used to etch the copper plate.

Blind stamp or embossing
Indentations in paper, usually text or symbols, that do not contain color. Our Oppenheimer Editions prints are blind embossed.

Blister
Small swellings that occur when a paint layer becomes too warm.

Bloom 
A fine cloudy discoloration which can form beneath the surface of varnish.

Cellulose
A substance constituting the chief part of the cell walls of plant material, trees and paper.

Chine-collé or chine collé
A technique used in the printmaking process, often etching or lithograph, where the image is printed on a thin sheet of paper that is mounted to a heavier sheet of paper. The heavier sheet of paper often has a plate mark. 

Cockling
A waviness that occurs in paper due to exposure to moisture.

Cold suction vacuum table
A device used for localized chemical and aqueous treatments to paper.

Condition report
A written documentation of the condition of a work of art itemizing the details of the art like the medium, and any accompanying labels or framing materials. A condition report most importantly documents damage of a work of art, and will notate any recommended conservation or restoration procedures. Artwork should be thoroughly documented with photographs before and after restoration. Condition reports in additional to being a conservation and restoration guide for the conservator, may also necessary when selling or transporting artwork.

Conservation (art conservation)
The preservation of works of art, including drawings, paintings, photographs, fiber art, and objects. Conservation prevents the further deterioration and/or stabilizes damage to works of art. Conservation techniques should always be reversible, and not permanently change a work of art. 

Consolidation 
Conservation treatments using a variety of adhesives to re-adhere loose paint or broken parts of the support and to prevent recurrence of delaminations.

Covalent bond
A chemical bond formed by sharing of one or more electrons between atoms; the type of molecular structure that forms cellulose in paper fibers.

Craquelure
A network of fine cracks in a paint surface.

Crazing 
In varnish, the cracking of the layer into such a fine network that the layer becomes opaque.

Chromolithography or Chromolithograph
A natural extension of one-color lithography, chromo (color) lithography involves using separate stones for each color. Before the use of chromolithography, prints were usually colored by hand. This new process was an ambitious development utilizing multiple stones. The difficulty of aligning the paper perfectly (registering) for each color was the primary challenge in the development of this medium. Chromolithography was developed during the second half of the nineteenth century. Its best-known early proponent was Julius Bien, who used this technique when printing the 1858-1860 Bien Edition of The Birds of America.

Cupping
Deformation of a paint film where the edges of cracks turn upward to create cup-like formations in response to environmental changes in temperature or humidity.

Deacidify
To chemically stabilize acidic paper; may be either an aqueous or non-aqueous treatment.

Delamination
The separation of a material into layers.

Double elephant folio
A bookbinding and printing term used to refer to very large prints and books. Double elephant folio size is the largest and can range up to 50" on the longest side. Audubon's Havell and Bien Editions are double elephant sizes. The 1981 publication of The Birds of America by the Audubon Society is often referred to as the baby elephant folio.

Dry peel
A restoration technique that is manual (non-aqueous and non-chemical) and employed to reduce or remove a mount material.

Dry treatment
Restoration treatments that do not involve water or chemicals.

Drypoint
Etching that involves the use of a sharp tool “burin” to physically incise lines into the copper plate. This differs from traditional etching which relies on a chemical process to etch the lines into the plate. The characteristic soft lines of a drypoint etching are created by the jagged edge of the lines cut by the burin, this rough edge is called the burr.

Embrittlement
A fragile condition that results from dehydration and prolonged acidic degradation.

Emulsion
A photosensitive (light sensitive) material which consists of a coating of silver halide or other photo-active grains in a gelatin layer, on photographic metal plates (for a daguerreotype), glass plates, film, fabric, paper or other surfaces.

Engaged face mat
An artwork is engaged when the facemat covers part of the artwork.

Etching 
In etching the lines in the copper plate are chemically etched into the copper by either immersing the plate into acid or otherwise applying acid to the copper surface. The plate is first coated with a solid ground of resin or wax. This ground is then scratched away with a needle that selectively exposes the copper in those areas. The ground resists the acid, allowing it to “bite” into only the areas that have been drawn on with the needle. The depth of the line is dictated by the duration of exposure of the plate to the acid. Although the technique of making the plate differs from engraving, the method of printing an etching is identical to that of printing an engraving, and both show the characteristic plate mark in the paper. The quality of the line created, however, is quite different and distinguishable to the trained eye.

Expansion
The result of change in the dimension of a sheet of paper due to excess humidity; more pronounced across the grain.

Fiber fill
Utilizing paper pulp to complete losses in a sheet of paper.

Finished corner or closed corner
A frame which is finished by hand after joining the moulding to create a seamless look. 

Fillet
A spacer device placed between the glazing and the mount in a frame which prohibits the glazing material from coming in contact with the artwork.

Flaking
Extreme cracking that causes the paint and/or ground layer to dislodge from its support, often through a combination of cleavage and cracking.

Flattening
A restoration procedure involving controlled humidification and controlled drying under pressure.

Floated face mat or with fillets
An artwork can be floated with a facemat or with fillets. With either of these options, the entire artwork is visible.  

Foxing
A type of stain in paper resulting from mold growth.

Fragmenting
When a sheet of paper has portions broken off or detached.

French mat
A mat with hand painted lines and color. 

Example of a French mat

Fugitive pigment
A pigment that becomes unstable when exposed to extreme environmental conditions.

Giclée
A generic term used to describe a variety of sophisticated large-scale ink-jet printing devices. Giclée is a French word meaning “spray” or “spurt”, which refers to how the ink or dye is applied to paper or fabric. A giclée printer is capable of producing millions of colors using continuous-tone technology. The extra-fine image resolution possible in this printing process permits retention of a high degree of fine detail from the original image, rendering deeply saturated colors having a broad range of tonal values.

Gelatin
One of many sizings that may be used to make paper less liable to bleed; a glutinous material obtained from animal tissues through continued boiling.

Handling crimps
Small creases that only cause minor disruption in the paper fibers. Often caused by the handling or gripping the artwork.

Heat-set tissue or dry-mount tissue
Any one of a variety of heat-reactive tissues used as an adhesive material.

Hinge
In framing, a folding or hanging device used to attach artwork to a mount allowing limited movement to occur.

Hydration
In restoration, the process of introducing moisture to an embrittled, dehydrated sheet.

Hygroexpansivity
Expansion or contraction of paper due to changing conditions of humidity.

Inpainting
Replacing missing pigment strictly within the borders of the loss.

Interrupted fiber
In a fold, when the paper fiber has ruptured or torn and the surface is no longer continuous.

Leaf casting
A process employing cold suction to complete voids with pulp.

Lifting
Layer separation whereby a painting’s surface is raised.

Lignin
A complex polymer; the chief non-carbohydrate constituent in wood that, in a living tree, binds to cellulose fibers and strengthens the cell walls. In processed wood pulp paper, the element that becomes acidic and breaks down the cellular structure.

Line engraving
Copper plate engraving was introduced in the fifteenth century. A thin copper plate is polished smooth. The lines are manually incised into the plate with a sharp, steel-bladed tool called a burin. The tip of the burin is V-shaped, thus allowing the thickness of the engraved line to be controlled by the depth of the incision in the plate. The plate is inked, and the ink is then wiped away. The ink remaining in the recesses of the engraved lines is transferred to paper under extreme pressure in a printing press. The resulting image is an engraving.

Lithograph or lithography
This technique is completely different from the other printing processes described here in that it does not rely on designs cut in relief or incised in the printing element to retain the ink. Lithography was discovered by a fluke occurrence observed and developed by a German printer named Senefelder. In 1797, Senefelder, out of paper, scribed a laundry list with a grease pencil on a limestone slab in his studio. Attempting to clean the stone, he found that the greasy area resisted the washing. The lithographic technique relies on this principle. First the drawing is made with a grease-based crayon. The stone is then wetted. The limestone absorbs the moisture and the crayon drawing repels it. Ink is then applied to the stone and the reverse occurs. The wet stone repels the ink, and the crayon drawing accepts it. A print is then made by pressing paper against the stone. This is done with a printing press designed for this purpose. The ease with which an artist can translate his talent into lithography stimulated a proliferation of art prints in the latter half of the nineteenth century. Stone lithography, as described here, is not to be confused with commercial offset lithography, a photomechanical process that does not involve the artist’s hand.

Mat burn or reverse mat burn
Discoloration under a facemat often caused by acidity. Reverse mat burn is discoloration of the area not covered by a facemat and is often the cause of acidity or exposure to UV light.

Mezzotint
An engraving technique that, like line engraving, is manual and does not involve a chemical process. In contrast to line engraving, the effect of the mezzotint is purely tonal. The copper plate is worked with a tool called a rocker. The rocker is a crescent-shaped instrument with many sharp points or teeth on the curved face or crescent. A rocking motion inflicts scores of points upon the plate until it is completely covered with them. If the plate were inked and printed in this state, it would yield an image of solid black. However, the copper plate is then slowly burnished by the artist, thereby smoothing areas of the plate. The smooth areas do not retain ink and become the lighter or white parts of the image. This is also known as a subtractive method.

Mulberry tissue
A long-fibered, unsized, acid-free Japanese paper which is used as a support mechanism for weakened paper or tears.

Octavo
Refers to a specific size of a book page. Refers to folding of a single sheet of paper eight times resulting in an approximately 6 1/2" x 10" page. Audubon's miniature edition is referred to the Octavo Edition.

Offset lithograph or lithography
Offset lithography is generally considered a commercial photomechanical four-color (CMYK) process.

Paper inlay
A solid piece of paper grafted to a sheet to complete a void.

Patina
The natural changes in appearance of a material due to aging.

pH value (power of hydrogen)
A method of measuring acidity or alkalinity, numerically equivalent to 7 for neutral solutions, increasing with increased alkalinity and decreasing with increased acidity. The pH scale commonly in use ranges from 0-14.

Pigment
A substance that provides color; usually suspended in a binding agent.

Plate mark
A depression produced in the printing process caused by the edge of the printing plate.

Preparator
A person who performs or supervises duties involving the handling of art objects.

Pressure-sensitive tape
A tape that is adhered by light pressure. Pressure-sensitive tapes degrade over time causing the adhesive to seep into the paper and cause adhesive staining.

Printer's crease
A crease caused in the printing process and generally considered original to the artwork.

Pulp
In paper making, the fibrous substance resulting from the pulping process.

Rabbet
An L-shaped cutout that forms the lip of a frame. The part that holds the picture and other framing materials.

Rag board or cotton rag paper
Paper or board made of 100% cotton fibers is generally considered archival and will not age as fast as wood pulp papers or boards

Raking light
Illumination from the side at a low angle, creating long shadows; used by conservators to reveal flaws in the surface of a work of art.

Recto
The front or face of a single sheet of paper or painting, or the right-hand page of an open book.

Reline
To adhere a piece of cloth, usually linen, to the back of an original textile or painting as a strengthening measure.

Restoration (art restoration)
A process by which a professional restorer of artwork with expert training reverses damage caused by age, environment, and/or accident.  The restoration process aims to return a work of art to it's original state by utilizing reversible techniques.

Reversibility
A material possessing the property that allows the it to be completely removed without damaging the original.

Resin
In painting conservation, a natural or synthetic substance which when dissolved or suspended in a medium becomes varnish.

Serigraph, silkscreen or screenprint
A printmaking process in which the image is created by forcing ink through a series of mesh screen that is like a stencil. Serigraphs most commonly have a very matte surface.

Shrinkage cracks
These occur when quick-drying layers are applied over soft, slow-drying paint. They appear in glazed areas that are thick with medium and sparse in pigment.

Size or sizing
A substance added to paper to create a degree of water resistance.

Strip line
A procedure used to support the margins of a painting when stretching it on stretcher bars.

Stipple engraving
Stippling a plate can be achieved with either an engraving or an etching process. A needle is used to individually incise minute depressions of varying depth into the surface of the copper plate. The forms and shapes are made up of thousands of tiny dots rather than lines. This process was developed in England but perfected in France in the early nineteenth century. Stippled engravings were particularly well suited for the à la poupée coloring method. The subtlety of tone and seamless transition of color achieved made this the preferred technique of the great French botanical artists like Redoute.

Substrate
The primary layer of material; can relate to a mount substance or the base material upon which a work of art is executed.

Surface clean
Any one of various dry, non- abrasive methods used to remove surface dirt, soot or grime from a sheet of paper or painting.

Thymol vapor treatment or thymolize
Thymol is a white crystalline aromatic compound derived from thyme oil or made synthetically that is employed in a gaseous state as a fungicide to kill mold.

Tide mark
A stain; a deposit which occurs at the point where water entered a sheet of paper and stopped.

Ultraviolet light
Primarily invisible light, ranging from the X-ray region, about four nanometers wavelength, to just beyond violet in the visible spectrum, about 380 nanometers. UV light causes deterioration and fading.

Van der Waals forces
Weak, short-range electrostatic attractive forces between uncharged molecules, arising from the interaction of permanent or transient electric dipole moments. One of the phenomena that bonds fibers together to form a sheet of paper.

Varnish
Any one of the various transparent coatings, synthetic or organic, used as a final layer on the surface of a work of art.

Verso
The back or underside of a single sheet of paper or painting, or the left hand page of an open book.

Void
A gap or area missing from the original paper or canvas.

Watermark
Text or symbols in paper that are visible on a light table. Watermarks are applied in the paper making process by creating thickness or density variations in the paper.

Wet peel
A method of reducing or removing a mount material involving aqueous treatment, but avoiding immersion in a water bath.

Woodcut or wood block print
The earliest method used for making natural history prints. The first woodcuts or wood block prints appeared in Europe in the fifteenth century. The face of a hardwood plank is carved or scooped out with specially designed knives. The woodcut differs from other printing methods in that the artist must create the image by working the negative space. The untouched areas remaining in high relief receive the ink and form the printed image.

Wood engraving 
The same technique as that of the woodcut. In the early sixteenth century, artists discovered that utilizing the end grain of a very hard wood, rather than the plank side, would result in a finer, crisper line. The method used in wood engraving does not resemble metal engraving in any way and should not be confused with it.

Wood pulp mount or paper
Wood fibers commonly used in the paper making process. Wood pulp papers and boards become acidic over time.