Frequently Used Archival Terms & What They Mean
This glossary is intended to help you make an informed choice of which materials you should use for the preservation of books, papers, photographs and other artifacts in your collection. Please note that this glossary is written in non-technical terms in order to help those lacking collections care training to meet the responsibilities required of them.
Acid: In chemistry, acid is a substance capable of forming hydrogen ions when dissolved in water. Acids can weaken cellulose in paper, board and cloth, leading to embrittlement. Acids may be present in materials either intentionally, by using inferior ingredients, or incidentally, by migration of acids form other materials or from atmospheric pollution. See also pH and acid migration.
Acid-Free: In chemistry, materials that have a pH of 7.0 or higher are considered acid-free. Sometimes used incorrectly as a synonym for alkaline or buffered. Such materials may be produced from virtually any cellulose fiber source (cotton and wood, among others), if measures are taken during manufacture to eliminate active acid from the pulp. However free of acid a paper or board may be immediately after manufacture, over time the presence of residual chlorine from bleaching, aluminum sulfate from sizing, or pollutants in the atmosphere may lead to the formation of acid unless the paper or board has been buffered with an alkaline substance.
Acid Migration: The transfer of acid from an acidic material to a less acidic or pH neutral material is known as acid migration. This may occur directly, when the two materials are in intimate contact. For instance, acid may migrate from packing or support materials, boxes, and other enclosures that house a document, book, or artifact.
Acrylic: A plastic noted for transparency, light weight, weather resistance, color fastness and rigidity. In addition to these qualities, acrylics are important in preservation because of their stability, or resistance to chemical change over time; a characteristic not common to all plastics. Acrylics are available in sheets, films, and resin adhesives.
Alkaline: Alkaline substances have a pH over 7.0. These substances may be added to a material to neutralize acids or as alkaline reserve or buffer for the purpose of counteracting acids that may form in the future. A buffer may be added during manufacture of during the process of deacidification. While a number of chemicals may be used as buffers, the most common are magnesium carbonate and calcium carbonate.
Alpha cellulose: A form of cellulose derived from cotton. The presence of alpha cellulose in paper or board is one indication of its stability or longevity. Non-cellulose components of wood are believed to contribute to the degradation of paper and board.
Archival (Archival Quality): This non-technical term suggests that a material or product is permanent, durable or chemically stable, and that it can therefore safely be used for preservation purposes. The phrase is not quantifiable; no standards exist that describe how long an "archival" or "archival quality" material will last.
Basis Weight: Basis weight is the weight of one ream (500 sheets) of paper, expressed in pounds.
Board: See Fiberboard or Solid Board
Buffering: Paper and paperboard products are divided between buffered and unbuffered. Calcium carbonate is the preferred buffering agent of preservation professionals. The buffering agent helps neutralize acids in the environment and helps to prevent those acids from attacking your collections. Because of concerns over the affect of buffering agents on certain animal-based products such as wool, leather and silk, these materials. Unbuffered storage materials are also recommended for certain rare photographic collections including dye transfer and cyanotypes. Note: Unbuffered materials can absorb acids from the environment and can, eventually become acidic themselves.
Calcium Carbonate: The preferred buffering agent of preservation professionals, Calcium Carbonate is added to paper pulp during the paper making process in order to neutralize acids. It's presence in folders, boxes, and other enclosures helps ensure that acids in the environment are neutralized.
Caliper: Caliper points, or simply points, is a unit for measuring the thickness of paper. One caliper point is equal to one one-thousandth of an inch.
Cellulose: The chief constituent of the cell walls of all plants is cellulose, and as such, is the chief constituent of many fibrous plant products including paper, board, and some clothes.
Chemical Stability: Not easily decomposed or otherwise modified chemically. This is a desirable characteristic for materials used in preservation, since it suggests an ability to resist chemical degradation (such as the embrittlement of paper), over time and/or upon exposure to various conditions during use or storage. Other terms used loosely as synonyms include inert, stable, and chemically inert.
Conservation: The treatment of library or archive materials, works of art, artifacts and museum objects to stabilize them chemically or strengthen them physically, sustaining their survival as long as possible in their original form.
Deacidification: A common term for a chemical treatment that neutralizes acid in a material such as paper and deposits an alkaline buffer to counteract future acid attack. While deacidification increases the chemical stability of paper, it does not restore strength or flexibility to brittle materials.
Encapsulation: A form of protective enclosure for papers and other flat object, this procedure involves placing the item between two sheets of transparent polyester film and sealing around the edges. The object is thus physically supported and protected from the atmosphere, although it may continue to deteriorate in the capsule if it were not properly deacidified. Because the object is not adhered to the polyester, it can be removed simply by cutting one or more edges of the polyester enclosure.
Fiber Content: A statement of the types and percentages of fibers used in the manufacture of paper, board, or clothe. The fiber content is important as it affects both the durability and chemical stability of the material.
Fiberboard: Paperboard made of laminated sheets of heavily pressed fiber.
Grain Direction: This represents the orientation of fibers in paper. Paper has a natural tendency to fold "with the grain" usually making it easy to determine the grain direction you are working with. Grain direction can be considered either short or long, depending on whether it runs parallel to the long or short side of the paper dimensions.
Inert: See chemical stability.
Lignin (Lignin-Free): Lignin is a naturally occurring substance found in plants that provides rigidity. In some circles, lignin is thought to contribute to the early degradation of paper and is often removed during the pulping process to create higher-grade paper.
Mil: This unit of measure is equal to one thousandth of an inch (.002).
Neutral: Having a pH level of around 7, being neither acidic nor alkaline.
Permanence: is the ability of a material to resist chemical deterioration but not a quantifiable term. Permanent paper usually refers to a durable alkaline paper that is manufactured according to ANSI Standards. Even so called permanent materials depend upon proper storage conditions to maintain their permanence.
pH (presence of Hydrogen): In chemistry, pH is a measurement of the concentration of Hydrogen ions, which is a measure of acidity or alkalinity. The pH scale runs from 1 to 14, with 7 being pH neutral; numbers below 7 indicate increasing acidity with 1 being most acidic, numbers above 7 indicate increasing alkalinity, with 14 being most alkaline. Each number away from 8 represents a ten fold increase. Paper with a pH below 5 is considered highly acidic.
Photo Activity Test (PAT): The PAT is an accelerated aging test that predicts the affects a material may have on photographic images. In essence, it predicts the type of interaction between an enclosures and photographic materials, and determines if any of the enclosures’ components, including adhesives, inks, paper and plastic, will adversely effect a photograph, negative, or other photographic materials.
Polyester: Polyester has long been recognized as a crystal clear, inert plastic sheeting that can be used to create long-term preservation enclosures. Mylar D, the trade name long recognized as the premier brand of polyester, was discontinued by the manufacturer in favor of another brand name, Mellinex – an equally high quality polyester. Other uncoated brands of polyester may also be considered adequate for preservation purposes.
Polyvinyl Chloride (PVC): This chemical, which is present in certain types of plastic, can emit damaging hydrochloric acids as it degrades, causing damage to materials stored within. PVC often emits a strong odor such as that given off by a new plastic shower curtain.
Polypropylene: A suitable inert plastic for archival storage usually associated with pages used for storage or display of photos and negatives within scrapbooks or albums.
Polyvinyl Acetate: A plastic usually abbreviated as PVA, which is a colorless and transparent solid. It is often used in adhesives which themselves are referred to as PVA or PVA adhesive. There are many PVA adhesives, some are internally plasticized and are suitable for use in conservation due to good chemical stability among other qualities.
Preservation: Describes the activities associated with maintaining library, archival, or museum materials for use, either in their original physical form or in some other format. Preservation is considered a broader term than conservation.
Reversibility: This term refers to the ability to undo a process or treatment with no change to the object. Reversibility is an important goal of conservation treatment, but it must be balanced with other treatment goals and options.
Solid board: This paperboard is made of the same material throughout. Solid board is distinct from a combination board where two or more types of fiber stock are used, or laminated, in layers.
Thermo hygrometers: This term refers to any number of instruments that measure temperature and humidity levels in a space that houses collections. Constant and controlled temperature and humidity levels are essential for the long- term survival of collections. Advances in technology have lead to more accurate and cheaper instruments including data loggers.
UV Filter: This material is used to filter out the ultraviolet (UV) rays out of visible light. Ultraviolet radiation is potentially damaging to library, archival and museum objects and is present in sunlight, fluorescent light, as well as incandescent light. Removing UV radiation from storage, use, and exhibition space can reduce the rate of deterioration on artifacts. UV filtering material is placed over windows and fluorescent light tubes, or used as glazing in framing and exhibition cases.