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P.V.A - See polyvinyl acetate.

paper hinges - Mounting technique using strips of paper affixed to the object being mounted and the backing mat by means of a fold (as in folded hinge) or other techniques (as in the T-hinge). Usually a reversible adhesive such as starch paste is used, and considerations are made in the hinge material and design to prevent damage to the original artwork.

passe-partout - A kind of "framing" in which no actual frame is used. The matted artwork is placed together with glass and backing in a sandwich condition and the edges are bound with tape.

perfect binding - A method of holding pages of a book together with glue using no stitching or sewing. The backbones of the gathered books are ground off, leaving a rough surface to which adhesive is applied. The books are usually finished with a wrap-around cover.

permanent adhesive - An adhesive that once used as a bond for materials is non-reversible.

pH - In chemistry, pH is a measure of the concentration of hydrogen ions in a solution, which is a measure of acidity or alkalinity. The pH scale runs from 0 to 14, and each number indicates a ten- fold increase. Seven is pH neutral; numbers below 7 indicate increasing acidity, with I being most acid. Numbers above 7 indicate increasing alkalinity, with 14 being most alkaline. Paper with a pH below 5 is considered highly acidic. Buffered storage materials typically have a pH between 7 and 9. See also acid; alkaline.

phased boxes - A four flap boxboard folder hand-scored and folded to fit specific dimensions of the object to be enclosed, usually a rare book, secured with tied ribbon or string.

photo sleeves - Enclosures for photographic prints, negatives, or transparencies. Usually made of transparent polyester, polypropylene, these are safe for photo storage. Can be made of unsafe PVC (vinyl), triacetate and other acetates.

photogram - A photo image produced without using a negative or camera by allowing an object to cast its shadow directly onto the light sensitive recording surface.

Photographic Activity Test (P.A.T): - Accelerated aging test using a specialized photo-emulsion in contact with a paper sample at elevated temperature and humidity levels to simulate the natural aging process used to check the archival quality of various papers and plastics products intended for photographic storage.

photographic emulsion - A suspension of a light-sensitive silver salt or a mixture of silver halides in a viscous medium (as in a gelatin solution) forming a coating on photographic plates, film, or paper.

photography - To write with light; the science, engineering, art and craft of producing relatively permanent images by the action of light (and similar electromagnetic radiation) on sensitive materials. In a narrow sense, the use of a camera to make images by the action of radiation on sensitive films or papers. In the case of digital imaging the action of light on CCD short for charge-coupled device, an instrument. Digital cameras, video cameras, and scanners all use CCD arrays.

photosensitive - The quality of reacting to light through chemical and electrical action.

pigment - Particles used to give color, body, or opacity to a semiliquid artists' material.

plastic mounts - Referring to 35mm formatted masks that both house and protect film transparencies, and are used for projection in slide projectors. Plastic mounts are an alternative to paper mounts, providing a more-rigid and potentially longer lasting mount.

plasticizer - A chemical added to plastic resins to improve flexibility, workability, or stretching. In plastic enclosures, plasticizers tend to volatize, or "outgas", and adversely affect photographs stored therein.

plate - (1) Illustrations prepared separately and inserted in a book when bound. (2) A master surface from which printing is done. (3) A photographic negative made on a glass plate.

plate finish - A hard, smooth finished paper.

platen - A flat plate (as of metal) that exerts or receives pressure. A component of the dry mounting presses.

platinum printing - A non-silver process using the light sensitivity of ferric oxalate to produce the precipitation of platinum, yielding a black image. Highly expensive and extremely permanent, platinum printing is known for its long and rich tonal scale. Requires considerable skill and a well-controlled lab/studio to produce quality prints. Like cyanotyping, it is an iron process.

PlexiGlas - Trade name for acrylic sheet material made by Rohm and Haas. See acrylic for other trade names.

ply - Any of several layers of solid paperboard.

point - Term used to describe the thickness or caliper of paperboard, a point being one thousandth of an inch. For example, .060" equals 60 points. See also mil.

polyester - A common name for the plastic polyethylene terephthalate. Its characteristics include transparency, colorlessness, and high tensile strength. In addition, it is useful in preservation because it is chemically stable. Commonly used in sheet or roll film form to make folders, encapsulations, and book jackets. Its thickness is often measured in mils. Common trade names are Mylar and Melinex by DuPont.

polyethylene - A chemically stable, highly flexible, transparent or translucent plastic with a low melting point. Used in preservation to make sleeves for photographic materials, when made of virgin material with no surface coatings or additives.

polymer - A chemical compound or mixture of compounds consisting essentially of repeating structural units.

polypropylene - A stiff, heat resistant, chemically stable plastic. Common uses in preservation are sleeves for photographs, slides or films; containers. Polypropylene has better clarity than polyethylene and less static charge than polyester.

polyvinyl acetate - A plastic usually abbreviated as PVA. A colorless transparent solid, it is usually used in adhesives, which are themselves also referred to as PVA or PVA adhesive. There are dozens of PVA adhesives, some are "internally “plasticized" and are suitable for use in conservation, due to greater chemical stability among other qualities.

polyvinyl chloride (PVC) - A plastic, often abbreviated as PVC. It is, not as chemically stable as some other plastics, since it can emit damaging hydrochloric acid as it deteriorates, and therefore has limited application in the preservation of books, photographs, and paper. Some plastics called vinyl may be polyvinyl chloride.

porosity - Of paper, the minute openings in the fiber through which air and light might pass.

portfolio - (1) A case for holding loose papers, consisting of two covers joined together at the back. (2) A group of prints or drawings issued by an artist.

positive - A photographic image on paper, film, or glass which exactly corresponds to the original subject in all details. See also negative.

post binder - Binder system typically with three posts in the form of rings that may be opened and closed. Pre-punched presentation sheets are made to be "bound" within the binder or album. The binding by the posts are reversible, as sheets may be added or taken out as desired.

posterization - Separate black-and-white negatives, each of which records either the highlights, the middle tones, or the shadows of the same image. In printing, usually each negative is used with a different color.

preservation - 1. The maintenance of objects in their original condition through retention, proper care and, if the object has been damaged, restoration. 2. 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.

pressure-sensitive - As in adhesive labeling products, materials backed with permanent or reversible adhesive, placed on smooth release paper. The label or material is peeled from the backing sheet, exposing the adhesive which is then applied to a desired surface, adhering by contact and/or pressure.

printing out paper - The creation of a photographic image solely by the action of light on the emulsion without requiring chemical development. Early printing out papers (POPS) were contact printed by sunlight. When the image was judged complete, the remaining silver was removed by fixing. The cyanotype or blueprint is a POP process.

psychrometer - An instrument for determining relative humidity in which wet and dry-bulb temperature readings are compared with a chart that shows relative humidity.

pulp - Cellulose fiber material produced by chemical or mechanical means from which paper and paperboard is manufactured. Origins of this cellulose fiber are many and can include wood, cotton, straw, jute, bamboo, hemp, various leaf fibers, reeds, etc. There are many mechanical and chemical means of separating the fiber from its original sources.

purified wood pulp - Pulp that is processed from wood chips and broken down to a high content of alpha cellulose fibers by a bleaching process which removes impurities as in acidic lignin. The pulp is then processed to paper, creating an archival paper stock.