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halftone - Photomechanical process for reproducing continuous-tone photographs. Printed with one color of ink, the halftone process gives the appearance of blacks, whites, and grays by converting the image into a pattern of clearly defined dots of varying sizes. The halftone process creates the illusion of tone gradation. The largest dots appear darkest and are in the shadow areas of the image, while the smallest dots seem lightest and represent the highlight areas. Intermediate sized dots reproduce the various middle tones of gray in a photograph.

headband - In books, a small ornamented band of cotton or silk placed at the head or tail of a volume between the cover and the backs of the folded signatures.

high tack (adhesives) - Showing properties of strong, fast bonding, as in adhesives. Usually non-repositionable.

highlight - The brightest light accents in the subject of a photographic image. Therefore, the lightest of whitest parts in the positive and the darkest areas of a negative.

hinge - A small piece of neutral or acid free paper such as mulberry tissue that is folded in half and glued to the artwork and the backing or sometimes is not folded but laid across the top edge of the art and onto the backing. The term also applies to a strip of tape used to join the mat to a backing piece of board to create a folder mat.

hinging papers - Usually lightweight but strong Japanese papers of various weights combined with organic and reversible adhesives (as in wheat or rice starch paste). They form archival hinging system for attaching artwork to album pages or mounting board.

hololography - Recording on a photographic material the interference pattern between a direct (reference) light beam and one reflected or transmitted by the subject. The resulting hologram is usually viewed by coherent light, giving the appearance of three dimensions. Within limits, changing the viewpoint from which a hologram is observed shows the subject as seen from different angles.

humidity - Moisture condition of the air. Relative humidity is the percent of moisture relative to the actual amount which air at any given temperature can retain without precipitation.

hydration - In paper, when the chemical components in paper are combined with moisture and the paper expands.

hydrolysis - The decomposition of organic compounds, such as paper, by the interaction of water. The presence of acid or alkali ions compound or hasten the process.

hygroscopic - Absorbing moisture from the air. Most paper is mildly hygroscopic. Table salt as a example is very hygroscopic.

hypo (fixer, fixing bath) - Originally an abbreviation for sodium hyposulfate, but now used to refer to sodium thiosulfate, the chemical agent used in fixing baths to remove unexposed silver halides from silver emulsion film and paper. It is also more generally used to refer to a fixer solution which may also contain certain acids and/or hardening agents.

hypo clearing agent - A washing agent, specially designed to remove residual hypo fixer (especially sodium thiosulfate) from prints or papers, and shorten final washing times.