H
HALF-TONE
The representation of tonal gradation by an image composed of dots of varied sizes, the centers of which are equally distant.
HALF-TONE SCREEN
A transparent material consisting of evenly spaced lines that is placed between a photograph and the film to be exposed. The number of lines to the inch controls the coarseness of the final dot formation; the more lines used, the higher the quality. The screen that is used depends on the paper and the type of printing process used. In electronic systems, the screen is simulated by software.
HARDWOOD PULP
Pulp obtained from the wood of hardwood trees by various processes. The fibers are usually shorter than those of softwood pulp.
HEAD BOX
See Flow Box. An elevated tank or container equipped with an overflow appliance that, when supplied with excess of a liquid or fluid, can deliver same at a constant hydraulic head to a lower point.
HEAT-SET INK
Inks used for high speed web offset printing. Heat set inks must be rapidly set by the use of heat.
HOT MELT COATING
A method of coating with a 100 percent solids compound of wax, resin of polymer – or mixtures thereof – heated to a fluid state and applied to the substrate by, for example, a roll, gravure or extrusion procedure with a subsequent chilling device.
HYDROGEN PEROXIDE BLEACHING
A procedure in which pulp is bleached in an alkaline environment with hydrogen peroxide, sometimes using oxygen reinforcement. The method considerably reduces the need for chlorine-containing chemicals in the final bleaching of chemical pulps.
