Contributors: Lawrence Houston
Calendering is the process by which the surface of the paper is rendered less fibrous and spongy in appearance. This is accomplished by means of pressure applied to the paper at the end of its manufacturing process. In paper making machinery, this is normally carried out by chilled, smooth iron rolls at the dry end of the paper making process. In handmade paper, calendering is often accomplished by sandwiching the paper between steel plates and applying pressure with weights or in a hydraulic press. In some processes of calendering, the paper may be dry or dampened and the steel is heated or chilled in order to achieve the desired effects. Calendered finishes range from the slightly toothier machine finish, to English finish, and lastly to the highly glossy supercalendered finish. Heavily calendered paper was often used in stationary bindings, as the glossy surface finish prevents inks from bleeding.
Calendered cloth, supercalender, calender
Synonyms in English
Similar to sizing or burnishing, calendaring tries to decrease the absorbency of paper to provide for better print quality and for manuscript paper. Heavily calendered paper can take longer to moisten when undergoing aqueous treatments and such treatments can be deleterious to the finish that the calendering process provides.
The compressive forces exerted during calendering reduces the thickness of the paper, and calendered paper can end up with a density that is 50% greater than untreated sheets. Improper calendering during the manufacturing process can cause defects due to the intense pressure and dimensional changes effected in the cellulose fibers. Banding from areas of different moisture content can appear, as can wrinkles from improper tension during rolling. The metal rolls, being iron, can offset oxidation and abrasion products onto the paper and contribute to foxing or other aesthetic defects. If moist paper is pressed with too high of a nip load in the calendering process, reduced opacity or ‘calender blackening’ can occur. Calendering typically results in a slight to moderate loss to the burst, tensile, and tear strength of paper along with a reduced stiffness. However, it can occasionally impart a moderate improvement in fold endurance. The loss of strength from calendering is considered to be due to a lower bonding strength.
Charles, L. A., and Waterhouse, J. F., 1987. The Effect of Supercalendering on the Strength Properties of Paper. Appleton: Institute of Paper Chemistry.
Burnett, R., 2003. “The nature of strength reduction when newsprint is calendered,” TAPPI Journal, Mrach 2003, Vol. 2(3).
Etherington, Don., and Roberts, Matt T., 1982. Bookbinding and the Conservation of Books. Washington: Library of Congress.