n.a substance used to fill the pores in fibrous materialsMinogue 1945, 115The rags, after a minimum of washing and cooking, were converted into a reasonably pure and stable form of cellulose fiber that did not require excessive chemical bleaching, mineral filling, or rosin sizing to produce acceptable paper . . .Poole 1977, 164–165Since about the middle of the last century, when wood pulp was substituted for linen and cotton rags in the papermaking process, and when alum-rosin sizing was substituted for gelatin sizing, there has been a steady deterioration in the quality of paper used for printing books, of paper used for business correspondence, and of printing and writing papers generally.Garlick 1986, 100Until the 19th century, sizing was most commonly applied to the surface of a finished paper in a separate hand operation. However, in the late 17th and 18th centuries, some papermakers did experiment with adding a sizing agent to the paper pulp.Ritzenthaler 2010, 380Sizing: Material added to the pulp or applied to the surface of a formed sheet or roll of paper to give it water- and ink-resistant characteristics. Sizing agents include gelatin, rosin, glue, starch, and synthetic resins. Chemical nature of size (acidic or alkaline) affects stability of paper.NEDCC 2015aSizing makes a paper’s surface more suitable for writing, and can be done either by coating the paper by hand or dipping it into a tub of sizing solution. After about 1650, alum was added to the tub of gelatin sizing to harden it and keep it from spoiling. Alum is extremely acidic and leads to acid hydrolysis of the paper. ¶ . . . ¶ In the early 19th century, alum rosin sizing was introduced to speed up the papermaking process. The paper was sized in the vat before the paper sheet was formed by adding rosin to the vat, followed by alum. This combination created sulphuric acid as a byproduct of the reaction between the alum and the rosin, and in-the-vat sizing meant that acids were distributed throughout the paper, rather than just on the surface. Cumulatively, this resulted in more acidic, weaker, and more brittle paper. Alum rosin sizing was widely used by the 1840s, and in the 1870s a cheaper (and even more acidic) form of the sizing was developed.
Archivists are likely to encounter sizing as a substance applied to paper.