There are two major types of sizing: engine (rosin) and surface (tub). Rosin sizing is applied to almost all papers and especially to all those that are machine made, while tub sizing is added for the highest grade bond, ledger, and writing papers. Tub sizing consists of gelatin glue and / or starch and is generally only used for handmade papers. Rosin is an amphipathic molecule, having both hydrophilic (water-loving) and hydrophobic (water-repelling) ends. The rosin coats the paper fiber and forms a film, with the hydrophilic tail facing the fiber and the hydrophobic tail facing outwards. This creates a water-repellent situation, which causes the water-based ink to remain outside on the paper surface.
There are three categories of papers with respect to sizing: unsized (water-leaf), weak sized (slack sized), and strong sized (hard sized). Waterleaf has low water resistance and includes absorbent papers for blotting. Slack sized paper is somewhat absorbent and includes newsprint, while hard sized papers have the highest water resistance.
As the process of sizing had and has the intent of making the paper suitable for printing, it would seem slightly ironic that some processes of sizing would make printing paper a problem for the continued existence of that paper and those who would preserve them. Sizing processes started early on in the paper making processes, with historians citing that items, such as starch, were early sizing agents used on paper. Dade Hunter in Papermaking through Eighteen Centuries corroborates this by writing, “The Chinese used starch as a size for paper as early as A.D. 768 and its use continued until the fourteenth century when animal glue was substituted.” The early modern paper mills Europe, which produced paper for printing and other uses, the sizing agent of choice was gelatin, as Susan Swartzburg writes in Preserving Library Materials, “Various substances have been used for sizing through the ages, from gypsum to animal gelatin.” Hunter describes the process of sizing in these paper mills in the following:
The drying completed, the old papermakers dipped their paper into an animal size that had been made from the parings of hides, which they procured from the parchment-makers. It was necessary to size that paper so that it would be impervious to ink, but sizing was more needed in writing that in printing papers. Many books of the fifteenth century were printed upon paper that had not been sized, this extra treatment not being essential for a type impression. The sizing was accomplished by a worker holding a number of sheets by the aid of two wooden sticks, and dipping the paper into the warm gelatinous liquid. The sheets were then pressed to extract the superfluous gelatine. This crude method of sizing the paper was extremely wasteful as many sheets were torn and bruised beyond use. The sizing room of the early paper mills, was, for this reason, known as the ‘slaughter-house.’
With the advent of the mass production of paper, the type of size used for paper production also changed. As Swartzburg writes, “By 1850 rosin size had come into use. Unfortunately, it produces a chemical action that hastens the decomposition of even the finest papers.” In the field of library preservation it is known “that acid hydrolysis of cellulose and related carbo-hydrates [sic] is one of the key factors responsible for the degradation of paper during ageing.” Some recent professional work has focused on the specific in the degradation involved in the deterioration of paper that has had a rosin sizing process, and what amount of rosin affects the deterioration process, in addition to work on developing permanent paper and sizing agents that will not eventually destroy the paper. An issue on the periphery to the preservation of paper and sizing, is washing, which is described by V. Daniels and J. Kosek as, “The removal of discolouration ... in water is principally effected by the dissolution of water-soluble material; this is usually done by immersing paper in water.” In such a process, surface level items applied to the paper, such as size in early paper making processes as seen above, have the possibility of being removed from the paper, which might have some item specific interest in a special collections library. With later processes in paper making being more akin to “engine sizing,” as H. Hardman and E. J. Cole describe it, “Engine sizing, with is part of the manufacturing process, has the ingredients added to the furnish or stock prior to sheet formation,” the concern for the removal of size is less, and as such, most literature focuses on the more pressing issue of preserving acidic papers and similar issues.