2 OVERVIEW OF DRYING AND FLATTENING

The treatment of drying and flattening is not performed unless another treatment occurs first. The most common initial treatment is washing, either to remove a stain or to rejuvenate the sheet by reducing free acidity. A sheet may need flattening, but the cockled or curled sheet must first be wet (humidified) before it can be flattened. The use of the term “drying” assumes the associated step of flattening.

After a paper is wet, the process proceeds by these steps: the fibers swell, hydrogen bonds within fibers are broken, fibers are plasticized, fiber-to-fiber bonds in the sheet network are broken, and the aggregate sheet is plasticized. The structural conformation of the fibers is lost because the structural bonds fixing the components to one another are gone. The surface aesthetics of the sheet are lost. The disruptions created by wetting are reversed when the process is reversed, that is, when the paper is dried. However, the original mechanical conditions that molded the paper are no longer obtainable. The original surface texture is lost.

When the wetting process is reversed without control or restraint, it is often called “free drying.” The free-drying process commonly yields a cockled sheet. Often, speed of drying is associated with degree of cockling, with anecdotal evidence suggesting that very slow free drying results in a fairly flat sheet. Cockles are due to uneven drying. When one area is wetter than the adjacent area in a sheet (i.e., the sheet displays small puddles of water), local planar distortion or cockling results. Slow drying encourages the dispersion of high water concentration to regions of low water content and therefore even drying. Controlling the speed of drying is therefore a treatment variable which should result in diminished cockling.

Drying treatments are concerted efforts to control or engineer the redried surface of the paper. Conservation drying treatment is the process of pressing, with minimal pressure, a paper between absorbent surfaces while it dries. Intuitively, we know that a dry sheet has a fixed shape. Pressing involves holding a sheet in a defined shape (flat) while water is removed. Commonly, water is removed by contact with blotters. The capillaries and absorbent dry fibers in the blotters absorb the large quantities of water in the wet sheet until an equilibrium water content is reached. Blotters are changed to keep the migration of water moving out of the sheet. Eventually the blotter in contact with the sheet reaches equilibrium with the air in the room.