This information is reprinted from the For Beginners column of Hand Papermaking Newsletter #38 (April, 1997).
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The basic ingredient of handmade paper is plant cellulose, which is hydrophilic--it loves water. This is an advantageous quality while paper is being made, but can lead to unfortunate consequences after the paper is made. The extreme porosity of an untreated sheet of paper means that printing or drawing inks and water-based dyes will soak into the paper spreading quickly and randomly. Sizing the paper will reduce or eliminate this bleeding and feathering by encasing the cellulose fibers. As an added benefit, sizing protects the fibers from breakdown due to oxidation. In addition, since other commonly used fillers can inhibit bonding between fibers thereby weakening the sheet, sizing can have adhesive qualities and add strength.
Papermakers in the East used various starches to size their paper. Up until recently, Western papermakers used alum and rosin, which are highly acidic. And prior to that, they made a gelatin size by boiling down the remains of slaughtered animals. Luckily for us, improved methods have developed since.
While one can make natural starches from potatoes, rice, or gelatin, it is more common and easier to simply purchase chemical sizing from papermaking suppliers.
Sizing is added directly to the vat (internal) or applied to the sheets after formation (external). Both methods have their benefits, and some papermakers even use a combination of both techniques.
Internal sizing is an easy way to apply a coating to individual fibers, but still allow for some absorption of oil or water based ink or dye. All the paper made from one batch of pulp is equally sized.
External sizing is more time consuming, but has some advantages. Individual sheets can be sized differently, for instance, if destined for different uses. Surface sizes do not envelop individual fibers but add a protective coating to the sheet itself, inhibiting any absorption or bleeding. While this is a helpful quality for calligraphers, some do not care for the appearance of handmade sheets with too much surface sizing.
Apply external sizing to well-dried paper using one of three methods: painting, spraying, or tub sizing. Use a large soft brush for quick application, or a simple plant sprayer if the size is fairly thin so as not to clog the sprayer. Or, run dry sheets through a tray of sizing liquid, or soak them in the tray for several minutes followed by a light pressing.
Personal experimentation is the key to successful sizing in your own studio.
Copyright 1997 Hand Papermaking, Inc.