Unsized paper is called “waterleaf” paper. It is usually composed of hydrophilic cellulose fibers, meaning they ‘love water.’ This is a good quality while the paper is being made, but it can lead to unfortunate consequences once the paper is made. The extreme porosity of an untreated sheet of paper means that printing or drawing inks and water-based paints will soak into the paper spreading quickly and randomly. This is called ‘bleedthrough.’ Sizing retards some of the paper’s absorbency.
Sizing paper will reduce or eliminate this bleeding and feathering by enveloping 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 adds adhesive qualities and strength to paper.
Early Sizing Materials
Papermakers in Asia used various starches to size their paper. Early Arabic papers were surface sized with wheat starch, wheat flour, or mixtures of starch and chalk and glazed to produce a shiny, burnished surface. Early Spanish papers, developed under Arabic influence, were coated with thick starch.
Gelatin was used as a surface size in Europe at least as early as 1276 (at the Fabriano mill) and continuously after that. Until the last century, some form of gelatin or glue sizing was the most common form of surface sizing. From the sixteenth century on, gelatin was frequently used in concert with additives or preservatives, especially alum (historically, potash alum or aluminum potassium sulfate; modern-day, aluminum sulfate). Used in concentrations as high as 20%, it stabilized the viscosity of the size, improved ink resistance, and prevented spoilage of the gelatin solution.
The first alum-rosin size used in the West was introduced in Germany (1807) and later adopted elsewhere in Europe (1835). Initially, this size was to prevent ink feathering on writing papers, but its use spread to many types of papers and became all but universal. It is primarily an internal size. Before that, they made a gelatin size by boiling down the hides of animals. Natural starches are made from potatoes, rice, or wheat.
Methylcellulose and alkyl ketene dimer (AKD) are popular sizes today for hand papermaking. They are stable, inexpensive, easy to obtain, and resist vermin. They affect paper in different ways that are subtle or obvious depending on the amounts used.
Surface sizes typically consist of vegetable starch (potato, rice, or wheat) or gelatin from animal bones and hides. Gelatin size can improve the strength and flexibility of the paper.
Methylcellulose seems to shrink upon drying. Cast pieces from pulp internally sized can dry to a bony version of the mold design. Methylcellulose is good glue for adhering sheets together and is helpful as a hardener.
According to some papermakers, carboxymethylcellulose (CMC) may cause yellowing of the paper, and they attribute it to creating a slightly plastic sheen on the surface of their paper.
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.
External Sizing (Sheet)
External sizing is more time-consuming but has advantages over vat or internal sizing. Individual sheets can be sized differently for different uses. Surface sizes do not envelop individual fibers but add a protective coating to the sheet, inhibiting absorption and bleeding.
Apply the size to well-dried paper using one of three methods:
Brushing—Using a sizeable soft-haired brush apply the size in strokes in one direction, followed by another coat (if desired) in the perpendicular direction to the first coat.
Spraying—Thin the size with water and spray on while warm so as not to clog the sprayer.
Tub Sizing—Place dry sheets in a tray of size, soaking them for several minutes, followed by a light pressing.
Gelatin Size Recipe
A two to a three-percent solution of gelatin makes a good size. Mix ¾ to one ounce (20–30 grams) of technical gelatin with one quart (liter) of cold water. Allow the gelatin to soak for several hours or until the particles are swollen. Warm the solution (do not exceed 140° F. or 60° C.) and stir until completely dissolved.
Tub Sizing Directions
- Pour the sizing solution into a shallow tray. A tray slightly larger than the material to be sized should be used with enough size in the tray to cover the material to be sized. If the size is gelatin, keep the size warm while in use at about 104° F. (40° C.).
- Place a support sheet, such as a polyester web material, in the tray first to aid in the removal of the paper.
- Immerse single sheets with support sheets on top and bottom. Unfold book gatherings before immersion and immerse each sheet one at a time, forming a stack of leaves; about 75 to 100 leaves can be sized in a stack. Add enough size to the tray to cover the stack.
- Remove single sheets once they are saturated and blot off any excess size. For multiple items, remove the entire stack with support sheets top and bottom, drain the stack, place between blotters and pressing boards (laminate pressing boards or other moisture-resistant surfaces), and nip in a press for about one minute.
- For single sheets, change the support material by flipping the item from wet to dry and placing it on a drying rack for air drying. For a stack of book leaves, peel each leaf off the stack with care and transfer it to drying racks covered with a polyester support material. A spatula helps lift the corner of the paper.
- Air dry the sheets until it begins to curl, i.e., dry to the touch but with considerable humidity still present. Place the sheet between blotters or felts and under light pressure, depending on the texture and surface finish, for final drying. In the case of book leaves, the final alignment of the folded leaves into gatherings can be done as they are removed from the drying racks, and then press the leaves in groups of gatherings with blotters inserted every two to three gatherings. An alternative method is to remoisten the leaves after drying and realign them, gathering them for final pressing. The latter provides more working time and may be necessary for large groups of leaves. Great care must be exercised to avoid loss of surface characteristics in the final drying under pressure.
When tub sizing dried paper, allow the sheets to mature for several weeks before sizing. Aging the sheet makes it stronger, more stable, and less likely to disintegrate in the sizing solution.
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