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Additives

White china clay added to the vat in small doses gives the paper a nicely paintable, almost porcelain finish. Whiteners, dyes, pigments can be used in casting- or sculpting pulp. Too much of any additive interferes with bonding, but again because of the sheer amount of pulp used, the consequences of too much can be minimal. Black dyes and pigments are an exception - there just does not seem to be a way to achieve a deep, rich black handmade paper without a huge sacrifice of strength and flexibility. (Manufacturers and suppliers furnish proportion guidelines for the additives they make and sell.) 

Mixing acrylic paint into the pulp for coloration is a terrible idea - it makes useless mush. Embedding anything into casting-pulp is an equally bad idea. A stick, fiber or sparkle can interfere with the relief pattern, even flatten out an area around it. Retention aid is necessary to keep imbedded objects in/on the paper if and when they are used. 

Common Additives

While some may think that the best paper comes from the vats containing only pulped fiber and water, there are many beneficial chemical additives which are used in hand papermaking. These materials are added to the pulp either during the mixing stage or in 
the vat itself before sheet forming. 

For Japanese nagashizuki papermaking, an essential additive is neri, which is used to slow drainage through the screen. It is known  as a formation aid or deflocculant. The Japanese pound the roots of the tororo aoi plant (a member of the hibiscus family) to form this viscous liquid. Other natural sources of formation aid have been used in Chinese and Korean papermaking, and okra and similar substitutes are used by some Western papermakers. 

Synthetic formation aids are used more frequently now, especially PNS and PMP, as tororo aoi in particular does not store well and is hard to come by. In recent years, papermakers have experimented using formation aid with Western-style sheet forming to achieve various decorative effects. 

The most traditional additives in Western papermaking (ignoring bleaching agents, which are used to whiten fibers but are then rinsed out before sheets are formed) are sizes. These inhibit the water-loving qualities of the fiber in paper so that writing or painting on the 
paper does not feather and bleed. While the oldest sizes were made of gelatin into which dried paper is dipped, sizes added to the vat, know as internal sizes, were developed in the late 18th century as time-savers and soon replaced gelatin sizing as machine papermaking took hold. 

Various kinds of size have been developed for different qualities of paper. Alum rosin sizing was used extensively for both hand- and machine-made paper. Unfortunately this size, while economical and much easier to use that gelatin, creates paper which is usually acidic. 

Many of the books produced since the early 19th century are now rapidly decaying because the paper has become so brittle. Most machine papermakers are now turning toward sizes which help produce paper in a more neutral pH range. The synthetic sizings most hand papermakers use today are also less acidic or can be used with other additives which neutralize the acidity. 

Colored paper can be made from colored rags, but most hand papermakers use dyes or pigments to change the color of a pulp. There are many different ways to color fiber and many variables involved in selecting the best method for a particular fiber and desired effect. In general, colorants are mixed in with pulp before it is added to the vat. Most pigments require an additive called a retention aid or agent to help them adhere to the fiber. 

Other additives used in hand papermaking include fillers and whiteners. Fillers occupy some of the gaps between fibers in paper and make the paper somewhat dense and more opaque. They may also serve as buffering agents to make paper less acidic. Calcium carbonate and magnesium carbonate are the most commonly used fillers. Whiteners, like titanium oxide, also end up in the spaces between fibers, but their specific purpose is to create a brighter finished sheet. Because both of these types of additives may inhibit fiber-to-fiber bonding, they may diminish the strength of paper made from pulp to which they have been added. 

While most paper fibers are benign in their health effects, many chemical additives are not. Powdered pigments, in particular, can be quite toxic and, once airborne, are easily inhaled. Many additives are poisonous if ingested. Certain ones can also cause rashes and other skin reaction on hands and arms dipped into the vat. Precautions are, therefore, urged in dealing with all of these chemicals: good ventilation and masks are recommended when using powdered pigments; gloves or the use of a skin guard may be appropriate to avoid contact sensitivity. 

Because of the variety of additives available, the hand papermaker is urged to research the subject carefully. Further reading is recommended.