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Cooking with Caustic

March 16, 2021

This information is reprinted from the For Beginners column of Hand Papermaking Newsletter #99 (July, 2012).

It is possible to construct a papermaking practice without cooking or using caustics if one purchases partially processed fibers in half-stuff sheet form or recycles existing paper fibers. However, to get more experimental with papermaking fibers it is likely that you’ll want to cook some of these fibers to help break them down and to neutralize their acid content.

Fibers one cooks might include dried fibers purchased from a papermaking supplier (kozo or hemp) or hand collected plant fibers. Your cooking setup for fibers should be separate from your kitchen cooking setup. A hotplate set up in a well-ventilated space (outdoors if you are using lye) will prevent the sometimes strange smells of cooking fibers from permeating your studio or kitchen, and will protect your lungs from the potentially irritating fumes. [I once had a friend who was obsessed with pu-erh tea, which I could never drink because it reminded me so strongly of the smell of cooking flax.]

Caustic generally refers to a strongly alkaline substance that displays corrosive properties—most commonly, in papermaking, soda ash or lye. Lime is also used in some Asian countries. These substances are useful in breaking down particularly tenacious plant fibers such as raw hemp, and in neutralizing the pH of pulp fibers by breaking down the acidic lignins that make paper non-archival. Lye is the stronger of the two caustics. It is generally recommended to begin by cooking with soda ash and only turn to cooking with lye when soda ash does not do the job.

Because these materials are corrosive, it is important to protect yourself by wearing rubber gloves and eye protection and by observing a few rules. Use a cooking pot made of stainless steel or glass, or an (unchipped) enameled pot. Other materials can react with the caustic, causing your cooking pot to break down. When using lye, always cook outside. Cooking with soda ash should be done in a well-ventilated space to prevent fumes from irritating your lungs.

The caustic should always be added to water, rather than the other way around, to prevent dangerous explosive splattering.

Fibers should be soaked overnight prior to cooking. Water should be brought nearly to a boil in the cooking pot. One should use about 7 liters of water for every pound of fiber. The caustic can be measured out by weight or volume. Standard measurements include using 20% of the dry weight of the fiber being cooked if using soda ash (3.2 oz per pound) or 9% of the dry weight if using lye (1.4 oz per pound). This amount should be added to 1 cup of hot water (again, do not add the water to the caustic) before adding it to the pot. Other measurement standards include using 1 tablespoon of soda ash for every quart of water, or measuring the pH of the cooking liquid until it is between 10 and 11.

Add the soaked fibers and simmer, stirring and checking the fibers every 30 minutes. You can expect a cook time of one to four hours for many fibers. Consider how easily the fibers pull apart. You should be able to separate and pull them apart by hand, but you also want them to retain their length and not become mushy.

Once the fibers are removed from the heat, they should be rinsed very thoroughly to remove all caustic—until the water is clear and the pH is neutral.

Recommended caustics and measurements for various fibers can differ from papermaker to papermaker. For additional information about cooking with caustics, consult:

Timothy Barrett, Japanese Papermaking: Traditions, Tools, and Techniques (New York and Tokyo: Weatherhill, 1983). Includes information about cooking with lime.

Arts-in-Company’s Papermaking web resource

Catherine Nash’s Resources for Papermaking
Includes a chart of recommended caustics and cook times for 41 plant fibers.

by Mary Tasillo