|Table of Contents for Caveman Chemistry: 28 Projects, from the Creation of Fire to the Production of Plastics|
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Not every thin, flexible writing material is paper. You've probably heard of parchment and vellum and papyrus and you might think that they're kinds of paper but they're not. To begin with, papyrus is made from the papyrus plant, a kind of plant that grows in the shallows of lakes and rivers in Africa. It's made by splitting thin sections of the bark, placing those sections side by side to make a layer, laying another layer cross-wise on top of the first one, and then pressing the layers together to make a sheet. Egyptians were making papyrus from about the third millennium BC and it was exported to Europe and the Middle East, but not to China. By contrast, parchment and vellum are made from animal skins, parchment from sheep skins and vellum from the skins of calves, lambs or kids. Parchment is produced by splitting the skin, separating the wool-side from the in-side; only the in-side is used for making parchment. Vellum is made from the whole skin of the young animal. Both parchment and vellum are scraped with a knife, rubbed with lime, stretched and sanded to make a thin, flexible and strong surface for writing on. Iranians were making parchment from about the second century BC, but it was unknown in China.
The Chinese had been writing on bones, stones, pottery, bronze and bamboo from the second millennium BC. And it was bamboo that dictated the form that Chinese writing would take. Craftsmen cut bamboo into tubes of a certain length, split those tubes into strips maybe 1 cm wide, and then sewed the strips into rolls. Since a reader rolled the scroll from side to side, the strips were running up and down and that's why Chinese is written from top to bottom. About 700 BC, silk cloth began to be used as a writing medium. As with bamboo, silk cloth was kept in rolls and so it was natural to keep writing vertically.
Paper is different from any of these other writing materials. Unlike parchment, vellum and silk, paper is always made from plants, namely from cellulose. In Chapter 6 Venus told you all about cellulose; plants make cellulose by stringing sugar molecules end-to-end and so cellulose is naturally long and thin. But unlike papyrus and bamboo, in which the cellulose remains parallel, the cellulose in paper is running every which way. One way to get the cellulose out of the plant stems is to shred them, a process called maceration. To macerate raw plant stems, linen, hemp, or cotton rags, you can soak them in water, boil them, beat them with a mallet, or put them in a kitchen blender. When you are finished, you have a kind of cellulose stew, or pulp, with lots of long, thin cellulose fibers floating around every which way. But you can do an even better job of pulling the fibers apart if you understand what holds them together. While plants use cellulose to build up those parts that hold them up—stems and trunks and stalks and such—those cellulose fibers are glued together with a material called lignin.
While maceration alone is capable of tearing cellulose fibers apart to produce a mechanical pulp, the separation would be more easily accomplished if the lignin could be removed. Whereas a condensation reaction puts two molecules together by removing water, a hydrolysis reaction cuts a molecule apart by inserting water. Alkaline solutions hydrolyze both cellulose and lignin, but the hydrolysis of lignin is faster than that of cellulose and this is the key to their separation. When plant stems are boiled in alkali, the insoluble lignin is hydrolyzed to water-soluble compounds and white cellulose fibers are left floating in a brown alkaline broth. Straining the fibers from the broth and washing them with water completes the separation. You could use potash from burning wood, soda ash from burning sea-weed, ammonia from fermented urine, or lime from burnt limestone. But remember from Table 8-1 that it takes 1000 parts of wood to get 1 part of potash. Fermented urine is not as strong as potash or soda ash, which leaves lime, which you can make from common limestone. At the time of the Book of the Supports of the Scribes, lime was the alkali of choice for chemical pulping. Now that you understand pulping, it's time to find out how to make paper from it.
To make paper from pulp, you need a mold, which is either a piece of cloth or fine screen attached to a wooden frame. You could dip the mold into a tub of pulp and lift it out and some of the pulp would stick to the mold, but most of it would just run down the sides and back into the tub. However if you put an empty wooden frame, called a deckle, on top of the mold, the deckle keeps the pulp from running over the sides. With the deckle, the water can drain out of the pulp and you're left with an even layer of pulp on the mold, maybe a millimeter thick, with the fibers running every which way. If you were to let that pulp dry on the mold, you would have paper, no doubt about it.
But then your mold would be tied up all day until the pulp dried and all for one sheet of paper. To get around that, you couche the paper onto a felt, that is, you transfer it to a piece of cloth. You can stack those felts into a stack called a post, and press that post with a press to squeeze most of the water out. And once that water has been squeezed out, you can slap that damp paper onto a wall or board to dry. Once dry, you have a smooth, flexible writing material suitable for chronicling heroic deeds, inciting revolution, or sharing pancake recipes.
Locate MSDS's for slaked lime (CAS 1305-62-0), soda ash (CAS 497-19-8), and caustic soda (CAS 1310-73-2). Summarize the hazardous properties of these materials in your notebook, including the identity of the company which produced each MSDS and the potential health effects for eye contact, skin contact, inhalation, and ingestion. Also include the LD50 (oral, rat) for each of these materials.
Your most likely exposure is eye or skin contact. Caustic soda, the product of reacting soda with lime, is caustic; in case of eye contact, immediately flush them with water and call for an ambulance; in case of skin contact, wash the affected area with cold water until it no longer feels slippery.
You should wear safety glasses and a rubber gloves while working on this project. Spent caustic soda solution can be flushed down the drain, but be careful not to clog the drain with pulp. Leftover pulp may be thrown in the trash or saved for later use.
|Research and Development|
Well, I guess if you are in a class or something, you might want to know what will be on the quiz.