Science in the Service of Man: The Making of Paper

Beyond all doubt, paper has played an important part in transmitting human knowledge and in preserving civilization and its heritage from generation to generation. Before paper existed, it was not possible for instruction to spread and become general. It is therefore no exaggeration to say that paper is the great instrument that has eased the spread of learning and culture among mankind, and that it is one of the cornerstones of our modern civilization.

Paper has an ancient history. The Chinese knew how to make it from silk, and in the British Museum there is a Chinese manuscript dating back to the year 25 A.D...

The Arabs, however, made great advances in the art of its manufacture, for they managed to substitute cheap cotton for rare silk. This work of theirs was a great step forward.

Paper manufacture today relies on wood as its basic source and raw material; it therefore requires countries where extensive forests grow and where an effective system of afforestation exists, so that the trees cut down can be replaced with new ones. It is nevertheless possible to produce certain kinds of paper from hemp, rice husks, straw, and linen threads. Yet however its kinds and sources may differ, paper always consists of "cellulose" fibers (fibres de cellulose). Cellulose is an organic substance found in all plants: cotton contains about 95 percent of it, and wood about 40 percent.

The kinds of paper differ not so much according to the source of the wood as according to the method by which they are made. There is, for example, newsprint, whose manufacture is comparatively simple; there is glossy, shining paper; there is writing paper; and there is stiffened paper, "cardboard," and the paper used for making bags and for wrapping goods, etc...

Since it is not possible to take in the methods of making all these kinds, I shall content myself with giving the details of the method followed in making the ordinary paper we use for writing or for printing books, merely touching on the manufacture of a few other kinds at the end of the article.

The first step in making paper is to cut the wood into small pieces, no one of which exceeds a centimeter or a centimeter and a half in thickness. This is accomplished by means of large wheels fitted with sharp steel blades. After the cutting operation the wood passes over a sieve to remove whatever sawdust clings to it; then the small wooden pieces are placed in large vessels in readiness for cooking, to separate the cellulose from the other woody substances, which make up about 60 percent of the weight of the wood. To achieve this end, the wood chips are mixed with a 10 percent solution of natron — that is, a solution composed in the proportion of 10 grams of natron to 90 grams of water — and the whole is placed in the special vessel; the temperature is then raised high for six hours, under a pressure equal to about 15 kilos to the square centimeter. At the end of the six hours the non-cellulose woody substances have dissolved in the natron solution, leaving the cellulose behind, since it is insoluble in that solution.

There is another method that leads to the same end. It consists, in brief, of mixing the small wood chips with a solution containing a gas called sulfur dioxide (bi-oxide de Soufre) in place of the natron. To obtain this sulfurous solution, sulfur is burned within closed vessels, sealed except for tubes at their top; the burning of the sulfur yields the gas referred to, which passes through the tubes that carry it to the wood-cooking vessel, where it mixes with the water to form the desired sulfurous solution. The temperature is then raised likewise, under a pressure like the first, except that the cooking lasts ten hours instead of six.

Whether the natron solution or the sulfur-gas solution is used, the cellulose is separated by these two operations from the other woody substances and remains in the vessel in the form of a gray-colored paste. It is then washed with hot water to remove whatever natron or sulfurous matter still clings to it. But its gray color makes it as yet unfit for use; it is therefore bleached by means of a substance resembling Javel water (Eau de Javelle), made by causing chlorine gas to react with slaked lime, which yields a white powder inclining to gray, called bleaching powder (Poudre de blanchissage). When this powder is mixed with water and the cellulose paste, it removes the foreign colors and gives the cellulose fibers a pure white color. The water is then drained off these white fibers, and they are washed well with clean water until every trace of the bleaching powder has gone.

Now that the cellulose is in a pure state, it is mixed thoroughly with a little "rosin" (collophane) — the substance used for rubbing the violin's bow — and with aluminum sulfate, to give the paper strength and firmness. When this stage is finished, the cellulose paste has become a paper paste, ready for the final step in its transformation into paper.

The paper paste, mixed with a great deal of water, is placed in a special vessel with an opening in its bottom, from which the paste issues at a known rate according to the size of the opening. The paste falls onto a steel screen, and most of the water passes out through its many holes, leaving behind a paste denser than before. During this draining the screen is pushed forward, and it is also shaken from side to side, so that the cellulose fibers may interlace and hold fast to one another.

This steel screen moves forward at a speed of about a hundred meters a minute. At the end of its course there are two cylinders that press the cellulose between them, removing most of the water still left in it. It is now no longer a paste; it has become moist paper.

To dry this moist paper it must pass over a number of heated cylinders, the paper winding around them. When it is thoroughly dry it passes over six further cylinders, heated as well, but whose task is not drying but pressing, so that the cellulose fibers may adhere well to one another; the paper thus gains in smoothness and becomes fit for writing, for this pressing operation closes up every gap between the fibers. Without this operation we should get what we call blotting paper, since the many cells between the cellulose fibers absorb the ink. Blotting paper, then, follows all the stages of the manufacture of ordinary writing paper except this final pressing stage.

To make glossy, shining paper, ordinary paper is coated with a special solution containing calcium carbonate or barium sulfate mixed with a substance extracted from milk and called casein (Casein), which is the protein (Protein) of which milk is composed.

As for the brown-colored paper used for paper bags and for wrapping various goods, it is made as ordinary paper is made, by cooking it with the sulfurous solution mentioned above, except that in this case it is cooked for a shorter time. The non-cellulose woody substances therefore do not all dissolve as they do in the cooking of ordinary paper; a large part of them remains mixed with the cellulose fibers, and the result is a brown-colored paper, but one stronger and better suited than ordinary paper for use in making bags or wrappers.