Fourth. From the press felts the paper is carried to the "dryer felts," which in turn carry the paper to the "dryers," which revolve and by means of the felt carry the paper along to the next dryer, and so on. The dryers are hollow iron or steel cylinders, heated by means of the exhaust steam from the engines which run the machine. More or less steam is allowed to run into the dryers, according to the quality of paper being made.
Fifth. As soon as the paper has been carried over all the dryers, during which time it becomes, perfectly dry, it is run through a set of so-called steel "chilled rolls," at the end of the machine, which are under pressure and which give the paper a fairly smooth surface for ordinary type printing. If a rough surface is desired, the paper is simply wound on reels from the dryers.
Super-calendered papers are those which have a high finish and smooth surface, and are used for cuts, lithographic work, magazine papers, and ordinary illustrations. To calender paper, it is run through a series of alternate "chilled" and "paper" rolls. The chilled rolls are made of steel and have a very smooth and even surface. The "paper" roll is made of circular discs of thin, but strong manila paper, clamped together on an iron shaft, and then put under hydraulic pressure, this pressure being increased constantly until it reaches one hundred tons of pressure to the inch. The rolls are sometimes kept under this pressure for five or six weeks, and then are turned on a lathe into a true and smooth cylinder, and finally burnished by being revolved against each other.
A "cotton" roll, used at times in place of the "paper" roll, is made in the same manner, except it is made of pieces of cotton cloth instead of thin manila paper. There is a heavy pressure on these rolls, and the paper goes through at a high rate of speed. When an especially smooth surface is wanted, steam is run on the paper as it unwinds, dampening it and giving the web a surface like that on ironed linen.
"Coated" paper is treated differently, being covered with a fine coating, which, after super-calendering, gives the paper a glazed and smooth surface for fine half-tone illustrations. Clay, mixed with casein, the product of skimmed milk, or glue, is the chief material used for coating. It is put on the paper by means of large brushes. Then it is dried by fans and passed through a long passageway heated by steam to a high temperature. After being reeled, it is allowed to stand for a while to harden; then is run several times through the calenders to get the smooth surface. If a high, glazed finish is necessary, steam is put on while running through the calenders. This gives a very bright surface for fine lithographic work. For the best coated papers, instead of clay, sulphate of lime and sometimes sulphate of barium is used, with glue or casein. Formaldehyde, a chemical compound, is used to prevent decomposition in the coating materials; and soda or borax is used to "cut" or dissolve the casein or glue.
If the paper is to be printed "from the web," that is, from the roll, it first has to be trimmed to the correct width, then wound tightly under a high pressure to a certain thickness, then the rolls are packed up in wrapping paper ready to be shipped. Some rolls contain as much as five miles of paper. When the paper is to be put up in sheets, it has to be cut to exactly the correct width and length on the cutting machine. It is all very carefully sorted—the imperfect sheets being thrown out—counted and packed in wooden cases, or done up with strong wrapping paper in bundles, ready to be sent to the printer.[Back to Contents]
PRESSWORK
By Walter J. Berwick
Books are printed in "forms," or sheets, of four, eight, twelve, twenty-four, or thirty-two pages at a time, the number being determined to a great extent by the size of the type page and by the class of the work.
An ordinary twelvemo book, without illustrations in the text, is usually printed in forms of thirty-two pages, on what is known as a single-cylinder flat-bed press, which prints only one side of the paper at an impression. For large editions, the size of the sheet of paper is sometimes doubled and sixty-four pages printed at a time. The class of work in question may also be printed on perfecting presses which print both sides of the paper at one time, and in this way as many as one hundred and twenty-eight pages are frequently printed on one sheet, there being sixty-four pages on each side. Large editions of books having small pages, such as small Bibles, are often printed two hundred and fifty-six pages (one hundred and twenty-eight on each side) at one time.
High grade, illustrated books are always printed on one side of the sheet at a time, the reverse side being printed after the first impression has dried properly. Thus a smooch, or "offset," the result of handling the paper before the ink has become dry, is prevented.