FIBER CHARACTERISTICS.
A few moments’ consideration of the changes which the fibers undergo from their condition of isolation as they exist mixed in the vat, to their status as components of a sheet of paper, will help to make clear much that seems obscure about the behavior of a sheet of finished paper, as well as to explain the reason for the different processes executed on the paper-machine.
The fiber is a hollow, collapsed tube, the ends bruised and frayed by the treatment in the beating and refining engines. Absorptive in nature to a marked degree, it swells with the water it takes up and is limp and flaccid. As the mold is raised horizontally out of the vat in the process of forming sheets, all the fibers which had been suspended in the water which passed through the meshes of the mold are caught like so many fish in a net, and lie spread in a limp, impressionable mass over the surface of the mold until they are transferred by the “coucher man” to the felt. Little alteration can take place in the general position of the fibers after they have been “couched,” consequently the formation of the sheet is the most important stage of the process. As the water is pressed out, each fiber contracts to some extent, and, from a consistency like gruel, the formed sheet passes to a more stable state, wherein it can be gently handled without disintegrating.
FOURDRINIER MACHINES, CRANE & CO.
A good view of the surface-sizing vat is obtained in the machine on the right hand. The paper is being slit just before its introduction into the vat.
As the drying proceeds there is a marked shrinkage in the dimensions of the sheet, caused by the shrinking of each individual fiber, until the fibers are thoroughly set, enmeshed one with the other.
The addition of size glazes over each fiber and makes it less susceptible to moisture. The addition of clay permeates the structure, filling up the interstices. Up to a certain point the clay does not materially weaken the structure, as a certain percentage of empty air space would exist without it. Beyond that point the clay will fill places that conceivably would be filled by fibers, and having no adhesive strength, the structure of a sheet overloaded with clay is weakened in proportion to its overload.
While the fibers are more or less moist, they are susceptible to alteration in structure, and may in this state be flattened by calendering to a smooth surface, and the presence of clay helps to fill in the microscopic valleys between the fibers so that the surface becomes level to human vision.