( Originally Published 1920 )
WOOD is, at present, by far the most important source of fiber for paper; it so far exceeds in quantity all other fibers used that it may be said to be the universal raw material for the industry. Its source is the forest, which also has to furnish timber for building purposes and other wood-using industries, as well as play a part of prime importance in the prevention of droughts and floods and in the equalization of climatic conditions. The perpetuation of our forests in healthy and usable condition is, therefore, of extreme importance to every citizen. This is quite a contrast to the days when the country was first settled; then they had to be destroyed to clear the land for agriculture or as a means of protection.
The forests in the different parts of the United States vary considerably in their characteristics and rate of growth. The north-eastern states formerly contained large quantities of white and red pine, spruce, hemlock and fir, as well as hardwoods such as beech, birch, maple, poplar, basswood and others. The best of the soft woods were long ago removed for lumber and since the rate of growth is slow in this region the available supply has been unable to keep up with the demand. This is evidenced by the very inferior grade of pine lumber now obtainable and by the almost total absence of some other grades from the market. Logs for pulp making are coming to the mill in much smaller sizes than formerly and kinds which were not considered suitable are now accepted and used without question. The soda pulp manufacturers, for example, used poplar almost exclusively for many years after the process was developed, but are now also using beech, birch, maple, basswood, elm, oak and nearly any of the other hardwoods which are obtainable.
The southern states are often spoken of as the southern pine region because of the predominance of longleaf, shortleaf, lob-lolly and Cuban pines in that section. There are also a number of hardwoods available, such as red gum, tupelo, sycamore, elm, oak, etc., which are suitable for the same uses as the hardwoods of the northern forests. Southern forests are favored by a long growing season and good reproduction, so trees reach a usable size in a much shorter time — 20—25 years as compared with 60—80 in New England. In spite of this, the rapid growth of the paper industry in the South is already forcing up the price of pulpwood in this section.
The forests of the Rocky Mountain areas have not attracted the interest of papermakers to any great extent, though the type of wood and its rate of growth in some portions of the region would seem to justify its use and perpetuation. The Pacific Coast forests, especially those west of the Cascade and Sierra Nevada Mountains in Washington and Oregon, contain enormous quantities of timber valuable both for lumber and wood pulp operations. These are being used at a rapid rate, but as reproduction and growth are especially favorable in this section, conditions seem better for a continual supply. Alaska also has great possibilities, but though it has been talked about for years it is only just being developed and its potential is not yet fully known.
The growth of the paper industry has been so rapid that the consumption of wood for pulp in the United States has grown from 2,000 cords in 1870 to about 26,522,000 in 1951. About 10 per cent of this last amount was imported from Canada. In more recent years the data are given under only two headings, "soft-woods" and "hardwoods," but according to the U. S. Forest Service, the kinds and the number of cords used in 1947 are as follows :
Spruce and true fir 3,995,172
It is estimated that the sawlogs produced in the United States averaged about 5 billion cubic feet in 1948 and 1949, compared with about 1.5 billion cubic feet for pulpwood produced during the same period. This indicates the serious competition of these two wood-using industries for our rapidly diminishing supply, and it is even worse for the lumberman than it seems, for the pulp maker takes smaller logs and thus cuts off in their youth trees which, if left to mature, would make good timber. Taking every-thing into consideration it appears that the paper industry is responsible for only about 10 per cent of the timber removed from the forests. The loss from insects, fires, diseases and wind is slightly more than that.
The continuation of our paper industry, as at present organized and operating, depends on a perpetual supply of wood. The problem for all concerned seems to boil down to a careful balance between growth and use, which means reforestation of cut-over lands and selective cutting of available timber. This will never be done by individual small owners, at least in sections where growth is slow, because the one who plants the trees will seldom live to derive any benefit from their sale. The only way out seems to be for a paper company to own sufficient land so that it can perpetuate its own supply of pulpwood by proper forest management. Just as an example, consider what this would mean in the north-eastern hardwood forests for a mill producing 100 tons of soda fiber per day from 153 cords of wood. Since such a mill usually operates every day it would require about 55,000 cords per year and since the average rate of hardwood growth in this region is about 0.2 cord per acre per year the land required would be approximately 275,000 acres, or 430 square miles. This does not take into consideration losses from forest fires or insect infestation, and, moreover, a mill producing only 100 tons of fiber a day is at present a rather small unit.
Perpetuation by ownership of land is, therefore, an expensive undertaking, which might indicate that eventually the pulp industry will be in the hands of a relatively few concerns which have the funds and foresight to carry through such a program. It will be interesting to see whether this will come to pass, or whether the use of materials other than wood may upset the prediction. At least it is self-evident that the forests in any country can supply only a certain amount of pulpwood, which will automatically limit the amount of pulp and paper produced. It is said that Sweden is already at the point where her pulp industry cannot be increased because the limit of her forest capacity has been reached.
Wood reaches the pulp mill in several forms, the commonest probably being 4 foot logs. Some mills receive longer logs and cut them by gang-saws or "slashers" into pieces of the length desired; others use large amounts of slabs and edgings from saw mill operations, while some of the mills on the Pacific coast are able to handle much larger logs than the eastern mills, both with regard to diameter and length. Some pulpwood is brought to the mills by truck, some by rail and some is floated to the mills down the rivers, or, in some sections, in large rafts guided by tugs.
Wood which has been floated in rapid streams loses a good part of its bark on its way to the mill, but a considerable part of that received dry still retains its bark. In the early days of pulp mills the bark was removed by hand-peeling when the sap flow made it relatively easy to remove. That made cutting, barking and delivery to the mill seasonal operations. Later came the knife barkers which removed the bark ( and incidentally considerable wood) from the individual logs, and later still came the drum barker through which the logs pass in the presence of water. As the logs rub against each other and against angle irons on the inside of the drum the greater part of the bark is loosened and washed off. A more modern method of barking is by streams of water under very high pressure (up to 1400 pounds per square inch ), directed against the log as it is rotated. This development is more applicable to the large logs of the Pacific coast where it is used on logs up to 24 feet long and 6 feet in diameter, but it may eventually be adapted to smaller logs in other parts of the country. The most recent development in bark removal is to kill the trees by injection of certain chemicals which cause the bark to loosen and fall off.
The last three methods of barking save considerable wood over the knife barkers, but drum and hydraulic barking have introduced a problem in the disposal of the waste bark. Unless some space is available where the wet bark can be dumped as filling, the preferred method seems to be to put it through presses which are capable of reducing its moisture content to about 50 per cent, and then use it as a fuel. The fuel value of the pressed bark is said to more than offset the cost and maintenance of the press. Considering the desirability of getting more humus into our soils it seems a pity that some cheap method of converting bark to humus cannot be worked out, so that it can be delivered to the gardener or farmer at a reasonable price.
Another problem which faces many pulp mills is that of decay of their wood. This is especially serious in those sections in which the woods operations are seasonal and a considerable supply of logs has to be kept on hand to insure continuous mill operations. In large piles of wood which are held over from one year to the next some decay is bound to take place, for there is always a zone in the pile where moisture conditions are especially favorable for fungus growth, and here decay is most rapid. The outside of the pile becomes too dry for serious decay while the portions well within the pile are too wet for most fungi to grow. Stacking the logs in well ventilated tiers prevents decay by keeping the wood well dried out, but stacking is expensive, and requires a large storage area, so the common method is to make a series of conical heaps and use them in rotation. In the south even this cannot be done safely, because the conditions of temperature and humidity so favor decay that wood cannot be kept more than a few weeks without the appearance of "blue stain," which is one of the first evidences of fungus growth, or without a marked loss in the yield of pulp which can be obtained.
From the standpoint of the pulp-maker, bark and decayed wood represent loss in several different ways. They occupy space in the digesters which should be available for good wood; they consume a portion of the chemicals, which is thus doing useless work, and they contribute very little fiber which is of value. Of course decay, and the resulting loss, vary in degree—ranging from the first sign of attack to a condition in which the wood is reduced to such a brittle, friable mass that it is almost completely lost during the mechanical processes of preparing the chips for the digesters. Furthermore, the losses from decay are not alike in all of the pulping processes; the effect of decay is greater in alkaline cooking processes than in sulfite cooks, and is still greater in the groundwood process because this does not sterilize the pulp and the contamination contributed by the wood may continue as decay in the finished pulp. Nevertheless in all processes bark and rotten wood contribute their quota of dirt to the pulp, and if high grade papers are to be made, both bark and rot, but more especially bark, should be avoided. In spite of all this some mills continue to use all, or part, of their wood in the unbarked state.
In perpetuating a wood supply, losses from forest fires as well as from serious insect infestation must be given careful consideration. Neither of these can be entirely avoided, and the problem of using wood killed from either cause frequently occurs. The question seems to be whether the charred portions of the fire-killed wood can be avoided sufficiently so that the pulp will be clean, and also whether both kinds of dead wood can be harvested and used before a serious amount of decay can take place.
For making groundwood the preparation of the wood is completed when the bark and rotten portions are removed and the clean wood is cut to the right length for the grinder. For all of the chemical processes the logs have to be reduced to small pieces so that uniform penetration of the cooking liquor may be assured. This is accomplished by means of a "chipper," which is a heavy steel disc with knives projecting from its face—the distance to which they project determining the length of the chip. The logs are fed down a spout so located that the knives shear off slices of the wood at an angle of about 45 degrees with the axis of the log. This produces "cards" or "hands" of chips which are readily broken into smaller pieces. Chippers have increased in size and capacity from the original with two knives, to present ones with as many as twelve. A chipper with eight or twelve knives will do the work of several of the old type, and will deliver more uniform chips because the knives are in almost continuous contact with the logs, and the latter have less chance to jump in the spout and thus shorten the chip. Still more recently chippers have been built for the Pacific Coast mills which will handle logs 24 feet long and as much as 34 inches in diameter. One such chipper has six knives on a disc 153 inches in diameter, running at 257 revolutions per minute and driven by a 1500 horse power motor. Its capacity is equivalent to about 400 cords per hour.
The chips as they come from the chipper contain some fine material, spoken of as "sawdust," some slivers and chunks from the ends of the logs, and many unbroken cards of chips. To insure uniform chips for the digester they are put through a crusher which breaks up the cards and then are passed over screens which separate the material into sawdust, good chips, and slivers which are too coarse for the digesters. The sawdust is generally burned as fuel and the slivers are sometimes put through a rechipper and returned to the screens.
As already mentioned, the kinds of wood which are used for pulping are much more numerous than formerly. This is due in part to the exhaustion, or growing scarcity of the preferred woods, and partly to improvement in methods of pulp preparation, especially in bleaching, which have made it possible to use woods which. were formerly thought to be useless. In general there is still a preference for certain kinds of wood in the different chemical processes. The sulfate and soda processes can cook almost any kind of wood, but sulfate pulp is usually prepared from coniferous wood while soda pulp is more often made from deciduous (broad leaved) woods. Probably this dates from the time the processes were first put into operation, the type of wood which was first used being continued unquestioningly. Whatever the reason, the sulfate process employs chiefly such woods as spruce, jack pine, the southern pines, Douglas fir and some of the other western conifers, while the soda process, starting with poplar, has expanded its wood use to include beech, birch, maple, ash, oak, chestnut, cottonwood, willow, magnolia, gum, and other locally available wood of this general type.
In general the coniferous woods have long fibers and are used for preparing pulp for strong papers, or to mix with short fibers in order to bring the strength of the paper up to an acceptable point. There is considerable difference in the quality of the fibers from different conifers and this seems to be due to the wood itself rather than to the cooking process employed. Thus it has so far not proved possible to prepare by the sulfite process exactly equivalent fibers from eastern spruce and western hemlock; and in the same way northern spruce and jack pine yield sulfate fiber which differs in properties from that made from the southern pines. The cause of such differences is not fully understood as yet. It may be due to several things, such as rate of tree growth, proportion of spring and summer wood, structure of the cell wall or to differences in the chemical constituents of the woods. Whatever its cause, such a difference in fibers is a factor to be considered in attempting to make paper of the same character out of fibers produced in different sections of the country.
The proportion of sapwood and heartwood is another factor of some importance in pulp making. Heartwood is darker in color, and harder than sapwood and contains more resins; the lighter sapwood, however, contains more fermentable material and is more subject to decay. Sapwood is preferred for pulping because it is more easily cooked. In some woods the difference is so great that the sapwood can be cooked by the sulfite process while the heartwood cannot. Since the change from sapwood to heartwood takes place only after the trees have reached a certain age, the younger trees of some of the pines can be cooked by the sulfite process, while older trees of the same species are not satisfactory.
Springwood and summerwood also influence the quality of the pulp. The fibers produced in the spring are thin-walled and tend to collapse into flat ribbons, and thus make a dense, closely-knit paper. The fibers growing during the summer are thick-walled, stiff and wire-like in character and tend to produce papers with high bulk and rather coarse surface formation. The proportion of these two types of fibers in wood is of considerable practical importance in the utilization of the pulp, and in one investigation of southern pines it was concluded that a difference of 10 per cent in the springwood content of a given species would result in a greater difference in pulp quality than that exhibited by different species of pines. The coarse annual rings shown by many of the southern pines indicate a relatively large proportion of summer-wood and this may account for the difference between pulp from these trees and that from northern spruce, and even jack pine, in which the annual rings are much less pronounced. While differences such as this are well recognized it is impractical to sort the logs as to quality, and about all that can be done is to use the woods which are available locally.
It has been mentioned that the coniferous woods have long fibers in comparison with the short fibers of the deciduous woods. It is impossible to establish the average length of fiber from any given kind of wood because it varies considerably in any species with the rate of growth, and this depends on the character of the soil and the amount of moisture available. A rich soil and plenty of moisture produce trees with longer fibers than those from trees grown in poor, dry soil. Fiber length also varies in different parts of the same tree. In both trunk and branches the average fiber length increases from the center outward until the tree has reached its maximum height, after which the fiber length changes little. The fiber length in the branches is usually less than that in the trunk.
The fiber length in wood is thus fixed by nature and nothing can be done by man to increase it during the pulp making processes. The best he can do is to so handle the wood that as few fibers as possible are shortened during the chipping of the logs, though any such change is of relatively minor importance.
The moisture content of wood varies with the kind, the time of cutting, and the position in the tree, and after delivery to the mill it will change according to the conditions under which it is stored. The top logs in a pile exposed to summer sun and weather will contain much less moisture than the bottom logs in the same pile. The term "air dry" as applied to wood chips prepared from stored wood may mean almost anything from 6 to 40 per cent by weight of moisture. Shrinkage is rather closely related to the moisture content of wood, since loss of moisture causes a large contraction in volume. Unfortunately the two do not change in any fixed ratio.
The weight per cubic foot of solid wood varies greatly with the kind and with its moisture content. Since a block of wood of definite dimensions develops shrinkage cracks on drying, it is difficult to measure its dry volume accurately. It is customary to express the weights of woods as pounds oven dry per cubic foot of green volume. On this basis some of the woods commonly used in pulp making will be found to vary from about 21 to 35 pounds. Among the lighter woods are aspen, basswood, black willow, balsam, silver and white firs of the Pacific Coast, white pine and Engelmann spruce—all of which will average less than 23 pounds per cubic foot. The heavy woods which are used in pulping—and which will weigh from 30 to 35 pounds per cubic foot—include shortleaf, longleaf, and loblolly pines, sugar maple, red maple, yellow and paper birch and beech. It is obvious that the weight of wood to fill a given size of digester will vary enormously according to the kind which is being used.
These points—moisture content and weight per cubic foot—must be taken into consideration if pulp of uniform quality is to be produced, because the amount and strength of the alkaline cooking agents must be adjusted to compensate for variations in the wood. Where wood of only one kind is used, or when the wood comes to the mill within a very short time after cutting, as in the southern pine kraft mills, this problem is not serious. In the north-ern mills, especially the soda mills, where a large variety of woods is used, and where long storage may cause marked variations in moisture content, considerable variation in the pulp may be caused if proper adjustments are not made.
The resin or pitch content of wood is of some importance, especially in the sulfite process, since it may remain in the pulp and cause sticking on the wires and press-rolls of the paper machines. There is no positive knowledge regarding the exact amount of resin in wood, because it varies to a great extent with the kind of solvent used in its determination. At its worst it is a small amount -0.45 to 3.6 per cent by weight in fresh wood—and it decreases during storage and seasoning of the wood. Even the hardwoods contain small amounts of material of a fatty or waxy nature which are extractable by ether or alcohol and for this reason might be classed as resins.
The chemical characteristics of woods vary considerably with the species as well as with the methods of analysis.