Popular Science Monthly/Volume 45/July 1894/Kiln-Drying Hard Wood
| KILN-DRYING HARD WOOD. |
By O. S. WHITMORE.
AMONG the many changes that have taken place in the manufacture and handling of lumber, there is none more marked or interesting than in the method of preparing lumber for use by getting rid of its natural or acquired moisture. For a century and a half after sawed lumber came into use, none but natural means were used for drying it, preparatory to its consumption in the building and kindred arts. Even in comparatively modern times, when sash, doors, and blinds were made by hand, and flooring and ceiling were dressed and matched in the same manner, if a person concluded to build some time in the future, the stock for these purposes was often bought after being weather-dried as much as possible and stored away in barns, lofts, and garrets, where it was not seldom left for years.
There can be no denying that this stock made excellent work, though it became not infrequently discolored from want of a circulation of air, which fact became so well understood that when at last attempts were made to shorten the drying period by artificial means, they all embodied some attempt, more or less crude, to create a circulation, to the end that the air that had enveloped the lumber until it had absorbed a portion of the moisture should be thrown or forced out of the drying room or building.
The first attempts at artificial drying did not contemplate general stock or drying lumber for shipping, the first dry-houses being usually nothing more than a one-story frame structure built over a low cellar excavated in the side of a hill, with a slatted floor above and a latticed cupola on the roof. A brick or tile furnace or arch was built in the cellar, from which extended a number of sheet-iron pipes which, while conveying the smoke to the chimney, also acted as radiators. The furnaces were built so as to be stoked from the outside through an arch in the wall on the down-hill side. After the introduction of cast-iron stoves, they were often substituted for the brick or tile furnaces, and in some cases these in turn were superseded by wrought-iron cylinders like steam-boiler shells.
The material to be dried was stacked upon the slatted floor in loose piles, through which the heated air from below could circulate more or less freely. For years these dry-houses—they were not then dignified by the name of kilns—were only used in connection with certain manufactories for drying stock already cut up for tubs, pails, and other wooden ware; small boxes, chairs, and other furniture material; turned work and Yankee-notion stock in general; no regular lumber stock being subjected to the process.
Occasionally a little lumber for interior finish, where an extra fine job was desired, was run through the dry-house for a final drying, and later, after machine-made sash, doors, and blinds began to take the place of the old hand-made goods, being generally made from air-dried stock, they were sometimes put through the dry-house before being wedged and pinned.
These dry-houses contained such an element of fire risk that they were generally built in isolated positions as close to water as possible. Even then they were a constant menace to all surrounding property as well as to their own contents. Lumber, except in small pieces, dried in them was apt to be checked and warped or twisted more or less, and was not at all satisfactory save in the one feature of being free from moisture.
The fire risk at last became so great where the establishments requiring the dry-houses were situated in towns, and the restrictions of underwriters so onerous, that along in the fifties some crude attempts were made to substitute steam for the furnaces by conducting the exhaust from the engines running the works into the cellars.
It is not definitely known when or by whom the first attempts were made, but it is a fact that as early as 1855 the trial was made by a manufacturer in northern Massachusetts. But the experiment did not prove very satisfactory, for the reason that the steam had to be carried quite a long distance; the science of protecting steam pipes so as to prevent condensation was not as well understood as at the present day; the engine was none too large and the boiler capacity limited, and there was more or less back pressure.
But so certain were the experimenters that they were on the right road that they kept up the trials, though, from causes stated, making but little attempt to use the steam during the winter months. Success seemed near, when the panic of 1857 came on, and the house met with reverses that stopped all further experiments. Some additional attempts were made in the New England States during the next four years, but in a rather desultory way, when, the war of the rebellion coming on, the inventive genius of the country seems to have been turned in other directions and the subject of drying lumber slept for some years.
A patent was granted to Hannah and Osgood, November 27, 1866, for "an improvement in the method of drying lumber" and other patents followed in rapid succession, a full history of which is shown by the records of the Patent Office. But it does not appear that any really successful kiln was built until the year 1875, when one was erected at Stillwater, Minn., and a little later one in Chicago, if the records are correct, for Pond and Soper, though Turner Brothers had one built about the same time. The dates as to when the first steam drier was put in successful operation are a little foggy, claims being made both for Stillwater, Minn., and St. Albans, Vt.
The question of the artificial drying of hard-wood lumber has assumed such importance that all, both manufacturers and dealers, must be interested in the subject. To air-dry hard-wood lumber by simple, natural means involves the loss of interest on immense sums of money invested in the lumber while it is awaiting the slow and not always satisfactory or sure process of Nature; while, on the other hand, it is a well-admitted fact that unscientific and hence unskillful drying by artificial means often involves a loss greater than the other.
The earliest attempts at the artificial drying of lumber made no difference in the matter of varieties of wood or quality of stock. All kinds and qualities were run in promiscuously, and all subjected to the same treatment. The only theory acted upon was that the lumber, being green or wet, must be dried in the shortest possible time. To effect this result it was only thought necessary to create as great a heat as possible within the limits of safety, and to raise it to the maximum degree in the very shortest time, the limit being often raised to a reckless height, not infrequently reaching the point of actual partial carbonization to an extent that killed the life of the lumber so treated. Often the kiln would be hastily opened for the removal of dry stock, while it was under full headway, with the heat up to the highest point, and green and often frozen lumber hurried in to receive at the very outset a blast of heat as near the point of combustion as it was possible to raise it with any degree of safety.
This, of course, has reference more particularly to the days of dry hot air and furnace heat, though the same was true of the earliest attempts at steam heating. Nothing was known or thought of the effect of thus subjecting lumber to a high temperature at the very first stage of the drying process, and nothing was known of the effect of high temperature upon different varieties of wood or the same variety under different conditions, whether entirely green, or partially or wholly air-dried. One thing only was known—that heat would drive the moisture, whether natural or acquired, out of the lumber, if it was only applied hot enough and long enough.
The physiology of wood, or what is now known as timber physics, was poorly understood by any one, much less by the men who were making the experiments; for in general they were plain business men, with only ordinary business education, and with no pretensions to scientific knowledge.
Thus little or nothing was known of the chemistry of woods and absolutely nothing of the effect of heat upon the gums, juices, or fibers. But while these men were not up in the sciences, they possessed what perhaps in this instance stood them in as good stead—hard common sense and quick perceptions, that permitted them to learn rapidly by experience and by quickness of observation to note the results upon the woods of various conditions in the course of their experiments.
Thus it was discovered, by a more or less costly experience, that in all the long list of varieties of timber hardly any two could be subjected to precisely similar treatment with the best results to both; and it was further found that difference in the source whence the same variety came often required a variation in treatment.
The next and perhaps the most important discovery made, and probably at the expense of the greatest amount of spoiled lumber, was that a temperature too high at the commencement of the drying process produced unsatisfactory results, and often ruined or greatly reduced the value of high-grade and costly material. Before this fact was discovered, so uncertain had been the process in its effects, other than in producing apparently dry lumber, that an actual prejudice arose against submitting upper grades to the artificial process until fairly weather-dried; it being found that if a portion of the moisture on and near the surface was evaporated by the natural heat of the sun, the effect of plunging the lumber at once into a high temperature when put into the kiln was less injurious.
Investigation and experiment proved further that this was a perfectly natural theory and one by which Nature herself worked constantly. It was fully and satisfactorily shown that lumber sawed and piled in the winter so as to take advantage of the first cool, dry days of spring, not only dried in better condition, with much less danger of sap stain, checking, and warping, but that it actually dried more completely to the very center of the piece, and in a shorter time.
This was found to be especially true of thick lumber, it proving to be a fact that, while the winter-sawed thick stuff would often, in favorable seasons, become remarkably dry to the very center. stock of the same thickness, if sawed and piled during the months of extremely hot weather, would have to be carried over until the approach of another summer, the effect of the season of extremely damp atmosphere seeming to be to liberate the internal moisture, which somehow appeared to be imprisoned by some (at that time) unknown force, and which, being so liberated, was rapidly carried off by the cool, dry days of the following spring.
It had probably always been known that lumber would dry, and did dry, most rapidly during the season of high winds, but the fact had been generally accepted without asking for a scientific reason. But when it dawned upon the minds of the experimenters that there must be one, it led to the further discovery that air in motion of a low temperature would produce better results than air of a high temperature if kept stagnant, and that the ordinary atmosphere, with its natural temperature, if above the freezing point and with a low degree of humidity, if kept in constant motion or circulation, would dry lumber well and rapidly without the aid of artificial heat.
These points once definitely settled and understood, led to researches that immediately led into the domain of wood chemistry and physiology, and the experimenters and inventors became to a degree chemists and physicists. Thus a special education was obtained before they were able to say they had solved the problem of drying lumber artificially; fairly accurate knowledge on the following points being gained:
1. (a) Different varieties of wood, and (b) wood of the same variety grown in different localities, requiring radically different treatment.
2. (a) That too high a degree of heat applied at any stage, and (b) especially during the first, injured the lumber more or less, according to kind, and retarded or prevented perfect drying.
3. That with a perfect circulation of air of a low degree of humidity, a high temperature was not necessary to produce good results except as to time.
4. That the results, good or bad, depended very largely upon the chemical changes produced by heat upon the natural gums and juices of the wood.
5. That all these points became much more pronounced in the case of hard woods, and hence the necessity for special machinery and arrangement of the kiln.