Popular Science Monthly/Volume 15/August 1879/Dry-Rot in Timber

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NEARLY everybody has heard of dry-rot, and knows that it is something which causes the destruction of wood in a manner different from ordinary decay. Some suppose the effect to be due to peculiar insects that gnaw timber to powder, and others have no very definite notions as to what produces it. Carpenters, ship-builders, lumbermen, and house-owners often find, by the rapid destruction of their property, that, whatever its cause, it is a very serious matter, and they seek to be protected from the evil, though taking little pains to inform themselves of its real nature and conditions. The subject is, however, one of curious scientific interest, and has now come to be pretty well understood. An excellent work upon it has recently been compiled by Mr. T. A. Britton, an eminent British architect, who has ransacked all sources of information; and for the materials of the statements which here follow we are indebted to this book.

To understand the nature and effects of dry-rot we must first glance briefly at the structure and properties of wood. The mass of the trunks of timber-trees consists of slender, short fibers, with tapering ends, which overlap each other; but this overlapping does not prevent the passage of sap through them. At first these fibers are hollow, but are gradually filled by the deposition of solid matter from the sap within them. The strength of wood is due to the shortness and overlapping of the fibers, and to the presence of this deposit. Woody fiber pervades the tree from the tips of the roots to the extremities of the branches, and is the chief organ of circulation. A current of sap passes upward through it, from the roots to the leaves; and another current, containing the products of leaf-action, passes back from the leaves, and is distributed for the uses of the tree. As wood grows older it grows darker, particularly in the center of the stem or heart. This darkening is due to the deposit within the fibers; and when a tree reaches maturity the fibers are so filled as no longer to join in the general circulation. Now, this inner or heart wood is less liable to decay than the outer or sap wood, and sap, as is well known, is the agent of destruction. Sap is water with sugary, saline, albuminous, mucilaginous, and gummy matters dissolved in it, and such solutions ferment easily and rapidly. Fermentation is a state of vegetable matter in which the various molecules, sugary, oily, albuminous, etc., exert their peculiar attractive and repulsive powers, forming new combinations, which at first change and at length destroy the texture of the substance of which they were formerly a part. Every one knows the smell of pure, fresh wood. If you bore into wood in which the sap has just begun to ferment, you get a vinous smell, which is soon followed by the smell of putrefactive decay, unless means are taken to arrest the chemical changes that are in progress. This decomposition of wood containing sap is ordinary rot or wet-rot. It is the most general and the most fatal cause of decay in wood; but it has attracted less attention than the more startling but less common evils of dry-rot, and the destruction of timber by insects.

The seasoning of wood, whether naturally or artificially, is simply the evaporation of its sap. Decay can not occur in well-seasoned wood if it is kept dry. It matters little whether wet is applied to timber before or after the erection of a building: it can not resist the effect of what must arise in either case; for heat and moisture will produce putrid fermentation. In basement stories with damp under them, dry timber is but little better than wet, for if it is dry it will soon be wet, and decay will only be delayed while the timber is absorbing moisture; and the amount it receives will depend upon the closeness of the deposit within its fibers. This moisture dissolves the substances held in solution by the sap, and fermentation begins, with its usual train of consequences.

Dry-rot is one of these consequences. Ordinary decay must have begun before dry-rot sets in. When the moisture in wood begins to ferment, whether it be the natural sap, or the water absorbed by seasoned timber, the conditions are ripe for the inroads of dry-rot, which can no more occur without moisture than wet-rot. The immediate agent of destruction, in this case, is of vegetable origin. It takes its name from the dust to which it reduces timber. That degree of moisture which is favorable both to natural decay and to the growth of plants is essential to the process of dry-rot. The vegetation that produces it belongs to the natural group of fungi. This group is made up of plants having distinct vegetative and reproductive systems, and their best known representative is the common mushroom. If you examine the mold on which this singular plant is seen to grow, you will find it penetrated with delicate, whitish, interlacing filaments which are the vegetative system of the plant. This part of a mushroom is called the mycelium, and from it arises the reproductive portion which grows above ground. But the only part of this above-ground portion that is essential, and that is found in all fungi whatever, is just that part which escapes ordinary observation. Everybody has seen the umbrella-like cap with the radiating vertical plates on its under surface. These plates are covered by a membrane which has the same office as the seed-vessel of the higher plants. It bears the minute reproductive bodies of the fungi, analogous to common seeds, and called spores.

The only parts of a mushroom which are common to all fungi are the mycelium or thready, interlacing portion which grows underground, and the minute, microscopic spores which are cellular in structure and so small that thousands of them are required to form a body the size of a pin's head. The fungi differ among themselves in many ways; but mycelia and spore production always occur in them, and are their essential characters. Every plant of which this mycelium forms a part, spreading its web throughout the substance on, or in, which it grows, belongs among fungi. Most of the species are either quite invisible, or else their parts are so small as to be indistinguishable. But some sort of reproductive organs exist, and spores are always produced. The mycelium is often so minute as to traverse living plants and the pores of solid wood. It grows rapidly and causes quick decay. Potato-rot, the yeast-and vinegar-plants, mildews, rusts, and smuts of grain, and molds of all kinds, are part of this immense group of plants that lives upon decay and fills the air with its countless myriads of spores. These subtile, germinal particles are lodged everywhere. They are light as vapor and abound in air, in water, in dust, in sand, ready, when warmth and moisture favor, to burst into life. As has been said, the dry-rot fungi flourish upon the products of wet-rot. Different stages of decay produce food of different qualities, adapted to different species of fungi. One species takes up the process where another leaves it, and carries it further and further forward.

Dry-rot may begin its ravages in the interior of timber as easily as upon the surface. As atmospheric dust is filled with the spores of fungi, they may be conveyed by rain into the earth, absorbed by the roots of vegetables, and diffused with the sap throughout the whole plant. There are numerous species of dry-rot fungi adapted to different conditions of life and presenting different aspects. Nor are they restricted to timber. They may flourish in the earth, where they present a perfectly white mycelium, branching and interlacing like roots; and when workmen are employed on grounds which are affected by the dry-rot fungi their health is often disturbed. A few years since, while a London builder was putting up some houses at Hampstead, his men were never well. He afterward learned that the ground was affected by rot, and that, within one year after the house was erected, all the basement floor was in a state of premature decay. In cases of dry-rot, where the mycelium passes through substances from the external surface, it separates into innumerable small branches; when it proceeds from slime in the fissures of the earth, the mycelial fibers shoot in every direction and are very much tangled. Arising from the roots of trees, they look at first like hoar frost, but soon show regular toadstools. When they grow in very damp situations, they feel fleshy and extend equally around a circular space which they wholly cover unless obstacles interpose.

Excessive damp is unfavorable to this fungus, and its growth is more rapid in proportion as the situation is less damp, until the proper point for the growth of vegetation is reached. When the fungus extends to dry situations, its effects are more destructive to the timber on which it grows: it is very fibrous, and in part covered with a light brown membrane perfectly soft and smooth. It is often of great magnitude, projecting from the timber in a white spongeous excrescence, on the surface of which a profuse humidity is frequently observed. Sometimes it forms only a fibrous, thin-coated, irregular web on the surface of the wood. Excrescences of a fungiform appearance are often protruded amid those already described, and are evidences of a very corrupt state at the spots whence they spring. Sometimes they arise in several fungiforms, each above the other, without any distinction of stem; and in some corrupt states the small acrid mushroom is generated.

But there are two or three species of fungi that are chiefly concerned with the process of dry-rot. The Merulius lachrymans (often called the dry-rot) is a most formidable enemy of timber. When the section of a piece of wood attacked internally by dry-rot is examined through a microscope, and minute white threads are seen interlaced and matted together all through its substance, and when this cottony texture effuses itself over the surface of the timber, and in the center of it a gelatinous substance forms which gradually becomes tawny and wrinkled and sheds a red powder on the white, downy surface, you have the Merulius lachrymans. But, long before this last stage of growth is reached, the interior of the wood has perished. As soon as the cottony filaments are seen upon timber internally affected, we may be sure that an apparently solid beam may be crumbled to dust between the fingers. In his botanical description of this plant, Dr. Greville says it is "soft, tender, at first very light, cottony, and white; veins appear, at length, which are of a fine orange or reddish brown, forming irregular folds, usually so arranged as to appear like pores, but never anything like tubes, and, when perfect, distilling drops of water." Hence the term lachrymans. The folds or pores here spoken of are the reproductive portion of the plant. They are covered by the hymenium or spore-bearing membrane, which sheds its red powder upon the white mycelium. In this fungus the stem is entirely absent, and the cap is attached by its back. In the different species we may find the reproductive or spore-bearing portion in the form of a cup, or a goblet, or a saucer; of an ear, a bird's nest, a horn, a bunch of coral, a button, a rosette, a lump of jelly, or a piece of velvet. The Merulius is found in cellars and hollow trees, sometimes several feet in width.

Another fungus, the Polyporus hybridus, is an especial enemy of oak-timber. It is described by Berkley as "white, mycelium thick, forming a dense membrane or creeping branched strings; hymenium breaking up into areæ, pores long, slender, minute." This species makes great havoc in the navy. At one time it is said, in the memoirs of Pepys, thirty new wooden ships in the British navy, "for want of proper care and attention, had toadstools growing in their holds as v big as one's fists, and were so decayed that the planks dropped from their sides." In the beginning of this century, three 74-gun ships of the Royal Navy decayed in five years; three others in seven years; and a 100-gun ship in six years. Fungi have been seen growing between the timbers of a man-of-war strong enough to force a plank half an inch from the ship's side.

But, without attempting to discriminate among the fungi causing dry-rot, it may be stated generally that, in timber that has been only superficially seasoned, this disease often arises internally, and has been known to convert the entire substance of a beam, excepting an inch or two at the surface, into fine, white, and thread-like vegetation, which forms a thick, fungous coat at the ends of the beam, otherwise appearing perfectly sound. This has often been observed in large girders of yellow fir, which have seemed sound on the outside. Major Jones, R. E., states of a building in Malta, that "the timbers had every external appearance of being sound, but on being bored with an auger they were found internally in a total state of decay."

The first symptoms of dry-rot in timber are swelling, discoloration, moldiness, and a musty smell. As the disease advances the fibers shrink lengthwise and break, presenting many deep fissures across the wood; finally, the cohesion of the wood is utterly destroyed, and at the least disturbance it crumbles to powder. Before it has time to destroy the principal timbers in a house, it gets behind the skirtings, dadoes, and wainscotings, drawing in the edges of the boards, and splitting them both horizontally and vertically. When cleared of the fungus they look like wood that has been charred. Though affected but a short time, a slight pressure will break them asunder: and, when examined, the fibrous fungus will be seen closely attached to the decayed wood.

Timber that is floated down rivers and conveyed from place to place in ships is very liable to this disease. It is said of the exports of timber from Canada to England, that few cargoes in the log arrive in which, in one part or other of almost every log, you will not see the beginning of this rot, either as reddish, discolored spots, which, when scratched by the nail, show that the texture to some little depth has been reduced to powder, or else the white fibers themselves may be seen growing. If the cargo was shipped dry, and had a rapid passage, the case is not so bad; but when shipped wet, and the voyage has been prolonged, white fibers will be seen growing over nearly every part of the surface of every log, especially if they are of yellow pine, red pine, and oak.

When deal (pine planks) is shipped wet in Canada, it is also covered with a network of white fibers on its arrival in England, and even when shipped tolerably dry the fungus will be found upon some of the pieces. When they have been floated down our rivers and shipped as soon as they were taken from the water, at the end of the voyage they are often so covered with this network of mycelial fibers that force is necessary to separate them, and they will grow together again in the barges before being landed. If deals in this state are piled flatwise, a whole pile will become deeply affected with rot in six months. In some instances the rot penetrates to the depth of one eighth of an inch. The decay may be arrested by sweeping the surface of each deal, and repiling them upon their edges during dry weather. The ships which carry this timber, though built of good, sound, well-seasoned oak, must be carefully dealt with, or they will become affected. It is usual to scrape their surface as soon as they are clear of the cargo, and sometimes the hold is washed with a desiccating fluid. The effects of dry rot upon European deals are very similar to those exhibited by Canadian deals. Decay is more rapid in white deal than in yellow, for the white deal absorbs more water than yellow deal. In the same way yellow deal absorbs more water and decays faster than red deal.

An example of the rapid decay of timber from dry-rot was given by Sir Thomas Deane in 1849 before the Institution of Civil Engineers in Ireland. It occurred in the Church of the Holy Trinity at Cork. On opening the floors under the pews, a most extraordinary appearance presented itself. There were flat fungi of immense size and thickness, some so large as almost to occupy a space equal to the size of a pew, and from one to three inches thick. In other places fungi appeared growing with the ordinary dry-rot, some of an unusual shape, in form like a convolvulus, with stems from a quarter to half an inch in diameter. When first exposed the whole was of a beautiful buff color, and emitted the usual smell of the dry-rot fungus.

During a part of the time occupied in the repairs of the church the weather was very rainy. The arches of the vaults having been turned before the roof was slated, the rain-water saturated the partly decayed oak beams. The flooring and joists, composed of fresh timber, were laid on the vaulting before it was dry, coming in contact at the same time with the old oak timber, which was abundantly supplied with the seeds of decay, stimulated by moisture, the bad atmosphere of an ill-contrived burial-place, and afterward by heat from the stoves constantly in use. All these circumstances account satisfactorily for the extraordinary and rapid growth of the fungi.

The decayed state of a barn-floor attacked by rot is thus described by Mr. B. Johnson: "An oak barn-floor which had been laid twelve years began to shake upon the joists, and on examination was found to be quite rotten in various parts. The planks, two and a half inches in thickness, were nearly eaten through, except the outsides, which were glossy and without blemish. The rotten wood was partly in a state of snuff-colored, impalpable powder; other parts were black, and the rest clearly fungus. No earth was near the wood."

An indication of dry-rot in a damp pantry will be a coating of fine powder, like brick-dust, upon the shelves and earthenware, which consists of myriads of reddish spores shed by the dry-rot fungus. When these spores fall upon a wet surface, the red skin cracks at both ends, and fine filaments are sent out, which grow and ramify in all directions, and do their work of mischief with the timber of the closet.

Ventilation as a remedy for dry-rot in buildings is of doubtful service. If dry air be admitted in such a way as to absorb the moisture which sustains it, the fungus will of course be destroyed; but the trouble is that the circulating air will carry the spores along with it, and so spread the disease to unaffected parts. This is the great danger with dry-rot, while the wet-rot or ordinary decay is only communicated by actual contact. Another difficulty in ventilating for dry-rot arises from the fact that air, in passing through damp places, soon becomes humid, and loses its efficacy, or even does more harm than good. Intestinal decay is not reached by ventilation, for the air can not penetrate the spongeous exterior rottenness of timber so affected.

The temperature at which dry-rot proceeds most rapidly is 80° Fahr. At 90° it is slower, and at 100° slower still, and from 110° to 120° is generally arrested. Its progress is rapid at 50°, slow at 36°, and is arrested at 32°; but will return if the temperature is again raised to 50°. But in a constancy and equality of temperature timber will endure for ages. The wooden piles on which Venice and Amsterdam are founded remain sound because of the constancy of the conditions that surround them. Nothing is more destructive to wood than partial wetting. If it be kept always wet or always dry, and at a steady temperature, decay does not begin. It is recorded that a pile was drawn up sound from a bridge on the Danube that parted the Austrian and Turkish dominions, which had been under water fifteen hundred years. It has been remarked that the part of a ship which is constantly washed by bilge-water is never affected by dry-rot; and that the planking of a ship's bottom which is next the water remains sound for a long time, even when the inside is quite rotten.

As the decay of wood is chiefly due to the presence in it of sap, and as dry-rot can only thrive upon decaying timber, it is apparent that the best protection against both these evils is careful seasoning. When wood dries gradually in the air by the process of natural seasoning, it should be placed in a dry yard and sheltered from sun and wind. This method may be recommended for specimens of moderate thickness; and the time needed is two years for timber used in carpentry, which in this period loses one fifth of its weight. Four years are needed for timber that is to be used in joinery, in which time it will lose one third of its weight. It is important that timber be reduced to its proper size for use before seasoning; for, however dry it may become, when it is cut smaller it will shrink and lose weight. At first the seasoning should proceed slowly, and the pores upon the surface should remain open to permit the free evaporation of internal moisture. It should be set on bearers to admit a circulation of air all around it. The sleepers at the bottom of the pile should be perfectly level and solid; for timber bent in seasoning will retain the same form when dried. The time required for drying under cover is shorter than in the open air in the proportion of five to seven. Three years are required to season ship-timber; the timbers are shaped a year before they are formed, and then left a year in a skeleton shape to complete the seasoning.

Sappy timber that must be seasoned quickly, in cases where strength is not chiefly required, should be immersed in running water as soon as felled. It should be chained down beneath the surface, as partial immersion is very destructive. Boards placed end on at the head of a mill-race for two or three weeks, and then set upright in the air, and turned daily, are said to floor better than timber that has been years in dry seasoning. The longer wood has been under water the faster it dries. The process of water-seasoning is easily explained. Sap is denser than pure water, and it is inclosed in membrane. By osmotic action pure water takes the place of the sap and so renders the wood less liable to ferment. Again, pure water evaporates more readily than water which contains certain principles in solution, and hence water-soaked timber dries more rapidly. Timber steeped in water has some of its substance dissolved thereby. Boiling and steaming are said to prevent dry-rot by getting rid of spores and coagulating albumen.

  1. A Treatise on Dry-Rot in Timber. By Thomas Allen Britton. London and New York: E. & F. N. Spon.