A treatise on the origin, progress, prevention, and cure of dry rot in timber

by Thomas Allen Britton

SEASONING BY STEAMING AND BOILING, ETC.

For the purposes of joinery, steaming and boiling are very good methods, as the loss of elasticity and strength which they produce, and which are essential in carpentry, is compensated by the tendency to shrinkage being reduced; the durability also is said by some to be rather improved than otherwise, at least from steaming. If steaming be not carried on too quickly it will answer, but if it be pushed with too much vigour it is very apt to produce a permanent warping and distortion of the material. Oak of British growth may be seasoned by this process, as without this precaution it requires a long time to season. It has been ascertained, that of woods seasoned by these methods, those dried soonest that had been steamed; but the drying in either case should be somewhat gradual, and four hours are generally sufficient for the boiling or steaming process. The question of time will depend upon circumstances: some persons consider that one hour should be allowed for every inch in thickness. In some dockyards, salt water is used in the boilers, in others fresh, from considerations of convenience; and the fact is, plank boiled in salt water never gets rid of the salts that naturally enter the pores of the wood in boiling; and such being the case, the ship in which this plank is used is much more liable to the effects of damp than she would have been if the plank had been boiled in fresh water.

Boiling and steaming are likewise employed for softening woods, to facilitate the cutting as well as bending of them. Thus, in Taylor’s patent machines for making casks, the blocks intended for the staves are cut out of white Canada oak to the size of 30 inches by 5 inches and smaller. They are well steamed, and then sliced into pieces ½ inch or ⅝ inch thick, at the rate of 200 in each minute, by a process far more rapid and economical than sawing; the instrument being a revolving iron plate of 12 feet diameter, with two radial knives arranged somewhat like the irons of an ordinary plane or spokeshave.

How far steaming or boiling affects the durability of timber has not been satisfactorily ascertained; but it is said that the planks of a ship near the bows, which are bent by steaming, have never been observed to be affected with dry rot. With respect to boiling, Du Hamel’s opinion is not favourable as to its adding to the durability of timber; for when a piece of dry wood was immersed in boiling water, and afterwards dried in a stove, it not only lost the water it had imbibed, but also a part of its substance; and when the experiment was repeated with the same piece of wood, it lost more of its substance the second time than it did the first. Tredgold—no mean authority—considers that “boiled or steamed timber shrinks less, and stands better than that which is naturally seasoned.” Barlow is of opinion that “the seasoning goes on more rapidly after the piece is steamed than when boiled.”

At the close of the Crimean and Baltic campaigns the port of Cherbourg was almost completely cleared of staves sufficiently seasoned for making casks. The engineer at the head of the coopering department determined to boil in fresh water the newly-cut staves, and compare the time of their seasoning with that of other staves cut from the same forests, but not prepared; and the result was that after four or five months’ exposure to the atmosphere, the boiled staves were perfectly fit for working up, while to bring the others to the same point fifteen months were barely sufficient.

Steaming is understood to prevent dry rot. No doubt boiling and steaming partly remove the ferment spores, but may not destroy the vitality of those remaining. For, according to Milne-Edwards, on ‘Spontaneous Generation.’ he has seen tardigrades resist the prolonged action of a temperature of 248° Fahr., and has known them to survive a temperature of 284° Fahr. That low forms of vegetation are fully as tenacious of life cannot be doubted.

Boiling and steaming also coagulate the albumen at 140° Fahr. Although coagulated albumen is insoluble in water, the water solution is by this heating process sealed up in the wood, and the cohesion of the latter is said to be diminished.

The first essays in the art of drying wood artificially carry us back to a period now tolerably remote. Wollaston and Fourcroy both recommended the drying of wood in ovens. Newmann, a German chemist, suggested another method, which has since been adopted in a somewhat different form, i. e. steaming the wood. Newmann placed the wood to be dried in a large wooden chest, taking care to leave spaces between the pieces, and then turned on the steam from a boiler provided for the purpose. The condensed steam, charged with albuminous matter taken up from the wood, or rather from its surface, was run off from time to time, and the process of the operation was judged by the colour of the water. When the latter was clear and colourless the chest was opened, and the wood withdrawn for use without further preparation. The process would have been useful enough if superheated steam, which would have dried the wood by absorbing the moisture, could have been used, but the cost of the process would doubtless have been too high to permit of its practical application.

In 1837, M. de Mecquenem devised a method of desiccation, in which the pieces of wood to be dried were placed in a closed chamber, and subjected to a current of hot air, heated for the purpose by a special apparatus, and driven by a blower. The air entered by apertures in the lower part of the chambers, and escaped at the top laden with the moisture absorbed from the wood.

In 1839, M. Charpentier obtained a brévet d’invention for a process of drying wood in hermetically-closed chambers. The wood was subjected to the action of air heated by contact with metal plates covering the flue of a coke furnace. This air entered by conduits on the level of the floor of the chamber, and escaped at the top through apertures leading into the chimney of the furnace.

In the same year, M. Saint Preuve invented a process for forcing steam into pores of the wood, and, by condensation of this steam in the pores, sucking in a preservative preparation.

In 1847, MM. Brochard and Watteau’s process was introduced. It consists simply of filling the cylinder with steam, and making a vacuum by forcing in a cold solution of salt, &c.

The plan which has been for some years in use in England is the injection, by means of a ventilator, of hot air into the drying stove where the wood is placed: by this the temperature is gently and gradually raised until it reaches boiling heat. But, as wood is one of the worst conductors known of caloric, if this plan is applied to large logs, the interior fibres still retain their original bulk, while those near the surface have a tendency to shrink; the consequence of which would be cracks and splits of more or less depth.

Timber may be dried by passing rapid currents of heated air through it under pressure. This plan was carried out with the timber used for the floorings of the Coal Exchange, London. The wood was taken in its natural state, and in less than ten days it was thoroughly seasoned. In some cases, from 10 to 48 per cent. of moisture was taken out of the wood, and although the floorings have now been down a great many years, it is stated that very little shrinkage has been found, except in the case of a few pieces which were put down in the latter portion of the work, and which had not been submitted to the seasoning process.

The process of desiccation, patented by Messrs. Davison and Symington, in 1844, is of great practical value in reducing the time requisite for seasoning timber. It is peculiarly applicable to the seasoning of flooring boards and of the wood used in joiners’ work. Care must be exercised when removing the timber from the stove to the building in which it is to be used, that it be not exposed to the wet, nor even to a damp atmosphere for any lengthened period. The advantage of this process over the ordinary stoving consists in the temperature never being so high as to scorch the wood, by which the strength of the fibres would be injured; and in the facility for removing the vapour as fast as it is expelled from the wood, in consequence of the air being propelled through the stove at any required velocity and temperature. As compared with furnace and steam-stoving ordinarily employed to desiccate woods, the great superiority of this process is established by its seasoning the wood quite as rapidly, but much more thoroughly; and instead of wood being rendered brittle, as it is to some extent by stoving, this mode does not reduce the strength and tenacity of the wood. The principle of the invention is propelled currents of heated air; but the heat has to be regulated according to the texture of the various woods. Honduras mahogany might be exposed to a heat of 300°, and the whole of the moisture can be taken out in three days. Timber 9 inches square is considered by Mr. Davison a proper size for his invention. This process is described as “A method or methods of drying, seasoning, and hardening wood, and other articles, parts of which are also applicable to the desiccation of vegetable substances generally.” The first or principal part of the invention consists in drying, seasoning, and hardening wood and other articles—among which other articles are included generally all things made of wood, or chiefly of wood—by means, as has been stated, of rapid currents of heated air. The manner in which these currents of heated air are produced, is by an apparatus consisting of a furnace and a series of pipes withinside of a core of brickwork. On each side of the furnace, on a level with the fire-bars, is a horizontal tube; communicating and springing from these tubes are a series of eighteen tubes placed vertically and parallel to each other over the furnace. The outer end of one of the horizontal tubes communicates with a fan or other impelling apparatus for driving a constant stream of atmospheric air through the tubes. As the air passes through the tubes it becomes heated at a high temperature, and rushes out at the farther end of the other horizontal tube, and is thus conveyed to the place where it is applied. The materials to be subjected to the heated currents, such as logs, deals, &c., by outward application, must be placed in closed chambers, galleries, vaults, or flues, which are to be of any suitable form or magnitude; but it is recommended that they should be made of fire-brick, and have double doors or shutters for introducing or removing the wood. Honourable Mention was made of Messrs. Davison and Symington’s process of desiccation, by the jury, Class IV., Exhibition of 1851, England.

Some amusing instances are related of the efficiency of Davison and Symington’s process. Thus, a violin had been in the owner’s possession for upwards of sixteen years; how old it was when he first had it is not known. Upon being exposed to this process it lost, in eight hours, no less than five-sixths (nearly five and two-thirds) per cent. of its own weight. This there is every reason to believe was owing to the blocks glued inside, for the purpose of holding the more slender parts together. A violin maker of high reputation, having an order to make an instrument for one of the first violinists of the day, was requested to have the wood seasoned by this process; only three days were allowed for the experiment, in which the wood was seasoned and sent home. The two heaviest pieces were reduced in weight 2½ lbs. It is ascertained that, by this means of drying, the effect of age has been given to the instrument made from the above wood, and it was, in 1848, first fiddle in the orchestra of Her Majesty’s Theatre, London. The wood had been in the possession of its owners for eight years, and it was sent from Switzerland, in the first instance, as dry wood.

In proof of the value of this invention for the manufacture and cleansing of brewers’ casks, it was stated, in 1848, that since its adoption at Trueman’s brewery, Spitalfields, a saving of 300 tons of coals has been effected annually.

Flues or chambers for the heated air may be constructed in parallel lines, either in the floors or upright walls of a building, having narrow openings through which the heated air may issue in thin streams, and spread itself over the surface of the wood. If the openings are in the floor, the wood will require to be placed in an upright position; but if admitted in a horizontal direction, standards and skeleton shelves will be necessary to lay it upon. The great object, in all cases, is to bring the heated air as speedily as possible into contact with the wood, and to allow it, after it has done its office, to pass away as speedily.

Furnaces and apparatus for the production of rapid currents of heated air may be erected to prepare any quantity of timber or articles of wood at one time, but care should be taken that whatever the size of the outlet may be from the series of pipes or vessels by which the heat is generated, an outlet of at least equal dimensions is left for the free exit of the air and the vapours thrown off. It should also be observed, in constructing the open space in the floor or upright walls for the stream of heated air to pass towards the timber, that the superficial area of the whole of them combined does not exceed the dimensions of the principal outlet of the pipes at the extremity of the furnace, so that a free current of heated air may be allowed to pass uniformly throughout the chambers containing the wood to be prepared. The temperature proper to be given to the air, and velocity to the current in each case, will depend on the size, density, and maturity of the wood to be acted upon. The inventors found by their experiments that wood generally may be advantageously subjected to currents of air raised to a temperature of 400° Fahr., when the currents are impelled at the rate of 100 feet per second. But when the wood is in a green state, it is better to commence at a lower temperature, say from 150° to 200°, and gradually raise it to the high degree before stated, as the desiccation proceeds, an object which may, in some cases, be facilitated by carrying a cold-air drain from the fanner or other propelling apparatus, and attaching a damper to it, so that any quantity of cold air required to reduce the temperature of the hot current may, from time to time, be admitted. When, again, the wood is in the log or unconverted state, it should be bored or augured out in the centre, and the current of hot air caused to traverse it as well interiorly as exteriorly, whereby much time will be saved in the process of desiccation, and a more uniform result obtained.

Woods treated in this manner, and with the above modifications when requisite, part rapidly with their natural sap and any other aqueous matter which they may contain, and the fibres are brought closer together.

With respect to the time required to season the wood upon this plan, much must depend upon the original state of dryness it may be in, as well as the quality and temperature of the heated air forced into contact with it. It may suffice to remark that the wood may safely remain thus exposed till any escape of moisture ceases to be perceptible. This may be readily known, either by applying a mirror or any polished surface to the outlet, or by calculating the quantity of moisture removed from the wood, which will be found to range between ¼ and ⅟12th of its whole weight. For the purpose of ascertaining more correctly the amount of moisture removed from time to time, when the wood is placed in seasoning chambers as already described, an opening should be constructed in the chamber, in any convenient position, through which a specimen of the wood may be withdrawn and weighed.

Between 1848 and 1853, Mr. Bethell, who had paid much attention to the subject, obtained several patents, both in England and France, for stoves for drying wood. In his English patent of 1848, and the subsequent French one of 1853, we find a description of a peculiar kind of stove, on the following plan:

It consisted of a rectangular chamber formed of three walls and vaulted over, the whole in brickwork, with a certain thickness of slag in the centre, to prevent loss of heat. One extremity of the chamber was open to admit of the introduction of the wood by means of a truck running upon longitudinal iron rails. The opening was closed with a double door when the chamber was full. On the exterior of the opposite end of the chamber was a furnace to burn coal, coke, wood, or tar, according as it was desired to dry the wood simply, or, in the words of the inventor, to smoke it, i. e., to impregnate it with the antiseptic gaseous matters evolved in the imperfect combustion of certain tarry substances. The heated air or smoke entered through a flue running along the floor and branching at the end, and it escaped, or was pumped out, at the top of the vaults. Bethell considered that the interior of the chamber should be kept at a temperature of 110° Fahr., and that the duration of the process should be regulated by the condition of the wood. His experiments showed that this time varied from eight to twelve hours, the rapidity being attained at the cost of a relatively large expenditure of fuel. In point of fact, the draught was too great to permit of the utilization of the full amount of heat contained in the gaseous matter, which escaped at a temperature very little below that at which it entered. The heat produced by the fuel was badly utilized, and it is open to question whether, under any circumstances, large pieces of wood, such as sleepers, could be dried in so short a time as eight or twelve hours. The drying could only be effected by the use of a very high degree of temperature, tending to split the wood and weaken its strength. This view was confirmed by the results obtained in a long series of experiments made, in 1852-3, by an English manufacturing company, known as the Desiccating Company. A low temperature, and long continuance of the drying process, appear to be the conditions essential to the success of artificial desiccation, particularly with wood intended for cabinet-making, turning, joinery, ornamental work, &c., in which it is desirable, as far as possible, to prevent splitting, warping, and other changes of structure in the material. These results, it would seem, were not secured by the arrangements above described.

Some years since, a stove was constructed for Messrs. S. and J. Holme, very extensive builders at Liverpool, for the purpose of drying timber for floors, and other fittings of houses, &c., by the application of Messrs. Price and Manby’s patent warming apparatus; the want of seasoned timber, with the great number of men they employed, being a serious inconvenience and loss. In their large undertakings Messrs. Holme found a difficulty in keeping a stock of dry timber. The dimensions of the stove in which the timber was to be dried was 43 feet long, 11 feet wide, and 17 feet 6 inches high, and the cost of the apparatus was about 150l. It was calculated to hold about 30,000 superficial feet of 1-inch boards, which, upon the steam-pipe system, occupied full three weeks in drying. This apparatus of Messrs. Price and Manby, with rather less fuel, was considered to thoroughly dry each stove-full in ten days, thus saving a consumption of ten days’ fuel, independent of the advantages of expediting business. The average temperature was 104°, and as the continuous stream of pure air passing between the metallic plates was divested of its moisture, it carried off the dampness of the timber in an imperceptible manner. An experiment was tried, by having a flooring batten, 7 inches by 1¼ inch, cut from a piece of timber which had been floated, and was as full of water as it could be, placed in the stove; and when the temperature was 102°, it remained there five days, and when sawn down into ⅝ inch thick, and planed, it was found to be perfectly dry throughout. The heat was so gentle, and the evaporation so equal, that the timber was never rent, as when exposed to the air and a hot sun: in short, Messrs. Holme considered it one of the most perfect timber stoves that had been made.

It may be remarked with respect to desiccation, that the timber to be artificially dried is generally exposed to a great heat for a short time, rather than to a moderate heat for a lengthened one; and the air, saturated with the vapour thus produced, is generally very imperfectly removed. Wood so treated is almost sure to split, from the unequal contraction to which it is exposed; and the pores are also very liable to reopen on the wood being withdrawn from the stove, because there is no gradual and permanent change in their mechanical structure. It is only within the last few years past that the artificial desiccation of wood, before its impregnation with an antiseptic preparation in closed vessels, has been frequently adopted in practice.

We cannot give a better termination to the few remarks we have made about “steaming and boiling timber,” than by quoting the opinion of the late Sir Charles Barry, R.A., architect to the new Houses of Parliament, which we propose doing in the following manner:

“York Road, Lambeth, Nov. 30, 1844.

Sir,

In reply to your application, we beg to acquaint you that we are willing to undertake the ordinary works required in the finishings of the new Palace of Westminster. … The wainscot to be used in the joiner’s work is assumed to be from the best Crown Riga wainscot in the logs, and from pipe-staves of the best quality, in equal proportions, to be prepared for use by steaming, or otherwise.…

Grissell & Peto.”

Charles Barry, Esq.

Sir Charles Barry recommended this tender to the Treasury for acceptance; but we fancy that he was doubtful about the efficacy of steaming, as we think will appear from the following extract from an “Agreement between Sir Charles Barry and Messrs. Grissell and Peto, builders:

“First. That the wainscot is assumed to be from the log and pipe-staves in equal quantities; the prime cost of which, in inch boards, seasoned by steam, or other artificial means, so as to be fit for use, is calculated at 6½d. per foot superficial.

“Secondly. That if it should be found necessary to make use of thoroughly dry wainscot boards for the whole or any portion of the joiner’s work, seasoned by natural means (viz. exposure to the atmosphere), the prime cost of such boards, with the addition of a profit of 7½ per cent., is to be allowed for them, over and above the price of 6½d. per foot superficial, the prime cost of wainscot boards provided for in the contract, as above stated.” (Italics are our own.)