Fig. 694.

Decay and Preservation.—Timber decays fastest when alternately wet and dry, as in the piles of a wharf, fence-posts, and the like, or when subjected to a hot, moist, close atmosphere, as the sills and floor-timbers situated over some damp and unventilated cellar. Fig. 694 shows the decay caused by alternate wetness and dryness, while the parts above and below are still sound.

Wood lasts the best when kept dry and well ventilated. When kept constantly wet it is somewhat softened, and will not resist so much, but it does not decay. Recently, upon cutting a slab from the outside of a large log taken from the bed of a river, where it had lain for one hundred years or more, the interior proved as sound and clear as could be found in any lumber-yard. Undoubtedly, however, such long submersion lessens the elastic strength of timber after it is dried. That is not, however, an extreme example of durability. Wood has been taken from bogs and ancient lake-dwellings after being preserved for ages. Piles were taken from the Old London Bridge after about 650 years of service. Piles placed in the Rhine about 2000 years ago have been found quite sound during the present century; and piles are now regularly used, as you doubtless know, for the support of the most massive stone buildings and piers, but only where they are driven deep in the ground or below the low-water line. Many examples of the durability of wood kept dry are found in European structures. Timbers put into the roof of Westminster Abbey in the reign of Richard II. are still in place, and the roof-timbers of some of the older Italian churches remain in good condition.

Thorough seasoning, protection from the sun and rain, and the free circulation of air are the essentials to the preservation of timber.

Many preparations and chemical processes have been tried for the preservation of wood.

Creosote is one of the best preservatives known. Insects and fungi are repelled by its odour. The modern so-called "creosote stains" are excellent, not very expensive, and easily applied. They are only suitable for outside work, however, on account of the odour.

Coal-tar and wood-tar or pitch, applied hot in thin coats, are also good and cheap preservatives for exposed wood-work.

Charring the ends of fence-posts by holding them for a short time over a fire and forming a protecting coating of charcoal is another method which has been extensively used.

Oil paint will protect wood from moisture from without, and is the method most commonly in use.

In the case of any external coating, however, which interferes with the process of evaporation, as tar or paint, the wood must be thoroughly dry when it is applied, or the moisture within will be unable to escape, and will cause decay.

Lumber as well as the living tree has enemies in the form of insects and worms, but the conditions best for the preservation of the wood, as referred to above, are also the least favourable for the attacks of animal life and of fungi.

As soon as the tree has been felled and dies, decomposition begins, as in all organic bodies, and sooner or later will totally destroy the wood. The woody fibre itself will last for ages, but some of the substances involved in the growth soon decay. The sap is liable to fermentation, shown by a bluish tint, and decay sets in. Fungi are liable to fasten upon the wood. Worms and insects also attack it, preferring that which is richest in sap. Thus we see that the danger of decay originates chiefly in the decomposition of the sap (although in living trees past their prime decay begins in the heartwood while the sapwood is sound), so the more the sap can be got rid of the better. There are, however, some substances found in various trees, aside from those elements especially required for their growth, which render the wood more durable, like tannic acid, which abounds in oak and a number of trees, particularly in the bark. There is no advantage in getting rid of the turpentine and other volatile oils and the resinous deposits found in needle-leaved trees, particularly in the case of those woods in which they abound. Care should be taken, however, not to use a piece of pine badly streaked or spotted with resinous deposits in a place where it will be exposed, as the turpentine or resinous matter will be apt to ooze out and blister the paint.

Wet rot is a decay of the unseasoned wood, which may also be caused in seasoned wood by moisture with a temperate degree of warmth. It occurs in wood alternately exposed to dryness and moisture. Dry rot, which is due to fungi, does not attack dry wood, but is found where there is dampness and lack of free circulation of air, as in warm, damp, and unventilated situations, like cellars and the more confined parts of ships, and in time results in the entire crumbling away of the wood. There are several forms of dry rot. One of the most common and worst of dry-rot fungi attacks pine and fir. Fungi also attack oak. Creosote is used as a preventive, to the extent to which it saturates the wood.

Effects of Expansion and Contraction.—Cracks, curling, warping, winding, or twisting are due to nothing but irregular and uneven swelling and shrinking. Some kinds of wood shrink much in drying, others but little. Some, after seasoning, swell or shrink and curl and warp to a marked degree with every change in temperature and dryness. Others, once thoroughly air-seasoned, alter much less in shape or size under ordinary circumstances.

We have already seen that the heart side of a board tends to become convex in seasoning, owing to the shrinkage of the other side, and that if one part swells much more than another the wood becomes out of shape,—warped, curled, or twisted. If one part shrinks much faster than another, cracks usually result in the quicker shrinking portion. If you stick one end of a green board into the hot oven of the kitchen stove, the heated end will crack and split before the rest of the board has fairly begun to dry. We have seen illustrations of this in the seasoning process, as shown in Chapter III.

Exposure of one side of a seasoned piece to either dampness or heat will thus cause the piece to curl. The dampness swells the side affected or the heat shrinks it so that the convexity will be on the dampened side, or the concavity on the heated side, as the case may be.

If lumber were of perfectly uniform texture, hung up where it would be entirely unconfined and free to swell or shrink in all directions, and equally exposed all over the surface to exactly the same degrees and changes of heat and cold, dryness and moisture, it would simply grow larger or smaller without changing its form or shape. There would then be no curling, warping or winding. As a matter of fact, however, wood is not uniform in texture, but exceedingly varied, some pieces being extremely complex in structure; neither is it always free to expand and contract in every direction, nor equally exposed on all sides to the alternations of heat and cold, moisture and dryness.

To come to the practical application of these facts, we have seen (in Chapter III.) that boards for nice work should be planed down equally, as nearly as may be, from both sides; that the mere dressing off of the surface by hand will sometimes cause a board to warp badly; and that it is better to buy stock of as nearly the required thickness as possible, than to plane it down or split it. It should also be noted that when a board is being sawed in two or split lengthwise with a saw it sometimes springs together behind the saw with so much force that the crack has to be wedged open in order to continue sawing (Fig. 695). Sometimes the crack opens wider instead of closing (Fig. 696). You see from this that you cannot always be sure when you split a board that the parts will retain the shape they had in the original board. In working up large pieces into smaller ones, unexpected twists and crooks will often be found in the smaller pieces which did not exist in the original stock. Sometimes mahogany, for instance, will act in this way very markedly. Strips sawed off from a board, for example, will sometimes immediately spring into very crooked forms, as shown in Fig. 697 (which would not be exaggerated if the pieces were drawn of greater proportionate length).


Fig. 695.


Fig. 696.


Fig. 697.

Fig. 698.

In splitting stock flatwise, i.e., making two thinner boards out of a thick board or plank, a similar result often follows. The latent power set free, so to speak, by suddenly exposing the middle of a board, plank, or other timber to the atmosphere sometimes causes curious developments. It being necessary one day to split for a picture frame a large mahogany board, 1" thick by 2' square, with a circular hole already sawed from the centre, the pieces warped and twisted as the sawing went on (Fig. 698), until, just as they were nearly separated, the whole thing "went off" with a report like a toy pistol, breaking into a dozen pieces and scattering them around the shop.

In very crooked-grained wood you will frequently find uneven and undulating forms of warping and twisting that you do not find in straight-grained pieces, but such wood is often of the most beautiful figure for indoor work. Where the grain is crooked, cropping up to the surface as in Fig. 701, the cut-off ends of the fibrous structure, so to speak, are exposed in places to the atmosphere. These open ends, "end wood," thus brought to the surface are more susceptible to moisture and dryness than the sides of the bundles of fibrous tissue, which tends to produce unequal swelling, shrinking, and warping.

You will see if you look at the ends of logs and stumps that the heart is frequently not in the centre, in some cases taking such a devious course throughout the stem as to make the grain so crooked that no method of sawing will remove the tendency to warp or twist, just shown. Such trees may show a beautiful grain. Even in straight trees the pith is not usually quite straight, and is apt to take a somewhat zigzag course, due to the crooked way the tree grew when young (Fig. 699).

Fig. 699.

Imagine, for an exaggerated illustration, that you could see with X-rays the pith as crooked as Fig. 699. that shown in Fig. 700. Imagine that from this tree you could saw out the board indicated, keeping with it the whole pith or heart as if it were a wire rope woven in and out of the board, so that the appearance would be somewhat like that shown in Fig. 701. Bear in mind that the annual rings are layers of wood, so to speak, which may vary in thickness, growing around the heart. You will see that these layers, or rings, as they dip below or rise above the surface of the board, will cause the grain to form various patterns, perhaps somewhat as shown in Fig. 701, which makes no claim to accurately showing the grain in this case. In fact, all such variations of grain in lumber are due to the surface of the piece being at an angle with the layers.

Fig. 700.

Fig. 701.

In addition, the knots caused by branches, the twisting of the stems screw-fashion (as is seen in cedar), wounds, and other causes, often produce very crooked and tangled grain, and the wood of many broad-leaved trees is sometimes extremely complicated in texture, especially when all these irregularities occur in the same piece. It is the nature of some kinds of mahogany, from whatever cause, to have the fibres strangely interlaced or running in very different directions in layers which are quite near each other.

The warping, twisting, and cracking is obviated in many cases where it is objectionable (as in the wooden frames of machines, the tops of benches) by building up with a number of smaller pieces, of which you will often see illustrations. To do this to the best advantage, the pieces should be selected and put together so that, though the grain will run in the same direction lengthways, the annual rings at the ends will not run together as in a whole beam, but will be reversed or arranged in various combinations, so that the tendencies of the different parts to warp or twist will counteract each other. Instead of a single board, which would naturally become warped in one large curve, a number of strips can be glued up with the grain of the strips arranged in alternate fashion (Fig. 559), so that in place of one large curve the warping will merely result in a slightly wavy line.

Where but one side of a board is seen or used and where the full strength is not needed, warping and twisting can be largely prevented by lengthways saw-cuts on the back or under surface, as in a drawing-board, the crossways strength required being secured by the cleats. Doors and most forms of panelled work also illustrate these matters of swelling and shrinking (see Doors and Panels).

Fig. 702.

Shakes.—Heart-shakes are cracks radiating from the centre in the line of the medullary rays, widest at the pith and narrowing toward the outside, and supposed to be chiefly caused by the shrinkage of the older wood due to the beginning of decay while the tree is standing (Fig. 702). Slight heart-shakes are common, but if large and numerous or twisting in the length of the log, they injure the timber seriously for cutting up.

Fig. 703.

Star-shakes are also radiating cracks, but, unlike the heart-shakes, the cracks are widest at the outside, narrowing toward the centre (Fig. 703), and are often caused by the shrinkage of the outer part due to the outside of the tree drying faster than the inside, as it naturally does from being more exposed after being felled; but they are sometimes owing to the beginning of decay and other causes.

Fig. 704.

Cup-shakes are cracks between some of the annual rings, separating the layers more or less (Fig. 704), sometimes reaching entirely around, separating the centre from the outer portion, and are supposed to be caused by the swaying of the tree in the wind (hence sometimes known as wind-shakes), or to some shock or extreme changes of temperature, or other causes.

Combinations of the various shakes may be found in the same log.

A Few Suggestions about Working-Drawings.—Drawing is far too extensive a subject to be even briefly treated in a manual on wood-working, but a few general remarks on matters connected with working-drawings may be of help to some.

While an ordinary picture gives a correct idea of how an object looks, we cannot take accurate measurements from it. When we need dimensions, as in practical work, we must have some drawings which will show us at once the exact shapes, sizes, and positions of the various parts. In addition to the picture to give us the general idea, we have for working purposes what are called elevations, plans, sections, etc.

In such a case as that of the little house shown on page 242, the picture (Fig. 363) shows us the appearance of the building, but for purposes of construction, working-drawings should also be made. The view of what you would see if you stood directly in front of this house, with only the front visible, is shown in Fig. 364, and is called the front elevation. Stand opposite either side or end, and the view seen is represented in Fig. 364 as the side elevation. In the same manner the rear elevation is given. Next imagine yourself in the air directly above the house. This view is called the plan.[53] In this case, as the view of the interior is desired, the view is shown as if the roof were removed. If the sides or ends are not alike, as is sometimes the case, two side or end views may be needed. In the case illustrated, inside elevations are also given, to show the construction.

Elevations, whether one or several, must always be taken at right angles to the plan. Although commonly, in simple work, confined to representations of each side or end, they can be taken from any point of view that may be at right angles to the plan. They may be taken from the corners or at any angles that may best show any complicated details of the object. If the object is quite simple, one elevation and the plan, or two elevations without the plan, may be quite sufficient, as the elevation or plan omitted can in such cases be understood at once.

Always make your drawings full-sized when the object to be made is not too large. You are much less likely to make mistakes in taking your dimensions and measurements from a drawing the actual size of the object than where you have to take them from a smaller drawing, and you also can get a better idea from a full-sized drawing just how the object will look. It is a safe-guard, with a drawing which is symmetrical, to lay it out from a centre line, measuring to the right and left.

If you make a drawing of which each line is one half the length of the same line in the real object, it is called a "half-size" drawing, and is said to be drawn on a scale of 6" to the foot. If "one fourth size," the scale is 3" to the foot. The scale is often expressed as an equation, viz.: 2 in. = 1 ft., or ¼" = 1'.

If the drawing is not made with accuracy, it is necessary to put the dimensions upon it, and this is often done for convenience and quickness of execution in the case of drawings which are accurate.

Details inside of an object, that is, such parts as cannot be seen or properly shown in the elevations or plan, are often shown by dotted lines, as in Fig. 597. Sometimes dotted lines are used in the same way to show the back of an object, to save making extra drawings. Too many dotted lines, however, are confusing, so if the parts that do not show on the surface are not quite simple and cannot be clearly shown by dotted lines on the plan and elevations, it is usual to make another kind of drawing especially to show such details. This is called a "section" (Lat., sectio, from secare, to cut), and represents what would be shown if the object were cut apart or sawed through at the place where the view of the details is wanted. The surface supposed to be cut is usually indicated by parallel lines crossing the surface, independent parts, as those of different pieces, frequently being shown by changing the direction of the parallel lines, as in Fig. 504.

When both sides of an object are alike, labour and space are often saved by making a drawing of one side or one half only, from a centre line. The same way is sometimes adopted in making sections, and an elevation and section can sometimes be combined in this way in one drawing.

As soon as you become used to plans and elevations, you can by combining the plan and elevations in your mind quickly imagine the form of the object represented, and often, unless it is complicated, get fully as good a conception of it as from a picture, and a more accurate knowledge of its proportions and details, so that in many cases there is no need of having a picture at all in order to construct the object. It is often a convenience to have a picture, however, and frequently an assistance in forming a correct idea of something you have never seen. Where the appearance of the object is of consequence, as in the case of a house or bookcase, for instance, the picture is of the first consequence, for you must have a correct representation of the general appearance of the object before you begin to make the working-drawings. You will soon find that merely having an idea in your mind is not always sufficient from which to make working-drawings, although the first step in the process. You will often find that when the idea in your mind is put into the form of a picture, it does not look at all as you thought it would, and that if you had started at once on the working-drawings without first making a sketch or picture, the result would have been unsatisfactory and sometimes entirely impracticable.

Even making a sketch or picture that just expresses your idea will not always result in the completed object being just what you wish. Strange though it may seem, it is a fact, practically, that the completed object often looks quite different from what the sketch leads you to expect. That result, however, is something which cannot be helped, so you need not give it any attention, only do not be surprised if once in a while you find that what you have made is not just what you thought it would be. First make the best design you can, then accurate working-drawings, then work carefully by the drawings, and if the result is not always exactly what you expected, you can console yourself with the thought that your experience is only that of architects, designers, carpenters, and workmen in all lines, and that no one can foresee all the conditions by which a piece of projected work will be affected.

Oblique or parallel projections are often used, from which measurements can be made. Such projections are not true representations of the objects as they appear to the eye, but they are often used because readily understood and easily drawn. They often answer every purpose from a practical point of view. Figs. 120 and 344 are examples.

Another way of representing objects for practical purposes is that shown in Figs. 121 and 407, and known as "isometric[54] projection" or "isometric perspective." This method is incorrect so far as giving an accurate picture is concerned, for the object is always represented as being too large in the farther parts, because the inclined lines are drawn parallel instead of converging; but it is often very useful from a practical point of view, because by it all that is required can frequently be expressed in one drawing.

Isometric perspective will not readily give the correct dimensions except in the lines which are vertical or which slant either way at an angle of 30° with the horizontal,—i.e., you cannot take the other dimensions right off with a rule as from a plan, and therefore, so far as obtaining correct dimensions is concerned, it is practically not useful for other than rectangular objects; but so far as merely showing the general shape or conveying the idea of the form it can often be advantageously used in representing many objects containing curved lines. Isometric projection has the advantage of being easy of execution, and of being so pictorial that it is almost always easy to see what is meant.

A First-Class Bench.—The construction of the bench shown on page 101 is not difficult to understand, but considerable skill is required to make a really good one. The arrangement of the vise is shown in Fig. 705, which is an inverted view (as if looking up from underneath). The vise is kept parallel by the stout bars of hard wood, parallel to the screw, which slide through mortises cut in the front of the bench-top, and are further guided by the cleats screwed to the under side of the top, where it is thinner than at the front edge. In case of using such a vise where the bench-top is not so thick in front, the thickness can easily be made sufficient by screwing a stout cleat on the under side where the vise comes. In this cleat can be cut the mortises for the slide-bars. The end-vise or "tail-screw" shown in Fig. 143 involves rather more work, but slides upon a similar principle. Perhaps the best way for the amateur is to make the end-vise in the same way as the main vise, adding the movable stop.

Fig. 705.

There is no better way to make the front of this bench-top than to build it up of narrow boards on edge, planed true, and thoroughly glued and bolted together. The planing and truing can best be done by machine, however. If well put together, such a bench-top will defy changes of weather and will stand a great deal of hard usage. The back part of the top can be thinner, but can very well be built up if desired. An excellent way to fasten the frame of such a bench together is with bolts, by which the parts can be drawn to a firm bearing.

It is impossible to make such a bench too rigid. If so stiffly framed that it cannot change its shape, and if the top is carefully trued, you will have something which will be a great help to good work.

INDEX