Where a mortise is to be cut in a timber bearing weight, it should be cut in the neutral axis, where the cutting of fibres will weaken it least.
In the mortise-and-tenon of a table-rail, Fig. 267, No. 43, there should be a wide shoulder above the tenon of the rail so that the top of the leg above the mortise will not shear out. The mortise should be as near the outside of the leg as possible so that the inner corner of the leg may remain strong. The tenon should be strong enough to share the strain with the shoulders.
A dado joint, Fig. 266, No. 25, should not be so deep as to weaken the supporting board.
A tenon should not be so large as to weaken the mortised piece.
Pins or other fastenings, Fig. 267, Nos. 38 and 39, may weaken rather than strengthen a joint if they are so placed or are so large as to shear or crush their way thru the timber.
9. _Place each abutting surface in a joint as nearly as possible perpendicular to the pressure which it has to transmit._
Illustrations of this principle are as follows: the angle in a strut joint, Fig. 266, No. 62, should be equally divided between the two beams.
The thrust joint, Fig. 268, No. 63, in a bridge truss, is exactly at right angles to the pressure.
It is on account of this principle that a spliced joint for compression, Fig. 264, No. 4, is different from a spliced joint for tension, No. 5; and that a housed braced joint, Fig. 269, No. 66, is better than a plain braced joint, No. 65.
A joint to resist vertical cross strain is stronger when scarfed vertically than horizontally.
Goss, p. 132. Adams, p. 12. Rivington, Vol. I, p. 57.
[Footnote *: For general bibliography see p. 4.]
The function of stains is to change the color, and to enchance the grain and texture of the wood. Stains may be divided into four general classes, which are not, however, entirely distinct. (1) Oil stains, (2) Water stains, (a) made from anilines, (b) made from dyes other than anilines, (3) Spirit stains, (4) Stains due to chemical changes.
(1) _Oil stains._ Advantages: they are easily prepared, are easy to apply evenly, and they do not raise the grain. Disadvantages: they cover the grain somewhat, are apt to give a muddy effect, they do not penetrate very deeply into the wood, and it is impossible to stain hard wood dark with them and at the same time keep the grain and texture of the wood clear. A convenient form in which to handle these pigments is Devoe's "coach colors," ground in japan. To prevent evaporation from cans once opened, it is well to keep them partly filled with water and the water covered with a little oil. For use, the pigments are thinned with turpentine or benzine, in the proportion of one pound of color to one-half gallon of turpentine or benzine. Benzine is much cheaper than turpentine, but evaporates more quickly. The addition of a little boiled oil gives a body to the stain, so that when the wood is well rubbed down a soft lustre can be had without any further finish. The stain should be applied with a brush to the wood, which may then be rubbed clean with cotton waste. Oil stains penetrate hard woods better when the wood has first been fumed in ammonia. (See below, p. 211). Or, the addition of a little ammonia to the stain just before applying aids it in penetrating the wood.