9. _Place each ab.u.t.ting surface in a joint as nearly as possible perpendicular to the pressure which it has to transmit._
Ill.u.s.trations 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.
THE PRINCIPLES OF JOINERY
REFERENCES:[*]
Goss, p. 132.
Adams, p. 12.
Rivington, Vol. I, p. 57.
[Footnote *: For general bibliography see p. 4.]
CHAPTER X.
WOOD FINISHING.
STAINS.
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 cla.s.ses, 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 j.a.pan. 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 l.u.s.tre 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.
The pigments most used for oil stains are: burnt and raw umber, burnt and raw sienna, Vand.y.k.e brown, drop black, and medium chrome yellow.
These colors may be varied by mixing. For example, for a green stain, take two parts of drop black and one part of medium chrome yellow, and dissolve in turpentine or benzine. The addition of a little vermilion gives a grayer green. The green may be made bluer by the addition of Prussian blue, but the blue already contained in the black gives a soft, pleasant green.
For antique oak, add a trifle of burnt umber and black to raw sienna thinned to the right consistency.
For a reddish brown, thin burnt umber to the right consistency. This may be grayed by the addition of a little green.
A walnut stain may be had by adding a little Venetian red to asphaltum, thinned with turpentine or benzine.
_Aniline oil stains._ Advantages: the colors are clear and easily obtainable. Disadvantages: the colors are likely to be crude and too bright, and unless great care is taken the tones are metallic and not soft enough to suit wood. It is necessary to purchase colors soluble in oil. These can be had of William Zinnser and Company, 197 William Street, New York. Four colors are necessary to get the desired shades, Bismarck brown, dark yellow, dark blue, and black. Bismarck brown comes in powdered form at $2.40 per lb., dark yellow comes in powdered form at $2.40 per lb., dark blue comes in lumps at $3.20 per lb., black comes in lumps at $2.40 per lb. These may be dissolved in three ounces of turpentine to one ounce of boiled oil, to one teaspoonful of color, a process that will take place much faster if the mixture is heated. Great care must be taken, however, not to set fire to the turpentine. When cool, thin with turpentine to the proper consistency, apply to the wood with a brush and rub clean with cotton waste.
(2) _Water Stains._ Advantages: they are cheap and clear and do not obscure the grain as oil stains are likely to do, and they penetrate deeply into the wood, especially when applied hot. They may be made of any coloring matter that is soluble in water, and are particularly good for hard woods and for use in large quant.i.ties. It is possible to stain wood much darker with them than with oil stains. Moreover, the brushes used with them are easily taken care of. Disadvantages: they are difficult to prepare and they raise the grain of the wood. The former disadvantage may be overcome by buying them all prepared.
The difficulty of the raising of the grain is to be obviated either by washing the wood in water and, when dry, rubbing down with sandpaper before applying the stain, or rubbing down after staining and re-staining when necessary.
a. Water stains made from anilines. Aniline stains are likely to fade, but the addition of a little vinegar is said to hinder fading. For Mahogany, dissolve 1 oz. Bismarck brown in 3 quarts of boiling water.
Use when cool.
b. Water stains made from dyes other than anilines. The number of these is legion; some of the simpler are given.
Reddish Brown. Dissolve extract of logwood of the size of a walnut in 1/2 cup (4 oz.) of hot water. Apply hot to wood repeatedly until desired color is obtained.
Black. Dissolve extract of logwood of the size of a walnut in 1/2 cup (4 oz.) of boiling water. Add a teaspoonful of alum. Apply repeatedly until the wood is dark brown. Prepare acetate of iron according to directions for making dark brown, on next page. Apply this to wood already browned with logwood. If the grain is raised, sandpaper lightly, or rub with steel wool and then with boiled oil.
(3) _Spirit Stains._ These are expensive and hence little used. A few ill.u.s.trations are given.[12]
[Footnote 12: For detailed directions for treatment of different woods, see Hodgson, pp. 112-153.]
Black. Aniline black, cut in alcohol, gives a bluish effect but if the wood thus stained is rubbed with raw linseed oil, it becomes black.
Another Black. Dissolve extract of logwood in wood alcohol. Develop the color by going over the work with tincture of muriate of iron.
Golden Oak. Dissolve asphaltum in naphtha until it is as thin as water and makes a yellowish stain; or to equal parts of asphaltum, varnish, and gold size j.a.pan, add enough turpentine to thin to proper consistency.
Mahogany. Dissolve Bismarck Brown in alcohol.
Aniline stains may be cut in alcohol and mixed with equal parts of white sh.e.l.lac and banana oil (amyl acetate) and all applied in one coat.
(4) _Stains due to chemical changes_. Certain substances like ammonia, pota.s.sium bichromate, and acetate of iron, give chemical reactions on certain woods and make very effective and inexpensive stains. Moreover the artistic effect of some of them is unexcelled. When applied in solution they are likely to raise the grain.
The effect of ammonia, either the liquid or fumes, is much the same as the effect produced by aging or weathering. Ammonia also cuts the pith rays of oak and makes it possible for other stains to take hold.
For this reason it is much used as a preliminary treatment for oak finishes. The color effect is to lessen the yellow and increase the gray.
The method of application is simply to expose the wood for a day or more to the fumes of strong ammonia (28%) in a tightly closed box. If the surface of the wood is moistened with water just before exposure, it turns darker than if exposed dry. The stain penetrates so deeply that it may be sandpapered after the exposure without harm. After fuming and sandpapering the surface should be oiled to prevent finger marks.
Dark brown for chestnut, or oak, or mahogany. This is obtained with a solution of acetate of iron, made as follows: digest one part by measure of iron dust in 8 parts of glacial acetic acid. After the chemical action is well started, add several times as much water to keep the mixture liquid. When the chemical action has ceased, the stain is ready for use. If a lighter shade is desired it may be still further diluted.
To darken mahogany. Make a saturate solution of bichromate of potash.
Dilute a portion of it with water 1/2, or 1/3, or 1/4, or in any proportion according to the darkness required. One part of the solution to two or three parts of water gives a good color. Apply the solution to mahogany with a brush. This solution alone is likely to be too brown. The reddish tinge of the wood may be saved by mixing as follows:
100% solution of bichromate of potash 1 part Breinig"s mahogany water stain 1 part Water 2 parts Apply with a brush and wipe off the surplus.
Bichromate of potash on oak gives a rich brown.
Bichromate of potash on ash gives a rich red.
Bichromate of potash on black walnut gives a dark brown.
A decoction of logwood treated with tannin gives yellow red, with sugar of lead gives gray brown, with ferric nitrate gives black. A decoction of fustic extract treated with dilute nitric acid gives brown, etc.[13]
[Footnote 13: For other effects obtained by chemical changes, see table on pp. 185-189 in _Brannt"s Painter, Gilder and Varnisher_, and also _Woodcraft_ 9: 71, June, "08.]