Wood finishers demand a difference in the composition of fillers, paste and liquid, for open- and close-grained wood, respectively; but unfortunately they do not demand a difference between either kind in themselves, according to the kind of wood. Paste fillers are used indiscriminately for open-grained wood and liquid for close-grained wood.
To find the fillers best adapted for a certain wood, and to classify them in this respect will require a large amount of chemical work and practical experiments; but that it should be done is evidenced by the fact that both success and failure result from the use of the same filler on different varieties of wood. After once being classified (owing to the large number now on the market), they will not number nearly so many in the aggregate as might be supposed; as it will be found in many instances that two entirely different varieties of wood resemble each other more closely in their vascular formation and cell characteristics than do two other specimens of the same variety. It is a recognized fact that paste fillers whose base is starch or the like work better and give better results in certain instances, while those whose base is mineral matter seem to do better in other cases.
It is noticed that rosewood as a finishing veneer is obsolete. This is not because of its scarcity, but because it is so hard to finish without having been seasoned for a long time. In these days, manufacturers cannot wait. It takes longer for the sap of rosewood to become inactive, or in trade parlance to “die,” than any other wood. This is because it takes so long for the albumen in the sap to coagulate. Rosewood has always been a source of trouble to piano makers, on account of the action of the sap on the varnish. However, if this wood, previously to filling, was washed with a weak solution of phosphoric acid, and then with wood spirit, it might be more easily finished. The phosphoric acid would coagulate the albumen on the surface of the wood immediately, while alcohol would reduce it to an insoluble state. The idea here is to destroy the activity of the sap, on the same principle as sappy places and knot sap are destroyed by alcohol-shellac before being painted.
Oak is another wood which gives the painter trouble to finish. This may be accounted for as follows: Oak contains a sour acid principle called tannic acid. It is a very active property. Wood during the growing season contains more albumen; thus in the circulation of the sap a large quantity of soft matter is deposited on the lignin which lines the cells, which lignin, if it contains any acid matter, acts on the material of the filler. Tannic acid has a deleterious effect on some of the material of which a number of fillers are made. Starch and many gums are susceptible to its influence, making some of them quite soft. Oak, like most other timber cut at the season when the least sap is in circulation, is the more easily finished.
The vascular formation may, and no doubt has, something to do with wood finishing. Different species of wood differ materially in their vascular and cellular formation. Wood finishers recognize a difference in treatment of French burl walnut and the common American {776} variety. Circassian and Italian walnut, although of the same species, demand widely different treatment in finishing to get the best results.
The only way to find the best materials to use in certain cases is to study and experiment with that end in view. If, by aid of a microscope, a certain piece of wood shows the same cellular formation that another piece did which was successfully finished by a certain process, it may be regarded as safe to treat both alike. If observation on this line is indulged in, it will not take the finisher very long to learn just what treatment is best for the work in hand. How often it has been noticed in something of two parts, like a door, that the panels when finished will pit, run, or sag, while the sides will present a surface in every way desirable and vice versa. This is due to the difference in the cellular construction of the wood and to the cellulose, and cannot be otherwise for the parts have been seasoned the same time and treated exactly alike. The physiology of wood is imperfectly understood, but enough is known to warrant us in saying with a certainty that the chemicals in fillers do act upon the principles embodied in its formation.
Some tried formulas follow:
I.—Make a paste to fill the cracks as follows: Old furniture polish: Whiting, plaster of Paris, pumice stone, litharge, equal parts, Japan drier, boiled linseed oil, turpentine, coloring matter, of each a sufficient quantity.
Rub the solids intimately with a mixture of 1 part of the Japan, 2 parts of the linseed oil, and 3 parts of turpentine, coloring to suit with vandyke brown or sienna. Lay the filling on with a brush, let it set for about 20 minutes, and then rub off clean except where it is to remain. In 2 days it will be hard enough to polish. After the surface has been thus prepared, the application of a coat of first-class copal varnish is in order. It is recommended that the varnish be applied in a moderately warm room, as it is injured by becoming chilled in drying. To get the best results in varnishing, some skill and experience are required. The varnish must be kept in an evenly warm temperature, and put on neither too plentifully nor too gingerly. After a satisfactorily smooth and regular surface has been obtained, the polishing proper may be done. This may be accomplished by manual labor and dexterity, or by the application of a very thin, even coat of a very fine, transparent varnish.