CHAPTER VI.
WORKING OUT THE BACK.
Passing on, I draw your attention to the working out of the back.
I show you, fig. 10, what must be drawn on the back and belly (on the flat, of course) before a chisel touches the wood for excavation. The blocks at either end speak for themselves, they having been fashioned to shape out of Swiss pine, and planed and squared so as to be glued square where you see them marked, later on. And be sure they stand one-and-a-half inches high in the rough, for a reason I will give you later, and about five-eighths of an inch thick, to about the breadth you see on fig. 10.
| PLATE X. |
Before, however, you can do anything in hollowing out the back, you will have to provide yourselves with a bed in which your table must firmly rest while you do so. Therefore, purchase a block of dry beech or birch, about one-and-a-half inches thick, sixteen inches long by eleven inches wide, and lay your finished back in the centre of it, tracing the whole outline, button as well, distinctly thereon; and having done so, cut by the outline inside all round to the depth of about one quarter of an inch, and from this basis proceed to make, as nearly as possible, a counterpart of the model of your back, but reversed, of course. And get all the tool ridges well levelled with rough to fine sandpaper; and, when you lay in your table for cutting, place a strong piece of brown paper for it to rest upon, not only to prevent it in any way scratching the fine surface of your wood obtained at so much trouble, but it enables you to shake off it quickly any residue of coarse dust or small cuttings that will creep under the wood upon which you are working; and so you get on rapidly and cleanly.
You will notice that I have again drawn the guitar line, and at a distance from the outline, so that a sufficiently flat surface is allowed for the ribs to rest firmly upon later. And I cut all round this line just as an indication, merely as a starting point from which to work more deeply all over, until I arrive at a point when the calipers, [No. 34], test the thicknesses roughly—which is by the way. For I have first to cut three cross channels, at the upper, middle, and lower nodal points, fig. 11, at such a depth that I caliper good three-sixteenths of an inch at the centre of middle groove, one-eighth of an inch upper and lower, falling away very little to all edges for the present. And I draw a distinct pencil mark through each groove, so that I must be a poor workman indeed if I go through the wood through these bars, as I have known some novices to do, or cannot gauge pretty well all over by their aid before using the calipers.
| PLATE XI. |
But you who, when beginning to cut out back or belly, having no sort of experience whatever, must use every care possible, and keep calipers [No. 3] (double) going constantly, as, with their aid, you will at once see by the outside half the thickness registered at the inner; then you can pare away with gouges, small and large, and with spoke shaves, [Nos. 48 and 53], until you get a fairly sloping and somewhat level surface from bare three-sixteenths of an inch in centre to full three-thirty-seconds of an inch all round by the edges, as shown by calipers.
And thus I come to the first rough thicknesses of the back; and I damp the surface all over as I did the outside, and dry it carefully; for you will understand the necessity for this carefulness, there being some fear of slight warping from the true flatness now the wood is thin all over, if quick, artificial heat be adopted to draw the moisture.
Whilst this natural process is going on, you must take the large calipers and open them at three-sixteenths of an inch. Then hold the plate in the extended left hand by middle bout, inside upwards, calipers and a long lead pencil together in the other; and, beginning at the centre of the plate, draw the calipers carefully from this starting-point all over the rough surface, gauging with your eye for the present any irregularities of said surface; for I want you to mark every part where the points stick, first within a radius of three inches, gradually extending your field of operations, slightly tightening the calipers as you get farther away from your centre, until the edges are finally reached, when you use the double calipers, [No. 3], to ascertain the exact thickness at those places.
This being done, and the places marked levelled down, using spoke shaves, flat gouge [No. 50], and rough sandpaper 3, take again the large calipers and go over the whole as before, but more carefully; and do this time after time, until the plate is accurately gauged from five-thirty-seconds of an inch centre to the diminution of about a good sixteenth—say one-twelfth of an inch at the edges. My way of working has always been thus, in preference to using what people call "indicating calipers"; and my advice to you is, do likewise, for you not only get over your ground more nimbly, but you can get from your centre more accurately, I maintain, gradation of thicknesses. I give you what I have proved the best thicknesses for my backs, and am pleased to do so to all the world; but if you care to try a hair or two thinner in the centre, adding those hairs to the edges, do so; you will not lose in energy, but you will in timbre, a trifle.
Before finally quitting this hollowing out of the back, gauge for the last time, then use fine sandpaper, and leave no mark of any tool whatever, as by clean work you will be judged.
This question of thicknesses is an important one, but applies more to the belly than the back; and I shall have more to say on this head when I get to that soundboard, merely adding now that the back must never be weak in wood, yet, at the same time, never so strong that a woody tone is the result, inevitable, as the timbre quality is scarcely developed, and without that I never care for it.
It is desirable at this stage that I point out to you how the inner edges of the back are rounded before the ribs are fixed. I use file [No. 6], half round, flat side to the wood first, turning to the round side for finish. When at the corners, I employ knife [No. 8] in cutting where the file would not do it so well in the early stage, and this file not at all nicely for finish, so I employ a smaller one, [No. 9], to these corners, the other all over the rest of the wood, cleanly doing the work so that about one-sixteenth only of the inner edge is rounded off. Then No. 1 sandpaper is used to finish off the work done, and the next stage is glueing on the end blocks, preparatory to fixing the ribs as they get made—of which, later.
So, for the present, I leave the back, and take up the wood you will remember I selected for the front table, or belly, and devote to it a separate section; merely adding that in the course of my work I have so arranged all the thicknesses of the back that it answers to the tone C, which do not forget, as I shall have again to refer to it.