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.
[Illustration: 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.
[Illustration: 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.