| FIG. 34. | FIG. 35. |
After roughly shaping the head comes the delicate operation of "setting." This is also known as putting in the "spring" or cambre. The principle upon which the amount of curve is determined is that an imaginary straight line drawn from the face of the head to the face of the nut shall coincide with the stick at the point of its greatest deviation from the horizontal. There is no fixed distance from either end for this extreme point of deviation to occur. It is a matter that rests entirely on the judgment of the maker, who, if thoroughly experienced, regulates the curve by any variation in rigidity he may discover in the stick. Thus should his observations point to the fact that a certain portion of the stick is slightly weaker than the rest, there will he put the greatest amount of "spring." It must be understood, however, that a good maker never uses a stick that is palpably unequal. He will only take this trouble to correct infinitesimal weaknesses (discernible only to a hand of great experience) in wood of exceptionally good grain. It is astonishing how many violinists seem to think good bows are made by accident. Few know that there are some men who can make a fine bow.
The prime factor in the "setting" of a bow is heat, by the judicious application of which the straight rod is made to assume and retain the desired cambre. The heat used now-a-days is that produced by an ordinary gas flame. Dry heat is absolutely essential, as the slightest moisture draws all the pigmentary matter out of the cells in the wood and leaves the bow as colourless and mean in appearance as a stick of deal. As it is, with dry heat even, the amount of colour exuded by a good stick during this process is quite enough to stain the hands a deep purple.
The great point to be observed in "setting" a bow is to make sure that the fibres are all heated equally right through to the centre of the stick. If this does not receive sufficient attention the bow can not possibly retain its curve, for the inner fibres that have not been affected by the heat will always be trying to resume their original straight position, and are bound ultimately to overcome the resistance of the heated outer fibres, with the result that the bow either becomes straight or warped and twisted, most probably the latter. To understand that this must be so it is only necessary to remember that any elastic rod, a walking stick for example, can be held so as to form a curve but as soon as the pressure is released it immediately recovers its normal state. This is what happens with the unheated inner fibres in an inferior bow. The constant strife of opposing forces must result in victory for the active force of the inner fibres over the passive resistance of the heated outer fibres.
| FIG. 36. |
For the operation of "setting" the bow is left about half as thick again as the finished stick is intended to be: this to allow for scorching or burning the outer surface. When the "setting" is satisfactorily accomplished the stick is planed up round, after which the bottom trench is cut. This is the slot in which the screw-eye of the nut travels. Then the hole for the screw itself is drilled out in a lathe fitted with a "Cushman chuck." The next thing is to put on the "black face." This is a thin slab of ebony glued on to the under surface of the head, which helps to strengthen the head and forms a solid bed for the ivory or metal plate which forms the outer facing of the head. The ivory faces are cut out of the solid tusk to the shape shown in Fig. 36. They are glued on with the very best glue procurable and tied down with strong twine. This is another matter of extreme difficulty and delicacy, as ivory is a very stubborn material to work in and it is easy to crack it in forcing it down to the curve of the face, that is if it is sufficiently thick adequately to fulfil its original purpose as a strengthener and protector of the head. One often sees in cheap bows faces of ivory so thin as to show the ebony face through in a bluish tint. Such a face is of as much value to the bow as a piece of paper, but it was easy to put on!
Metal faces are growing more and more into favour but, personally, I prefer a substantial ivory face, for though the metal may be stronger in itself I think an ivory face well glued on is more homogeneous. The successive layers of ebony and ivory on the already hard wood forms a more equal gradation of density.
After both the faces are adjusted a circular hole is drilled in the head and then chiselled out square to form the top trench or box to receive the hair. The nut is then fitted. Many people imagine that even the best makers buy the nuts wholesale and fit the sticks to them, but good makers always make the nut for each bow as it is wanted. They can by this means better regulate the balance of the bow.