The success of this so much depends upon the natural talent of the operator for matching colours and mixing of different gums for obtaining as close as possible resemblance to the surrounding work, that any hard and fast rules concerning it cannot be laid down.
An alcoholic mixture is almost invariably used for the covering of raw repairs, time scarcely ever being available for the use of an oil varnish.
The most commonly used basis is a lac varnish. The ease with which this is dissolved and manipulated is a temptation to use it at times when it would be the least desirable for the imitation of old varnish. One great fault in connection with it is its retaining a glare on the surface when hardened, and the undesirable aspect is given of polished spots where repairs have been going on. There is only one way of counteracting this—by mixing other gums or resins that have less or but little glare when hard. Those of a very astringent quality should be avoided, as when dry their pulling power or contraction is very great, and a cracked surface not at all like the rich fused appearance of many of the old masters, but dry and uninteresting, will make itself too evident.
By a carefully-calculated mixture of soft and hard gums, with a little transparent colouring matter when necessary, a very good if not highly successful imitation of the surrounding work can be accomplished, such as will prevent the repair "catching the eye" too soon, for it must do so eventually when hunted for. The density of colouring and thickness of the principal component parts must always be studied, as the same depth of tint by a very thin layer as that of a thick one will not have the same effect, and one or the other, when making a contrast with the adjacent old surface, will be conspicuous as a failure.
A few hints may be acceptable as to material and the management of it. We will assume, for instance, that a portion of the upper table of a rather deep brown-red old master has been repaired, and a slice of comparatively light coloured or new wood has been inserted as a necessity, the grain as a matter of course having been matched to the best of the ability of the repairer.
The first step taken will be that of putting a nice clean even surface over the fresh wood, and in such a manner, that on passing the hand or finger over it, no lumps, edges, or rough spots are felt. Having brushed the dust or powdered wood away, the colour of the wood will have to be lowered or subdued, otherwise the whiteness will obtrude itself and stare through any carefully selected varnish. This, for good effect, will be found advisable with the repairing of any old instrument.
The precautions to be taken at this stage are respecting the quality or disposition of the stain (as we may call it). The stains ordinarily sold for colouring wood are quite useless for present purposes, as they are absorbed between the threads, leaving these by contrast very light and the reverse of what is desirable.
A very weak solution in water of bichromate of potash will lower the colour sufficiently for most purposes, and when quite dry the wood will (having swelled with the moisture) require the fine glass-papering again, after which, the next stage can be proceeded with.
The solution of nitric acid in water should be avoided, as, although giving a fairly good lowering of the tint, it destroys the soft parts of the wood, and, further, causes an odour that may cause annoyance to the musician and suspicion to the expert.
Another solution may be recommended, that of the black liquorice, which is a transparent brown, and naturally hard. Judgment will have to be exercised in the management of either of the above solutions that they are not put on too heavily.