Another operation may be explained here, although it is not generally used during the period from 1685 to 1700, that of shading or sand-burning (see Fig. 354).
The pieces to be shaded are held with a pair of tweezers and dipped into a bowl or pan containing very hot silver sand. The usual plan is to have a flat iron tray filled with the sand and placed over a small gas-ring. The corners only of the pieces are dipped, as where the wood touches the sand it burns to a deep brown, and from this point shades away, gradually, into the colour of the wood itself. It is obvious that only light-coloured woods can be shaded in this manner.
Fig. 355. Laburnum " Oyster-Piece." - (Cut transversely from sapling.)
The laying of the marqueterie veneer is the work of the cabinet-maker, not of the marqueterie cutter. The surface to be veneered is planed, scraped and finished perfectly smooth and level, and is then roughened with the " toothing plane " to afford a key. The underside of the veneer is toothed in like manner. The edge of the iron of a toothing plane is slightly serrated, and it is pitched nearly upright instead of at the usual angle. Its action, therefore, is rather that of a scraper than of a plane.
Having prepared the wood, the panel or flat surface is brushed over with hot glue of proper consistency, the glue being allowed to become quite cold. The marqueterie veneer is then placed on its surface, paper side uppermost, and secured, to prevent slipping, by headless " veneer " pins, which are allowed to project above the surface about two-thirds of their length. A flat piece of wood, slightly larger than the panel, technically known as a " caul," and made from soft wood such as pine, is then made very hot and placed on the veneer, handscrews being quickly applied to squeeze the whole together with great force. It is obvious that the contact surface of the caul must be level or the pressure on the veneer will not be exerted equally, and subsequent blistering will result.1 The pressure of the handscrews should be applied to the centre first, to drive the glue outwards. The heat from the caul penetrates through the veneer direction of the veneered side, but when hot glue is used, and the veneer applied immediately, this pull will be excessive.
1 To make sure that the pressure shall be from the centre to the edges, and thus to drive out the superfluous glue, a caul is usually made slightly convex on its under surface. A good plan is to place a piece of felt between the caul and the work, as this ensures perfect contact everywhere, by taking up any surface irregularities. To prevent this felt from sticking to the work, should any glue exude through the cutting of the ornament itself, it is usual to rub it with soap.
This casting or cracking of surfaces which have been veneered in the wrong way is a more important point than would appear, at first glance. Where original marqueterie is found with the veneer split in this way, it is strong presumptive evidence that it must have been laid by a workman unacquainted with the technicalities of his craft. When the heat is applied by the agency of the caul only, the veneer is secured by the hand-screws before it can penetrate through to the glue beneath, and there is no expansion, with subsequent contraction, and the work should stand. A modern innovation is the veneering press, where panels are placed between two large metal plates and squeezed together by powerful screws. A number of gas-jets placed underneath keeps the plates and the work hot until the pressure has been applied, after which the gas is turned off and the work allowed to cool. With large panels, it is important that the pressure should be applied to the centres first and to the edges afterwards, otherwise the liquefied glue will be imprisoned in the centre and will be unable to escape from the edges. It must be remembered that glue, even when quite cold, is still soft for a very long time after the panel has been veneered, and as the air is more or less kept from it by the panel on the one side and the veneer on the other, the process of hardening is a very gradual one. In this action of " setting," glue contracts very much, and it is necessary, therefore, that the layer of glue between the panel and the veneer should be squeezed out as thinly as possible. The excess, if allowed to remain, will make an imperfect joint, and cause blisters and bubbles. These blisters are very troublesome to rectify. They cannot be reduced by another application of the hot caul, as a certain amount of air takes the place of the contracted glue, which cannot be squeezed out, as it has no escaping vent. It is necessary to prick the surface, so as to allow the imprisoned air to escape before attempting to reduce blisters by the caul or the hammer.
A veneering hammer is in form like a blunted axe, with the head fixed at right angles to the handle instead of in a line with it. The hammer is rarely used for large flat surfaces, as the caul is much more convenient and certain, but for shaped surfaces, especially where there is curvature both ways, as in a "bombe" front, it is indispensable. The veneer to be applied is soaked with hot size, or glue, on each side, both to render it pliable and to facilitate the action of the hammer. The glue must be thin and very hot. The veneer being laid, the hammer is used, from the centre outwards, with a pressure applied with a circular action, the aim being to work the excess glue from the centre to the outside edges. An accurate knowledge of the correct consistencies of the glue, for both the hammer and the caul methods, is indispensable.