This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
There are 2 ways of fixing the veneer, known as "hammering" and "cauling," alike in that they are both methods of applying pressure, but differing in that the former is accompanied by damp heat, the latter by dry.
In either case, the wood to be veneered must now be sized with thin glue; the ordinary glue-pot will supply this by dipping the brush first into the glue, then into the boiling water in the outer vessel. This size must be allowed to dry before the veneer is laid. Suppose now that veneering by the hammer process is about to commence. The glue is in good condition and boiling hot; the bench is cleared; a basin of hot water with the veneering hammer and a sponge in it is at hand, together with a cloth or two, and everything in such position that one will not interfere with or be in the way of another. Then : -
(1) Damp with hot water that side of the veneer which is not to be glued, and glue the other side; (2) go over as quickly as possible the wood itself, previously toothed and sized: (3) bring the veneer rapidly to it, pressing it down with the outspread hands, and taking care that the edges of the veneer overlap a little all round; (4) grasp the veneering hammer close to the pane (shaking off the hot water from it) and the handle pointing away from you; wriggle it about, pressing it down stoutly, and squeezing the glue from the centre out at the edges. If it is a large piece of stuff which is to be veneered, the assistance of a hot iron will be wanted to make the glue liquid again after it has set; but do not let it dry the wood underneath it, or it will burn the glue and scorch the veneer, and ruin the work.
(5) Having got out all the glue possible, search the surface for blisters, which will at once be betrayed by the sound they give when tapped with the handle of the hammer; the hot iron (or the inner vessel of the glue-pot itself, which often answers the purpose) must be applied, and the process with the hammer repeated. When the hammer is not in the hand, it should be in the hot water. The whole may now be sponged over with hot water, and wiped as dry as can be. And observe, throughout the above process never have any slop and wet about the work that you can avoid. Whenever you use the sponge, squeeze it well first. Damp and heat are wanted, not wet and heat. It is a good thing to have the sponge in the left hand nearly all the time, ready to take up any moisture or squeezed-out glue from the front of the hammer.
The veneering "hammer" resembles an ordinary hammer in little but its shape, the manner of using it being altogether different. The form of the "hammer" too presents some variety. In Fig. 685, A is what may be termed the " shop " style of veneering hammer-head, while B, C are such as may be made by the operator himself. The form A can be purchased at a dealer's and fitted with a wooden shaft. The form B is made in the following manner: a handle a, 12 in. long and 1 in. thick, is inserted in a hole bored in the centre of a piece of hard wood b, 6 in. sq. and 1 in. thick, in the bottom edge of which a slit about 1 in. deep is cut with a thick saw, and into this slit is fitted a piece of iron or steel plate c, 6 in. long and 2 in. wide, secured by a couple of rivets.
This done, the corners of the top and bottom edges of the wood b, and the edge of the plate c are nicely rounded and smoothed. The construction of C is evident from the illustration; a is the handle; b, the head. The hammer, of whichever shape, is employed as a squeezer for pressing out superfluous glue; it is therefore held by one hand grasping the handle and the other pressing on the head, and is moved forward with a zigzag motion, each end of the head advancing alternately in short sliding steps.
It may sometimes happen that when the veneer is laid a fault may be noticed which renders it necessary to remove and relay the veneer. This is difficult to do without damaging the veneer. The best plan is to first thoroughly clean the surface by hot damp sponging; then dry and warm it by a fire, and while hot rub in linseed oil; hold to the fire again till the oil has disappeared, and repeat the oiling and warming till the glue beneath is so weakened that the veneer can be gently stripped off. Both old glued surfaces are thoroughly cleansed and roughed by the toothing plane before relaying is attempted. The projecting edges of the veneer can be taken off by a sharp chisel or plane when the whole is quite dry and firm, which end is attained by placing the work under weights supported on an even surface, and leaving it in a warm room. The difficulty with hammer veneering is that the glue is not kept always sufficiently hot and that therefore it does not get properly squeezed out at the edges, and sometimes so much hot water has to be used in the operation that the veneer swells and shrinks to a degree that spoils the look of the work. Still, with care, it is quite feasible to lay flat veneers up to 5 ft. long and 18 in. wide with the hammer in a satisfactory manner.
The working of the hammer should always be from the centre outwards. The sponge and hot water, or the heated flat-iron, is applied when the glue sets, or an air bubble gets entrapped so as to form a " blister." To veneer a convex surface, it is only necessary to wet the veneer on one side, when it will curl up so as to fit a convex object; it should be held in place by binding round with some soft string.
In veneering with a caul, the process is identical with that already described as far as the glueing; the difference commences in the mode of applying pressure to ensure adhesion between the body and the veneer. Cauls are made either of well-seasoned pine or of rolled zinc plate, with a surface exactly the converse of the veneer to be pressed. Hence cauling, while superior to hammering, and in some cases indispensable, is much more-expensive, as, except in the case of small flat work, a new caul is required for every new outline presented by the various veneered articles. The substance of the caul, especially in the case of wood, should be thin enough to bend slightly under great pressure; and it should fit somewhat more closely at the centre than at the edges, so that, when pressure is applied, it will pass progressively from the centre outwards. The object of the caul is to remelt the glue which has been spread on the body and the veneer, for which purpose it is strongly heated before application; pressure is then applied in various ways to expel the superfluous glue and increase the intimacy of contact. Small cauls of 1-in. pine for flat work may be pressed by means of wooden hand-screws, applied at short intervals, commencing always in the centre.