Convex mirrors need only be mentioned because they are sometimes supposed to be old and not made now. This is a mistake, as they can be got from almost any glass house. They are not much used.
When measuring for glass, the size of the opening, or what is known as 'sight' size, should be carefully taken, as, of course, nothing smaller than this will do. If the plate has plain edges, i.e., unbevelled, it may be got large enough to fit closely within the rabbet, without leaving any open space round the edges. However, as this might run it above a full inch and no advantage is gained by having more than sufficient within the rabbet, measurement should should also be taken of the full or rabbet opening behind. For all practical purposes, a quarter of an inch hidden is all that is required, though, beyond measurement, there is no reason why it may not be considerably more. To prevent unpleasant reflection from the rough edges, these should always be blackened. A mixture of thin glue and gas black is as good as anything. The front of the rabbet, against which the face of the glass lies, should also be blackened with the same or a similar mixture. This blackening only refers to plain-edged silvered plates, for if they are bevelled it is not necessary always, though there can be no objection to its being done, and some make a practice of treating all silvered plates alike in this respect.
Measuring for bevelled plates requires more accuracy, for, as the cost of the bevelling depends on the width, it is folly to have more than sufficient hidden. I may just refer to the erroneous notion sometimes held that the more of the glass that is within the rabbet, the more secure the plate will be. If the glass lay flat on the wood there might be something in this, but on account of the bevel it can only come in contact with the arris of the rabbet edge. What then is enough? Well, the ordinary custom is to make a bevelled plate § in. longer and wider than sight size. This practically allows rather more than | in. behind the rabbet, the remaining 1/16 along each edge being to provide against contingencies of rough edges. When ordering bevelled plates it is always advisable to state whether sight or plate size is given in order to prevent mistakes. If ordered sight they will be supplied as stated, 3/8 in. larger than the measurements given. For any except rectangular straight-edged plates measurements alone are not sufficient; an exact template or pattern drawn on a piece of paper or board should be supplied. It can easily be made by putting the wood or paper in the rabbet and ruling round the edge of the opening. This line should be described as 'sight,' otherwise the cutter is very apt to cut the plate exactly to it.
Transparent plates are more convenient when they fit exactly to the rabbet opening, but still there is no use wasting the bevel. If the rabbet is too wide it is better to fill it up with wood neatly fastened in.
The usual way of fastening silvered plates in is by means of glued blocks. With plain-edged plates no special precautions are necessary. The frame or part being glazed is laid face downwards on the bench top - see that all tools, nails, etc, are removed from within the opening - and the glass placed. If it fits close, a few blocks glued above it will prevent it falling backwards, and also prevent the wood backing, which is put on afterwards, from coming in contact with it. When gluing blocks in, be careful that the glue does not get on the back of the plate. If any drops fall on it, they should be wiped up without delay, for if left to harden, they are very apt to pull the silvering beneath them away from the glass. The blocks themselves may be from two to three inches long, and should be placed at intervals, say, of six to twelve inches, according to the size of the plate. Fig. 155 shows a tightly fitting plate with block, which if the backing is sunk within the opening must be below the level of the back surface. If, as sometimes happens, it is not convenient to have the rabbet deeper than just sufficient to hold the plate, or the back board comes directly in contact with it, it is advisable to have a few sheets of soft paper or a sheet or two of flannel between them. With very small plates this is not necessary, though with large ones it is a safeguard against injury. With plates silvered with mercury, the flannel should not be omitted if there is the slightest chance of the blind-frame - the backboard - coming in contact with them.
It is, however, comparatively rarely that the glass entirely fills the rabbet, and then wedge-shaped blocks, as in Fig. 156, are most conveniently used. These, when firmly glued in at intervals all round, prevent the plate moving in any direction. As the blocks are not always fitted with the greatest nicety, for which there is no occasion, a wire nail is often driven through them slantingly into the frame. When this is done, care must be taken that the nail does not chip the edge of the glass.
Fig. 155. - Glass close fitting.
With bevelled plates more care must be taken to adjust them closely, so that an equal width of bevel shows all round in front and that the mitres are exactly in their corners, for nothing looks worse than to see these all awry, and a wider bevel along one edge than elsewhere. In order to fit them accurately, it is advisable to have the frame so supported that the fitter can look underneath and see how the glass lies. If it is not quite in place, do not be tempted to move it with a chisel or screwdriver or any tool in fact, used like a lever. If this is done, the edge of the glass, especially if it is a large and consequently heavy one, is very apt to be chipped. With small plates there is little or no risk; but whatever the size, it is better to adjust by slightly hitting the edge of the frame with the hand. This will cause the slight movements necessary to the plate. When it is satisfactorily placed, put the wood blocks in their places without glue. This will prevent the plate being moved as each one is glued. The wedge-shaped blocks are almost invariably used with bevelled mirrors, and if there is a wide rabbet the lower edges must be cut, as shown in Fig. 157.
Fig. 156. - Glass not close fitting.
Fig. 157. - Bevelled Glass blocked.
Transparent plates, whether plain or bevelled edged, of course cannot be fitted in with blocks. Putty may be and sometimes is used, but it is by no means a nice way of doing the work, even though it is coloured to match the wood. An altogether superior way is to fasten the glass in as if it were a wooden panel, viz., with beads, as shown in Figs. 145 and 146 (p. 190). As has been said, the glass should fit closely, but if it is a trifle loose it may be secured by thin strips, which will be hidden by the beads. If, however, the space is too wide to allow of this, as may easily be the case with bevels, the best way is to neatly fill the rabbet by gluing a stop along each side and then fastening the bead on this, as in Fig. 158.
When fitting a plate, it should always be so arranged that any flaws may be as inconspicuous as possible. By a little judicious management of this sort they may often be quite undistinguishable when the glass is fixed up. It is, therefore, well to see what can be done with any plate before condemning it. Thus we may suppose an overmantel plate with some defect near one end. If this were placed at the bottom, the faults could hardly escape observation, but at the top they may be out of sight.
Glass is also sometimes used for shelves. When this is the case, any edges which may show should be ground and polished to give them a good finish.
Fig. 158. - Transparent Bevelled Glass fitted.
With these directions the novice ought to be able to fit and fix any glass in any piece of furniture in a workmanlike manner. This chapter may be concluded with the hint that any frame intended for glass must not be in winding. Glass is flexible to a small extent and might be forced to fit the winding, but in all probability the result at some time or other would be a crack across it. Also see that the rabbet is clean and free from hardened drops of glue, etc, and on the same level all round before the glass is laid.