Thin slices about 1/16 in (1.5 mm) thick cut transversely from the branches of lignum vitae, olive, laburnum, mulberry or walnut to yield an oyster-shell effect were often used in traditional furniture. The oysters can be cut and stacked with wood spacers between and weighted down, or short lengths of branch wood buried in dry sand, but the seasoning must be very gradual and prolonged if splitting and casting are to be avoided. They were either trimmed square or to an octagonal shape, carefully matched, coated with glue size to stop absorption, with a glued cover paper over to prevent them curling, and laid between cauls. Figure 301:7 shows the usual method of matching up but there is no reason why oysters of different sizes should not be laid in a random build-up (301:8). A modern example of oyster-shell veneering is shown in the photograph of a large cigarette-box in 304 designed and made by Mr Richard Fyson, which shows to advantage the careful selection, matching and patching of inevitable defects necessary in the finest work.
A material which is laid on and not in another material or ground is known as overlay, and typical examples are the applied frets on solid groundworks or substrates in Chippendale cabinets, and the lavishly applied overlays of ebony and ivory in Italian and Dutch period work. In applying these frets the groundwork must not be glued for it would be impossible to clean off the surplus glue between the frets, therefore the glue is rolled out on a flat surface, the fret pressed on the glue, quickly transferred to the ground, pinned and weighted down.
Overlays of boards thicker than a veneer and applied to a ground of common boards to save valuable material are usually referred to as facing-up work, ox facings.
Mother-of-pearl can be obtained prepared for use in various qualities and colours, blue, pink and green being the most expensive. It can be cut with a jeweller's piercing saw and filed to shape, but should be backed with a temporary veneer or sheet of paper before cutting as some varieties are very brittle. If laid with hide glue it should be roughened with a file and any slight curvature taken up with plaster of Paris mixed in the glue. Very little pressure must be used, sufficient only to press the shell home flush with the surface, and final finishing is done with 10:0 grit abrasive paper, from which the fierceness of the cut has been knocked off by rubbing two sheets together, followed up by pumice powder and rottenstone. The pearl can be engraved by brushing over with warm wax, scratching through the wax when cold and pouring nitric acid into the scratches, after which mastic suitably coloured is run into the lines to complete the design. Tortoiseshell can be cut and laid in a similar fashion to pearl, and can be cleaned off by scraping and papering and then polishing with dry whiting or rottenstone. It is usual to colour the ground under the shell, or insert gold-foil underneath to increase the brilliance. Ivory is obtainable in fairly large pieces and can be turned, cut and shaped with normal woodworking tools, laid with normal glues and the surfaces flushed off with scraper and file followed up with fine paper and pumice powder, and polished with whiting on a chamois-leather buff. Bone and horn are fair substitutes in small sections but lack the wonderful depth and ripple of real ivory, while white plastics are only poor imitations, although valid materials in their own right if treated as such.
A method of inlaying wood, metal and other hard materials in a groundwork/substrate of suitably coloured polyester resin, first
developed by William Mitchell at the Royal College of Art in London, is illustrated in the coffee-table shown in 305. The table, which was one of a series designed by the writer especially for this inlay, had the wood top inset 3/16 in (5 mm) in the yew framing. Cross-sections of various short ends of wood in the workshop— laburnum, Indian laurel, acacia, yew, grey sycamore, oak, brown oak and rosewood— were then sawn 1/4 in (6 mm) thick, broken up at random, spaced out on the ground/substrate and anchored with a little of the polyester resin used. A mix was then made up of resin, accelerator, hardener, and thixotropic medium, which allows a build up without creep or slide, coloured pale green with polyester pigments and poured over the assembly, working it into the gaps with a pointed stick. After hardening, the surface was levelled off flush with scrapers and belt sander, polished with silicon carbide paper and the whole table then sprayed with a clear catalyst finish. The completed table is successful because it treats the plastic as a material in its own right and not as a substitute for other materials, and although it has seen hard service over many years shows no signs of cracking, lifting or failure of any kind. Very oily woods (rosewood, etc.) should not be used for this type of inlay as the natural oil may inhibit the setting of the polyester resin. A photograph of the table-top with a broken-up laburnum oyster as a central motif is also shown (305B).