Monday 9 July 2018
As well as covering furniture with wood grain that is independent of its construction, veneers are often used to create patterns or sometimes pictures on surfaces.
Oyster veneers are built up from the natural ring patterns seen in the sections of sliced up branches. Parquetry is made by repeating geometric shapes and marquetry makes pictures from individual shapes. Banding and stringing are lines laid in grooves, or rebates on edges. Banding is often composite including veneers with crossed direction while stringing is generally cut narrower in the direction of its grain.
Any of these patterns can be set into the wood, termed inlaid, or alternatively included alongside a face veneer where they are overlaid.
Bandsawn veneers can capture special figuring from offcuts of wood that would otherwise be scrapped. Most workshops have a bandsaw that can be adjusted by the furniture maker to produce special veneers. However, the process does require a little time and effort because the factory adjustments and standard bandsaw fence are rarely up to the job.
A robust fence that will not deflect under pressure is needed. This can be made from 18mm MDF in double thickness, carefully screwed and glued at right angles to the base which is G-cramped to the bandsaw table. Ideally, if there is a screw hole in the upper blade guide, fit an adjustable bracing strut between the fence and guide.
The gap between blade and fence can now be measured top and bottom. If necessary tweak the table angle to make the gap perfectly even. Remember it is the inward-set teeth that will determine the thickness of veneer that emerges from the gap. 2mm is ideal for most special veneers.
Oyster veneers have nothing at all to do with shellfish other than the vaguely similar oval patterning of the heartwood which, when sliced, is reminiscent of an oyster shell. Walnut, fruit trees, laburnum or olive are normally used because of their strongly contrasting heart and sapwood colours and their better resistance to radial cracking.
A log will have higher moisture content towards the centre and so needs drying after it has been cut; however, being very thin with a large end-grain area, oysters will dry quickly in a week or two.
Normal methods of stacking between stickers with a heavy weight on top should minimise buckling. If the oysters distort badly while drying they can be softened with weak hide glue made flexible with glycerine. Sizing the end grain surface with weak glue also prepares it better for making a strong bond.
Oysters can be combined in any pattern that appeals, for instance, random, geometric or mounted individually, surrounded by plain veneer.
The oyster needs to be shaped on the bandsaw or with a fretsaw or scrollsaw. Templates are used where the shapes are repeated, otherwise each oyster can be used as a template to mark out its neighbours.
Where oysters butt against one another they need a gap-free joint which may need adjusting with a plane or spokeshave. The pieces can be held up to the light to check for mismatch between edges.
In any design of wood layout it is wise to think about wood movement effects. Tangential wood movement – the strongest type – around the oysters can produce splits in any direction, but these will not add together across separate pieces.
Small oysters are unlikely to split especially if they do not include too much sapwood. The only cumulative movement across a number of oysters is radial which is relatively small. Overall, any movement of the oyster veneer should be uniform and well contained.
Once a pleasing pattern has been made and all the edges fitted, the oysters need to be locked together ready for glue up. I find it best to lay the oysters face down on a board and join the backs tightly with masking tape.
With the aid of a second board, turn the matrix of oysters over and apply gummed veneer tape to the front.
Once the veneer tape has dried, remove the masking tape and lay the oysters on the groundwork in a bed of glue. Being all end grain, the surface will form a relatively weak bond and could easily be starved of glue so I find it best to use a fairly stiff mixture of powdered resin.
Ideally there should be no variation in thickness between oysters but in practice there will be some. The hollows will need padding out if a conventional veneer press is used. A vacuum bag press, however, will accommodate these imperfections, applying a consistent one kilogramme per square centimetre regardless of any humps and bumps.
The hexagonal pattern used in the oyster veneering illustrated here is also an example of parquetry. Parquetry is the technique of making geometric patterns with pieces of veneer or wooden floor tiles fitted together as on a parquet floor.
Parquetry uses a single shape or a small number of shapes repeated to build up the pattern. Identically shaped pieces can be book-matched or slip-matched alongside each other, or else rotated as in a tessellation pattern.
Many hardwoods have a deep lustre that changes with the direction of light. By arranging each piece of parquetry to exploit this, such as by rotating it compared to its neighbours, you can create an optical illusion with the appearance of 3-dimensional depth.
Parquetry pieces having the same shape should all be interchangeable. The best way to check this after sawing is to stack them up and hold a square against the stacked edges.
Marquetry is the art of making pictures with individually shaped pieces of veneer and forms the basis of Boulle work which includes metals and shell, and intarsia which includes inlay.
Traditionally veneers are stacked up in a pad and cut with a fretsaw or on a manually powered mechanical frame known as a marquetry donkey. The modern equivalent to this is the scrollsaw.
Dyed or stained wood must be sealed otherwise, under the influence of dampness absorbed from the glue or the residual moisture content, the colours will bleed from the edges into adjacent pieces.
Commercially produced marquetry is often made with laser-cut veneers based on CAD patterns. These can be expected to fit with immaculate precision.
At only 0.6mm thick, modern machine-cut veneers are easy to shape with a craft knife or scalpel, making the old technique of window marquetry attractive for occasional work.
Window marquetry is a simple technique requiring no equipment other than a sharp knife and a cutting mat. It gives the maker complete freedom to design and produce patterns with any complexity using any number of veneers.
A piece of veneer selected to form the outer border is marked with the pattern, then cut out to produce a shaped window.
The window is positioned over the next chosen piece of veneer which can be moved around to identify the best figuring and grain direction. The inner piece of veneer is then cut using the window as a template.
This piece is then glued edge to edge inside the window with PVA on an artists fine brush. Working inwards, the next stage of the pattern is cut inside this piece to form an inner window which in turn forms another template and so on.
Patterns need to be carefully chosen or adapted for window marquetry, but with a steady hand and good near vision the finished results can be excellent.
Banding is laid in broad lines, sometimes overlaid around the outside of a veneered panel, sometimes inlaid in a shallow groove.
Cross-banding has a grain direction perpendicular to its length, providing an attractive visual break. Veneers such as Macassar ebony, zebrano or rosewood that include natural colour banding are ideal for this.
Artificially patterned bandings are made by layering up pieces of wood, often laid at angles to give a diagonal element. After gluing, the composite board is sliced through on the bandsaw to reveal the patterns in the section, Blackpool rock style.
Commercially manufactured bandings are available in metre lengths with any number of patterns. While these are designed to dazzle the eye, they tend to be rather characterless and repetitive compared to individually made designs.
Grooves to inlay banding can be cut either with hand tools such as a finely set plough plane or with a router.
The grab property of hot hide glue applied by hammer veneering is particularly helpful to hold banding in place while it sets.
Stringing lines are normally cut from a single piece of wood or veneer so the grain follows the direction of the line. Purfling is a specialised form of fine stringing made from two or three contrasting lines bonded together, as used around the edges of stringed instruments.
Wide stringing can be cut on the bandsaw while narrow pieces need to be trimmed with a dedicated slicing gauge or with extra care using a sharp marking gauge. A thicknessing gauge can be used to scrape the surface to produce a uniform depth.
Straight shallow grooves and rebates for stringing can be cut by hand with a scratch stock or with a specialised inlay groove cutter. The corners of grooves are generally shaped freehand with a thin chisel.
A router fitted with a fine cutter can make intricate grooves to take inlaid stringing. The router can either follow a shaped template, be guided by a pivot arm to produce a circle or by an ellipse jig.
Broad string lines may need to be steamed to soften them before they can be fitted around tight curves.
After the glue has set inlays must be levelled, but there are a couple of points to watch out for.
Thick banding and stringing are glued in place proud of the surface and then planed or scraped flat. However, at this stage commercial materials made from dyed woods sometimes reveal that the dye has not fully penetrated.
Sanding open-grained woods often fills the pores with dust, which is barely noticeable when it matches the wood, but contamination with dust of a contrasting colour will look dirty. Using a cabinet scraper to level the surface creates no dust.
Before any fine sanding is applied the surface should be sealed with something like shellac or sanding sealer. After sanding, any dust needs to be removed thoroughly with a vacuum cleaner before further finishing.