Demi Lune Table archive
Tuesday 23 September 2008
John Bullar makes an attractive oak and ebony demi lune tableError loading Partial View script (file: ~/Views/MacroPartials/cwsGalleryImages.cshtml)
A 'demi-lune' is the name that was often given to tables shaped like this (or a 'half-moon' to us English-speaking folk). I was asked to make the table to stand in front of a mirror in an entrance hall. The use of African ebony (Diospyrus crassiflora) in combination with quarter-sawn native oak (Quercus robur) is a tradition for English furniture dating back to the early 1600s when the East India Company first imported ebony. The Arts and Crafts movement revived it some three centuries later.
Strictly speaking, perhaps, a demi-lune should be a true semicircle rather than half an ellipse. But, having agreed the table would match the full 47 3/16in (1200mm) width of the mirror but only be 15 13/16in (400mm) deep, I reasoned that the shape would have to be either an arc or semi-ellipse. A semicircle this wide would have been 23 5/8in (600mm) deep - a bit too obstructive in a hall. While arc-shaped tables can be quite elegant, I felt a half ellipse would look better in front of a mirror, its reflection providing an image of the other half, with a continuous curve between tabletop and reflection.
A rounded ebony bead underlines the oak rails and echoes the barrel-shaped edge on the oak tabletop above. The square ebony feet repeat the pattern of black marble squares in the owner's floor, where the table now proudly stands.
Stringing alongHalf ellipses are easy to mark out with a pencil stretching a slack string, which is secured between two map pins. By increasing the distance between the pins you can elongate the major axis and reduce the minor axis. Increasing the amount of slack in the string increases both the major and minor axis. If I make this sound technical I apologise - it isn't. Just by playing around with pin spacing and string length you can have any size and shape of ellipse you want!
So that the boards could be cut without including any sapwood or shakes from the heart, my tabletop needed an edge joint to make it wide enough. I arranged the joint between edges from the outside of the tree so the front and back edges would be of the hardest heartwood from the centre of the tree. This makes the edges a bit more durable and they also cut cleaner.
It is worth spending a bit of time playing around with possible cutting lines and getting to know the grain and ray patterns in the oak. If the wood is sawn through and through (plain sawn or flat sawn) you may wish to alternate the tree-ring direction to cancel out any future tendency to bowing, but boards cut through the centre of the tree, or quarter-sawn, do not have this tendency.
Cut the shapes of the top pieces oversize with a jigsaw. After surface planing and thicknessing, edge joint them as invisibly as possible, then cut the completed piece on the bandsaw accurately up to the curved edge line ready for hand-planing.
Paring the edgeParing the elliptical edge with a compass plane is fairly straightforward if you tackle it as a series of small arcs, continuously adjusting the radius control on the plane sole as you move round to the next arc. Having established a true and smooth elliptical edge with the compass plane I gave the curve a second dimension (a barrel edge) using a convex spokeshave.
There is a choice about where to place the front legs, which will vary both the appearance and the stability of the table. An ellipse is what we see when we look at a circular tabletop. With this in mind I chose to place the legs in line with where they would appear on a six-legged circular table, hoping that the semi-elliptical table and its mirror image would give the illusion of a larger circular table.
Fancy footworkThe legs are 33 3/8in (850mm) long overall. They start to taper on three faces from the base of the rails and finish at the feet, with tapered ebony feet or sabots (wooden shoes) made from lipping cut on the bandsaw.
The legs are square-sectioned all the way down, 1 5/8in (40mm) for the depth of the rails and 25/32in (20mm) at the base of the foot. The fourth face, which forms the front of the front legs and the outside of the back legs, has a flat surface from end to end. When the frame is assembled these outer faces stand vertically, so that the legs appear slightly splayed out even though they are not. Saw the legs to shape, then surface plane them, before cutting the recesses to take the ebony lippings on the bandsaw.
The ebony I used for the feet came from a 1in (25mm) wide billet. After planing one face with a blockplane, I used a bandsaw with a 6tpi blade to cut the ebony to 5/64in (2mm) thickness. I then repeated the planing and sawing for each slice (16 in total).
Don't cut the ebony too thin or it will be too delicate and curl when you glue it, also the legs need to have micro-chamfers down the arrises so thickness is needed in the ebony to accommodate these. Glue up one opposite pair of ebony lippings at a time with as many cramps as will fit. When set cut the other pair to a close fit, glue up then finely plane and chamfer the legs end to end.
Rail sandwichesEach curved front rail is laminated to its own shape to make up part of the semi-ellipse. Mark the shape of the rails out on the underside of the tabletop 1 1/8in (30mm) from the edge using a pencil marker gauge. Use this as a guide to the shape of the formers for shaping the laminated rail sections while they set.
You need two sets of male and female formers - one symmetrical set for the front and the other asymmetrical set for both sides. Cut them on a bandsaw from sheets of MDF then glue them together and smooth the contact surfaces to prevent them marking the oak. There is a moderate amount of tension trapped in laminated woodwork so it tends to open out slightly
when it is released (though far less than with steam-bending). This means the formers need a marginally tighter curvature than you plan for the finished product.
If you want to make the layers practically invisible they need to be thicknessed to remove the saw marks and present a flat surface against each other. Thicknessing such thin material can be troublesome.
(I normally attach it to an MDF board with double-sided tape to discourage the thicknesser from curling it up and turning it into matchsticks.) However, for rails like these, where one edge is hidden by the tabletop and the other is on the underside, arguably it doesn't matter if the glue lines are slightly visible - you can use gap-filling adhesive between bandsawn surfaces. Mind you, squeezing together six laminations with a generous helping of gap-filling glue between each is a bit like trying to eat half a dozen cream buns at once.
Once you have pulled the formers and laminations loosely together with a couple of cramps, grab as many cramps as will fit to apply even, firm pressure all over.
Beading the bendWith three curved oak rails formed, the edges planed and tenons cut on the ends, the next stage - running a curved ebony beading around the lower rail edges - calls for a bit of micro-laminating. The finished bead is 3/16in wide x 1/8in deep (5 x 3mm). Ebony is a brilliant material to work with tools, but it does not take kindly to being bent, so the beading is built up of three 3/64in (1mm) thick laminations before shaping. These laminations, unlike the rails, would show any glue lines rather badly so they must have good flat faces for gluing. I bought proprietary manufactured ebony stringing 3/16in wide x 3/64in thick (5 x 1mm) then prepared it by clamping one end and running along the surface with a finely set blockplane.
Dark side of the moonHaving prepared and cut three lengths of ebony stringing for each piece of bead (nine in total) progressively glue them onto the curved lower edge of each piece of rail, clamping in place continually as you work along. Use a scratch stock tool to shape the round edge on the bead after the glue has set. To make this tool take a short piece of hacksaw blade, say 1 1/8in (30mm) long, file off the teeth and grind the faces and edges flat and smooth. Make up a concave cutting edge by filing a 3/16in (5mm) diameter semicircular notch, 3/32in (2.5mm) deep in one edge of the piece of blade. Try to prepare this inside cutting edge as you would a cabinet scraper. Give it a finely ground inner surface square to the faces and sharp, square edges. Then run a fine steel shaft lightly over it to curl the edge into a scraping hook.
Make a square beechwood shaft to fit in a basic joiner's marking-gauge and cut a fine saw-slot in one end, about 2in (50mm) long, to hold the blade. Carve a semicircular groove across the shaft, slightly larger than the notch in the blade. Place the blade in the slot, making sure the straight edge of the blade is slightly recessed so it cannot mark the curved surface of the oak rails. Pinch it in place by tightening the thumbscrew and the scratch stock is ready for use.
Ebony responds well to scraping and if the tool edge is fine, the ebony surface will cut to a sheen straight from the blade. Patience is a virtue when using this type of tool; many light passes are best. Sneak up on the required shape rather than trying to force it! A touch of beeswax on the tool helps keep it moving with a light touch and no judder (I find myself using beeswax with nearly every hand tool, but so lightly that a block of it seems to last forever). Use a dry brush and vacuum to remove fine ebony scrapings as you go along so they don't find their way into the open oak pores.
In the frameEach leg has a sturdy bracing member to support it. This is provided by a 25/32in (20mm) thick rail across the back of the table, with tenons into each back leg and two radial rails with tenons into each of the front legs and doweled into the back rail. (They are doweled into the back rail so the exact alignment can be marked easily during a dry fit.) Together, the three concealed straight rails provide most of the table's rigidity. The curved laminated front rails are primarily decorative, only performing a secondary structural role.
First glue the back rail to the two back legs with sash cramps. When this has set, the front curved rails need to be dry-fitted and pulled up tightly to close the joints and find the exact position for doweling the radial rails into the back rail. I used a pair of Jorgensen heavy-duty canvas band clamps for pulling the legs and curved rails together, both for the dry-fit and again for the glue-up.
With the legs and frame rigidly assembled, fit the top in the conventional manner with buttons screwed to the top and dry tenons going into the frame.
Finishing touchBy planing all the components by hand, with a fine set blade before assembly, very little is needed in the way of sanding to give the surfaces a silky touch. I particularly wanted to avoid contaminating the open pores of the oak grain with ebony dust. I felt certain that ebony would not benefit from sanding, so I sanded the oak lightly by hand with fine grades of paper, being careful to stop at the ebony.
If I were making the table for a home where it would get wet, with swimming towels and that sort of thing dumped on it, I would feel obliged to protect it with oil treatment, perhaps mixed with weak varnish. However, in my view the yellowing that oil produces does not improve oak so when I can get away with a clear wax finish on oak, I prefer to. A simple wax finish with beeswax/carnauba paste provides a light sheen together with some protection, keeping the colours fresh and the grain open. I applied the wax with a nylon dish-scouring pad. Avoid steel wool on light oak as it can lead to trapped metal particles producing microscopic black iron/tannin stains. The owners can re-wax whenever they see the need.
The table design is undeniably borrowed from a classical pattern with its simple, clean lines and contrasts of textures and colours. The open-grained pale oak against amorphous black ebony give it a contemporary freshness, which has caught some admiring looks.