English walnut plinth

Plinth for Dulwich Picture Gallery in solid English walnut

This is an ambitious commission for a large plinth to support a wooden model of the Dulwich Picture Gallery. Funded by a private donor, the piece will be on public display. It’s intended to reflect the style of England's first purpose-built public art gallery, a Grade II*-listed building dating from 1814 and designed by John Soane (1753-1837). Soane produced a series of neo-classical masterpieces in London, among them his own house in Lincoln’s Inn Fields, now the Soane Museum.

The gallery wanted something austere-looking, so the plinth incorporates simplified elements of a classical architectural order: the cornice on top, surrounding the model; a frieze beneath it, cut into tooth-like dentils; four heavy pilasters, flat square columns, on each side; rounded Roman arches arranged as a blind arcade; and a base at the foot of the pilasters. Doors at each end will opening on to an interior storage space.

In keeping with the gallery's Georgian elegance, the staff chose English walnut (Juglans regia) as the wood for the plinth. Now rare and expensive, this was the timber of choice for English furniture during the ‘Age of Walnut’ – the 60-year period up to around 1720, when mahogany began taking over. Naturalised in Britain, it’s a grey-brown wood with a lovely smoky grain, more subtle than more commonly used American black walnut (juglans nigra).

The timber for this piece all comes from within about 50 miles of Dulwich. The three-inch (75mm) boards making up the pilasters come from a tree on the Grange Estate at Northington in Hampshire that was felled in late 2009. The thinner stock for the rest of the piece was grown on the Englefield Estate near Reading, Berkshire, and cut in spring 2004.

The whole unit will have to be occasionally moved from room to room at the gallery, but at around 150kg it’s going to be far too heavy to carry and for aesthetic reasons there can’t be any visible wheels. So we have a design engineer working on a unique system of retractable wheels hidden inside the base that can be steered and braked.

You can see pictures of the completed piece here.

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The model of the Picture Gallery, in a Perspex case, currently sits on of an antique table, part of the gallery’s collection of furniture. But it needs a custom-made plinth more in keeping with the surroundings and capable of being moved between the gallery and the cloisters.

 

English walnut - the same species that produces the nuts, shown growing here on a mature tree - is the perfect choice for the project. But it typically includes wide bands of sapwood, so to allow for wastage I buy altogether 21 cubic feet from two West Sussex mills.

 

I choose the two thick boards I need for the pilasters at W.L. West in Selham, near the National Trust’s Petworth House. They’re sawn through-and-through – across the whole width of the log – and come complete with the knobbly, fissured bark typical of old walnut trees.

 

I can just get the waney-edge slabs in the back of the car so they arrive at the workshop first. My first job is to skim off the surface, which helps release any residual moisture. The boards are too big to go on the planer/thicknesser so I plane them slowly by hand.

 

I spend an afternoon selecting the one-inch, one and a quarter-inch and one a half-inch walnut from the kiln-dried timber shed at English Woodlands Timber. They’re based not far from West’s in Cocking, near Midhurst, right on the South Downs Way long-distance path.

 

The EWT timber arrives in London by lorry a few days later, shrink-wrapped on a palette. It has to be unwrapped and carried into the workshop, where I pile it temporarily wherever there’s a bit of space - on my workbench, on top of a storage cupboard and on the floor.

 

I can skim this thinner batch on the machine, and once the dark, grimy outer surface comes off the grain pattern starts to show through clearly. Now you can begin to appreciate the beauty of the timber, its gently swirling figure and lovely fine texture.

 

There are 21 boards in this delivery and it’s a job to keep tabs on all of them. Once I can see them properly I chalk each one up for a different part. Anywhere the grain is tightly curved is earmarked for an arch. That will give it more strength than a straight-grained piece.

 

At this stage there's also a lot of technical drawing - refining the overall design and detailing the individual joints. Drawing by hand, in pencil, helps you visualise the construction, calculate the precise dimensions, plan the assembly and avoid problems before they arise.

 

The real joinery starts with making the pilasters. Almost every other dimension depends on how thick they are. I rough-cut the three-inch boards by pushing them freehand through the bandsaw, then hand-planing the sawn edge and running them along the bandsaw fence.

 

Once the eight pilasters are squared off I can start on the six arches. The skimmed timber has to be machined to the rough shape and size of each component, then planed by hand. It's a pleasure to watch ultra-thin shavings like these coming effortlessly off the smoother.

 

The arches aren't just decorative: they play a key structural role, linking the pilasters and providing rigidity. After marking up the curve with a beam compass I carefully cut just outside the line on the bandsaw then do an initial smoothing with a cabinetmaker's rasp.

 

It takes a 'compassed' spokeshave, with a convex sole, to do the really fine shaping. Spokeshaves are simple, miniature planes with two handles, perfect for creating curves. Antique originals do a great job, but I've also got a few newer models like this one from Veritas.

 

The wood I use for the arches is carefully selected. I look for pieces where the grain runs in a tight curve as close as possible to a semi-circle. As well as producing a striking figure, this gives each arch more strength and flexibility than if I used straight-grained timber.

 

When the arch fronts are ready I attach them to the half-dozen backs that I've already made up. Horizontal rails at the top and vertical stiles at the side create a 15mm gap between the fronts and the backs, into which solid wood infill panels will eventually slide.

 

I make up each infill panel from three or four individual narrow boards and hand-plane each one along the edge to create a gentle hollow. That way, when the boards are cramped for gluing they close up extra tight and there's less chance of the ends opening up.

 

The boards are selected for their colour and grain, and it's worth spending some time getting a good match to make each panel look as pleasing and natural as possible. You also have to take into account any possible distortion and movement in adjacent pieces.

 

It takes a couple of hours to flatten and smooth each newly made panel. I use four different planes - a no. 5 jack, then a no. 6 , a low-angle smoother and a coffin smoother. My old Stanley planes have replacement blades from specialist makers like Ron Hock or Ray Iles.

 

Now I can start on jointing the arches into the vertical posts. The first step is to match the individual parts for colour and grain, laying them out loosely on the workshop floor and deciding which arch goes where. You can begin to visualise how the completed piece will look.

 

There's a lot of marking up to do - with a 2H pencil, a Japanese knife and a range of marking gauges - before any cutting can begin. I work to within a fraction of a millimetre, even on big projects like this, so I clamp my metre rule to the wood to mark the exact dimensions.

 

Now I can set about chopping the mortices in the posts. These are big joints, so I divide each of them into a double mortice for added stability and strength. After chiselling them out I use the router plane to cut 11mm deep platforms for the haunches and bridges.

I start the tenons off with a big rip saw, designed to cut along the grain. The bridge halfway along will separate the two tenons, while the haunches at each end (which I won't glue) will minimise twisting and allow the arch to expand in the higher humidity of the gallery.

 

There's a big pile of offcuts left after sawing the tenons. There are 12 double tenons in all, plus 12 matching double mortices. It's slow and heavy work. Each paired joint takes three and a half or four hours to cut by hand. That's two weeks of workshop time for the whole set.

 

You can just about make out the 'sandwich' construction of the arches when the tenons are finished. I've cut them so that each one is a three-ply laminate incorporating wood from the curved front arch panel, the stile and rail behind it and the solid panel at the back.

 

Once the tenons are trimmed to fit the mortices exactly I can turn to the lower rails, which form the base of each arch. I 'shoot' mitred (45-degree) tenons where the rails slot into the corner posts, using this left-handed wooden mitre plane, built for me by Phil Edwards.

 

Looking from what will be the inside of the plinth you can see a mitred tenon in the foreground, the halving joints linking the central posts (or pilasters) to the lower rail, the double mortices higher up the post and, at the top, bridle joints that will later house a top rail.

 

I'm constantly reassembling and knocking down the arcades to test the fit and the dimensions at every stage of construction. It's gratifying when all the components seem to go together smoothly, and irritating when joints are out of square or a fraction loose.

 

The last pieces in the arcades are the grooved columns that will appear to support the arches. I cut the grooves with a pair of new Veritas plough planes - one left-handed and one right - and leave a gap at the top of each column to allow the arch to shrink or expand.

 

This detail shows how the new columns intersect with the arches. The narrow gap between the two makes an attractive little feature known as a 'shadow line', but I've left a tongue on the top of each column to make sure you can't see right through to the back.

 

The long lower rails now need a groove of their own to accommodate the infill panels that I made a few weeks earlier. The Veritas plough planes are quick and easy to set up, but they're too small to groove these wide rails, so I dig out my antique Stanley 45 to do the job.

 

Ploughing grooves is tricky when you're working with the complex grain you find in walnut and it's hard to avoid tear-out. If I get any breakage along the edge I cut out the damage, glue in a matching piece, then smooth the repair with a flush saw and a hand plane.

 

Finally I clean up the wall of the groove with this little Stanley 79 side rebate plane, completing a repair that will be virtually invisible. Grooving by hand like this is quiet and satisfying, but frankly it's a bit of a dying art and most cabinetmakers now prefer an electric router.

 

Now I can spend a bit of time on the panels, cutting them to size, rebating them to fit the grooves and matching their colour and figure to specific arches. Then I put the arcades together one more time to make sure each panel slides in easily and fits without rattling about.

 

Before asssembling the arches I need to oil the parts that will be hard to reach once they're glued up. I use a mixture of tung oil and other plant oils that really enhances the wood. Look at the contrast here between the oiled arch span and the unfinished panel behind.

 

The oil takes a week to fully dry hard, so I've got a little time before the glue can safely go on. I use it to start making the frieze and the panels for the two end doors. I've kept aside some really striking wood for these, but complex grain like this can be tough to plane.

 

With so many interconnecting components I've had to plan the gluing process, down to the last detail, since the start of the project. It begins with putting together the central arch in each arcade, then over the next two days I'll slide in the side arches and corner posts.

 

The final glue-up for the arches and posts is on an industrial scale, with sash cramps bolted together to double their normal length. The cramps need repeated fine adjustments until the different surfaces are all exactly in line with one another and precisely squared up.

 

Now I finally get to see the arcades in one piece. There's still a lot more work to do - the toothed frieze, bridled and dovetailed into the tops of the posts; the two end doors; the skirt around the feet; and the cornice on top - but nothing as complex as building these arches.

 

The frieze panels, visible at the bottom of the picture, are attached at 90 degrees to the top rails. The rails themselves are load-bearing, so to strengthen the corners where they meet I cut slots in the mitres where I'll slide in loose tenons to add strength and stability.

 

Further along each top rail there's another compound joint, made up of a through mortice and a wider housing, designed to slide over the bridle joint formed at the top of the central posts. It's quite a job to make all these joints accurate enough to work simultaneously.

 

Once the main locating joints for the rail are ready, the decorative frieze itself can be cut out. First I saw each tooth, or dentil, by hand and then I carefully chisel out the bulk of the waste wood in the notches, making sure not to break or disfigure any of the surface.

 

When I've gone in as close as I can with the chisels I use the hand router to smooth the floor of each notch so each tooth is clearly defined. Some of the wood - especially in wild grain around the knots - is very dense, and a narrow cutter works better than a wide one.

 

With the teeth all cut, the two long rails look more like complicated engineering components than a classical frieze. But once they're glued in place above the blind arcades I'm hoping they'll look harmonious and elegant, very much in keeping with the gallery.

 

Sash cramps smoothly pull the top rail joints into place when they're lubricated with adhesive. I lay 620-mm sticks of MDF alongside the columns while the glue sets. They act as spacers to make sure there's a one-millimetre gap below the frieze to let the arch expand.

 

The frieze works well visually, finishing off the arch with a typical classical motif. When the pilasters and the frieze are both oiled they'll blend in neatly with the arch. Completing the design will be a moulded cornice above and a skirt below, pegged into the frame.

 

First, though, I make the four cross-ribs that will connect the two sides of the piece. Without using any glue at this stage, I cramp the double tenons of each rib into their matching mortices on the long rails and take measurements to check that everything is square.

 

The whole assembly can be glued up once I've made the bottom rails for each end. These will have to support almost the full weight of the plinth when it's being wheeled around, so for stability and strength I reinforce the corners with little loose tenons slotted into the mitres.

 

The final part of the frame is the short frieze, which will complete the toothed panel around the top edge. Cutting out the dentils means making multiple saw cuts through the waste, carefully chiselling out most of the wood then finishing the cut with a hand router plane.

 

I won't glue in the completed frieze just yet, not until the door frames below the frieze are in place. These are made up with a bridled stile (or upright) and rail (horizontal), which together form the door jamb, and a mitred architrave in front where the door will hang.

 

Not everything goes to plan. I started planing one 45-degree mitre at the wrong end of the stile, so I carefully cut out a matching piece to make an invisible repair. The new section had to be glued, tightly clamped into place then sawn and planed down to the right size.

 

With the door frames done I can slide in the dovetailed frieze, locking the corners of the mitred top rails with loose tenons hammered into a slot. I'll need to keep the dovetailed posts 2mm proud until I install the wheels, which can only be done with the piece upside down.

 

Beneath the frieze, I'll hang a pair of lockable panelled doors in the remaining opening. At the very bottom the proposed skirt will be grooved so that it can be lifted up and out. That will expose the void and give the gallery staff access to the wheels and steering arm.

 

I've selected a very dark, highly figured piece of timber to make the door stiles, or uprights, with black streaks swirling across the face. Walnut is a naturally lustrous wood, and the shavings that come off the plane when I'm smoothing the stiles glint brightly in the sun.

 

The doors themselves are made in the traditional way. I plough a rebate all round the panels and a corresponding groove in the stiles and rails (horizontals). The rails are tenoned into the stiles, with a little haunch, or step, at the top of the tenon to close off the groove.

 

Hanging the completed doors correctly means locating the hinges to within a fraction of a millimetre. In the traditional way, the 'knuckle' of the hinge - where the two leaves pivot - is cut into the door stile, not the carcase, so the recess is chiselled at a 12.5 degree slope.

 

I aim for a visible gap around each door of maximum 0.5mm, so each of the central stiles has to be bevelled a little at the back so they don't knock each other. I'll also be fitting old-fashioned 'cut cupboard locks', striking plates, bolts and escutcheons (keyhole surrounds).

 

The slot for the escutcheon has to be carefully drilled down to about a 2mm depth, then shaped along the flared sides with a chisel. The shallow arc at the bottom is cut out with a tiny gouge. After sawing the keyhole the escutcheon can be held in place with a dab of glue.

 

Now attention turns to the cornice that will eventually house the model of the gallery. The corner mitre joints are pulled tightly together with ingenious glue blocks: little bits of scrap wood bevelled to fit the cornice and angled to take a g-clamp.

 

The blocks are glued in place with PVA but there's a paper gasket in between them and the walnut - just a single sheet of newspaper. After the cornice is glued together they can be easily knocked off with a chisel and the paper smoothed away with a shoulder plane.

 

The cornice is then carefully located on the frame and cramped into place. I'm leaving the upper rails and ribs exposed to save weight, rather than building a solid top. Before delivery I'll insert a light MDF frame to lift the model just high enough to clear the cornice edges.

 

Now I can turn the plinth on its side to attach the steering assembly to the lower rails, starting with the axle bearings. The whole system, manufactured in stainless steel and anodised aluminium, was designed and engineered by Paul Edmondson at PE Marine Designz.

 

Looking from above, you can see the axle shaft slipped into the main mechanism, which is made up of a friction-free, smoothly rotating slewing ring (used in aircraft manufacture) sandwiched between two metal plates. At rest, the nylon wheels are automatically retracted.

 

Looking from below gives a better idea of how the system works. You can see the main assembly pivoting freely in the green bearing blocks and the wheels showing beside the white blocks housing the handle. For this photo I've inserted one of the two short handles.

 

As well as the two short handles, there are two long ones. Each long handle (shown here) can rotate about 30 degrees. They'll be used for steering the plinth around the gallery. The two short ones, which can swing round 90 degrees, will be used for 'parking' it in an alcove.

 

Now it's time for the final part of the construction to go on: the skirt around the bottom of the plinth. The only practical way to attach this is to screw it to the feet and conceal the screws with end-grain walnut plugs. There are 20 holes on each side, in four rows of five.

 

The top of the skirt is bevelled at a shallow angle, which I produce with hand planes. I was originally planning to groove a 3mm 'shadow line' where the bevel meets the flat rail, to highlight the intersection, but the walnut tore up badly on my test run and I gave up the idea.

 

Once the skirt is screwed in I brush a little glue into each hole and hammer in the tiny walnut plugs. Ideally I'd have made my own, but I don't have a lathe and even then wouldn't have been able to taper them like these ones, bought from Plug It in Helensburgh, Scotland.

 

With the piece back on its side to apply more finishing oil, you can again see the steering system. When the unit is stationary, the assembly tips upwards, the wheels retract behind the skirt and the plinth rests on its wooden feet, which project 2mm beyond the skirt.

 

But when you insert a handle into the white plastic housing at either end and gently push down on it , the whole thing rises effortlessly on to the wheels. Here you can see the wheels protruding below the skirt, just far enough to lift the plinth 24mm (almost an inch).

 

A short end section of skirt, designed to slide open when you need to move the plinth, conceals the wheel assembly. This short skirt is made up of a loose panel, a wide top rail, a narrow bottom rail and two tenoned stiles with grooves on the inside and tongues outside.

 

Once the components are bevelled and glued the short skirt drops into a vertical groove that I cut earlier into the end of the adjacent long skirt. The short skirt hasn't been oiled yet, unlike its neighbour, so the contrasting colours let you see easily how they fit together.

 

Before re-hanging the doors in preparation for delivery, I cut a shallow hole inside one door at each end using a Forstner bit in the drill press. These holes house a little laser-engraved disc with my 'brand' on it - the modern equivalent of a traditional maker's mark.

 

The handles for the steering assembly are going to be stored inside the plinth, so it needs a floor made up of removable MDF panels that drop in between the rails. There's another MDF frame on top, inside the cornice, to boost the model of the gallery to the correct height.

 

At the last minute things can still go wrong. I'm using brass screws for the hinges, but they're so delicate you have to cut the threads with a steel screw first. Even then this one snapped in half, so I had to dig it out with a chisel and plug the hole with a walnut dowel.