Butterflies and Bevel Top Dovetails
Wednesday 02 February 2011
Ambrose Heal was an influential designer born at the right time into a family that owned a successful furniture business. In the early 1900s he used his position to develop the Arts & Crafts style from its rather exclusive bespoke origins into batch-produced high street furniture, accessible to the comfortably-off in the growing suburbs of London.
In this article we will look at some of the methods employed in his successful designs and how they can be developed with contemporary tools and materials to make our own designs of 21st-century furniture.
I hope you will be able to use some of these techniques directly or adapt and develop them to suit your own new designs just as Ambrose Heal borrowed and adapted styles and techniques from centuries before the Arts & Crafts movement.
Butterfly keysButterfly keys are loose-fitted components designed to lock boards together edge-to-edge. They would typically be spaced along tabletops every few hundred millimetres in rows following each edge joint. The keys are usually made from tropical hardwoods chosen both aesthetically, to contrast with the tabletop, and practically, for strength.
Ideally we can make solid wood tabletops using boards with carefully planed edges, butted together and glued without any mechanical joints, as indeed could Victorian makers. Why then was there a need for butterfly joints? For one reason visible joinery was an essential part of Arts & Crafts furniture and the butterfly certainly fulfils a visual function clearly showing how the boards are locked together. However, the use of butterfly keys also permits the employment of unstable woods of massive proportions without the risk of joints failing catastrophically.
Cutting keysThe edges of a butterfly key are normally pared at right angles to the face or can be very slightly tapered to facilitate a tight fit.
Butterfly keys are cut and then the sockets are individually marked around them using a knife and numbering each one to ensure a tight fit.
The sockets are chopped out using a bevel-edged chisel and then the sides and base are pared.
The key is fitted with glue and tapped securely into place so it protrudes by a fraction of a millimetre above the surface. The key is then planed flush with the face of the surrounding timber.
The butterfly joint is normally laid into a socket on the upper face of the timber where it forms a decorative feature. For balance, butterflies can be fitted on both the upper and lower faces of a tabletop, particularly if the timber is thick.
Butterfly joints can be used to restrain the checking of large slabs of bookmatched waney-edged wildwood or flitch-cut timber as used extensively in the work of George Nakashima.
Angled edge joints, such as for the lid of a trunk or the sides of a polygonal box, are made by bevelling the edges before they are joined. If the angle is shallow it may be practical to fit a flat butterfly key, as we have seen above, then pare the top of the key flush with the angled face on either side.
For steeper angles the splined butterfly provides a stronger mechanical joint. The splined butterfly is a strip of wood with a wedge-. shaped cross section slid into a housing slot that is cut through the angled faces of the wood.
The boards are prepared for joining by planing each of the mating edges to half the angle of the finished joint. They are secured using horizontal cramps to pull the bevelled edge joint together and vertical cramps to pull down a cramping board on top and stop the joint from rising. The bevelled edges may also be glued at this stage.
Routing housingThe router is fitted with a dovetail bit and a guidebush. This allows the router to be steered along the straight edge of the cramping board while cutting a triangular-sectioned housing slot in the wood beneath.
A short router bit with a wide angle should be chosen because the cross section of the slot will be elongated considerably by the geometry as it breaks through the wooden surfaces.
The next stage is to shape a strip of wood into a spline that will fit into the housing. This is best done on a router table using the same router bit as before so that the angle corresponds precisely to the cross section of the housing slot. With the two halves of the joint still cramped in alignment the spline is slid into the housing and glued into place.
With the glue set, the protruding corners of the spline are sawn off to reveal the butterfly shape of the joint. Again, strong tight-grained wood is used for the key, in a colour that contrasts with the faces of the surrounding timber.
Bevel-topped dovetailsThrough dovetails are normally constructed so the position of the shoulder line on each side of the joint corresponds to the thickness of wood on the other side of the joint. That way the joint closes flush, and is finished by planing the tails and pins smooth.
However, bevel-topped dovetails are purposely made longer than necessary so they protrude through the joint. The extra length is then shaped to provide a pyramid-like end to each tail.
Pared tailsThe procedure followed is the normal method for hand cutting through dovetails, which has been described many times in this magazine or can be found in books or on the internet. The only difference is that the shoulder line is marked so as to leave the tails over-length by about 6mm. Also the pins are slightly wider than they would normally be.
The joint can be trial assembled but must not be glued at this stage as it needs to be dismantled for chamfering the ends of the tails.
The sides of all the tails are chamfered as a row using a finely set block plane. Skew the plane for this so that it does not tear away the short grain at the top of the tails. The front and back of each tail is individually chamfered using a fine paring chisel.
Liming techniqueElizabethan cottage furniture was sometimes rubbed with caustic lime as a preservative and insect repellent, giving it a white hazy appearance and filling the open grain with caked powder. Arts & Crafts makers re-established the use of liming on oak, ash, and elm to evoke this traditional rustic look.
Modern liming preparations consist of white pigment in a wax paste or viscous liquid suspension.
Wax-based liming pastes are easy to apply, replenish and maintain. One disadvantage is that they are incompatible with other water- or oil-based finishes when applied over the wax. Also, the wax remains soft so will tend to rub off onto other surfaces.
Bronze brushBefore applying the liming paste the wood is wire brushed to open out the surface grain and remove dust. Steel wire brushes are too aggressive and they would tear the grain.
Steel would also contaminate the wood with iron residue which discolours oak in particular. Instead a bronze-bristled brush is used, always following the grain, or very lightly brushing in random directions across end grain.
Liming alternativesOrdinary household emulsion paint, known as latex paint outside the UK, while cheap and easily available also offers some significant advantages over proprietary products for liming.
It dries to a flexible solid which is compatible with other finishes. Although the paint itself is carried as a suspension in a water base, when dried it is water resistant and can be varnished over using modern water-based acrylic lacquers. These preserve the colour of both the wood and the liming.
The wood is prepared by opening and clearing the grain using a bronze wire brush, as before. Paint is brushed generously onto the surface and after a few seconds rubbed off again following the grain direction with a clean cotton rag. This fills the open grain while leaving the surface clean.
Liming contrastA different approach from liming the whole piece of furniture is to use paint to lime one side of a joint only. The other side of each joint is left bare. Try not to run paint onto the surfaces to be glued, but unlike wax, a little will not harm.
When the joints have been glued and the glue has set the piece can then be finished with a clear varnish. This leaves a contrast between the limed and unlimed areas. Alternatively different shades of paint colours can be used to create contrasting liming.