A traditional chair is made from separate elements - legs, a square frame or hoop for the seat and uprights to form a back - joined together and then with some form of cord, webbing or upholstery across the frame to sit on and lean back against. As a general type or form of chair, shell chairs have a solid seat and back … that is not with any obvious frame to the seat and back.
Collection of shell chairs in the design museum in Oslo
Plywood chair with arms. designed by Alvar Aalto. in the collection of Artek 2nd Cycle in Helsinki
In the 1930s Alvar Aalto experimented designing a number of chairs that had a sheet of plywood that was steamed and shaped to form a scroll, in some chairs of quite complex forms, that was then supported on a bent-wood frame but these are not strictly shell chairs as the term normally implies a complex shape moulded in all directions and forming a hollow to sit in.
Later chairs in either plywood or a sophisticated veneer given a more complicated shape such as the Ant Chair from 1952 or the Series 7 from 1955 - both designed by Arne Jacobsen and manufactured by Fritz Hansen - are certainly shell chairs.
The seat and back of a shell chair can be in shaped plywood, a more complex veneer, shaped metal or moulded in a man-made material such as glass fibre or plastic. Generally the shell is supported on a separate frame of legs that can be in wood, metal or plastic. In some chairs the whole form of seat and legs or even a plinth or pedestal can be moulded either as a single piece or more usually is made in separate sections that are then fixed together to give the appearance of a single moulded piece.
One quirky and occasionally fashionable form of shell chair - a sort of sub genre - is an inflatable chair and presumably, at a stretch, bean bags or very large floor cushions ape the form of a moulded chair if not the complex construction.
Egg Chair by Arne Jacobsen 1958
The arm of a Swan Chair by Arne Jacobsen
Probably the archetypal shell chairs are The Egg and The Swan Chair, designed by Arne Jacobsen in 1958 for the SAS Royal Hotel in Copenhagen and still produced by Fritz Hansen. The shell can be covered in fabric or leather and is supported on an aluminium base. The curved, pronounced outward angle of the arms on the Swan chair seems like an affectation until you sit in one. Then you realise that not only does the shape give an unusual sense of openness but here you really can use the arms as a support as you push to rise from the chair and stand up.
One particular problem, tackled in different ways by different designers, is the way that the base or legs are fixed to the shell. For comfort, most shell chairs are made to be slightly flexible, rather than stiff, and many chairs have complex buffers or dampeners that have to achieve a balance between making the whole chair too rigid, and therefore rapidly becoming uncomfortable to sit in for any length of time, and too much movement between the shell and the legs that makes it feel unstable.
A recent version from Fritz Hansen of the Series 7 chair by Jacobsen. The underside shows the spacers to hold the position of the chrome steel legs and the plastic cover over the plate that fixes the legs to the shell
The shell can be fully upholstered, like the Swan Chair, upholstered just on the inner face of the seat and back or have separate squab cushions. Both the Egg and the Swan chairs have remarkably complex upholstery that requires hand stitching or hand finishing.
Minuscule by Cecilie Manz (Photograph from Fritz Hansen)
At regular intervals, since the 1950s, furniture manufacturers have introduced new designs of shell … in 2012 Fritz Hansen produced the Minuscule designed by Cecilie Manz; in 2013 they introduced the Ro by Jaime Hayon, as a modern reinterpretation of the winged-back arm chair, and they have just announced the reintroduction of the Grand Prix or Model 3130 designed by Arne Jacobsen in 1957 but now produced with a wooden leg frame rather than chromed metal.
These three chairs illustrate the complex variety of shell chairs that are now made and shows how sophisticated and complicated the technology can be. The Minuscule has a shell made of polypropylene upholstered in foam and the leg frame is actually plastic, forming a distinct cradle for the shell. The shell of the Ro is formed in a hard polyurethane foam with glass fibre reinforcing the edges … following the technology developed for the Egg Chair. There is a choice of legs with either cast aluminium or a wood base. The Model 3130 has a shell made from a complex veneer.
The Ro designed by Jaime Hayon (Photograph from Fritz Hansen)