K is for Keystone: A-Z of Historic Architecture
A keystone is, fundamentally, a wedge. The keystone is a wedge-shaped piece of stone or brick that sits at the apex (the top) of a segmental arch. Without the keystone, the arch would collapse. Without a keystone, an arch is just a pair of near semi-circles that want to slip over one another and fall apart. Therefore, the keystone is both the ‘key’ to making this elegant structure work, and the key to understanding how it works.
Gibbs’ St Martin in the Fields, 1722-1726, with exaggerated keystones throughout including the flat-arched doorway at lower centre of the photograph. (Photograph: Paul Farmer, CC-BY-SA).
Masonry structures do not want to ‘span’ openings. This is because masonry has zero tensile strength: the collection of stones and mortar are very strong when loaded vertically (this is usually described as ‘infinite compressive strength’ as an engineering rule of thumb). Put any stress across masonry, and it will give way like a house of cards. This gives builders two options. Option one is just to get a solid piece of stone available in large pieces (like granite) and use this to span the gap.
Option two, however, is a more elegant solution. This involves a chain of wedges with a wedge at the top, diverting a downward stress through each angled piece. When correctly engineered, such an arch can tolerate huge spans. In large structures, the diverted stress forces create a sideways thrust, which is why medieval cathedrals need massive buttresses in all manner of places.
The keystone, like all the other wedges of a segmental arch, must be put in place over a ‘formwork’. This would typically be a wooden structure capable of holding the masonry in place while it is assembled together and while the mortar sets.
The structural importance of the keystone, and its pleasing wedge-shaped effect, have been exploited for decorative effect for centuries. The so-called ‘Gibbs Surround’, named after James Gibbs, (1682-1752), a Baroque and then Georgian master architect, is an important example. The Gibbs Surround takes the wedge shape of an exaggerated, large keystone, and repeats it in an off-and-on pattern the whole way around a doorway or window opening. Gibbs employed ‘his’ surrounds in many projects, and a familiar example is St Martin in the Fields.
However, Gibbs was far from the first architect to play with the decorative possibilities of the keystone. Mannerist architects working over a century before Gibbs were making ‘ironic’, sculptural use of the structural feature. A particularly intense example is one of the doorway openings of the Palazzo del Té, one of the masterpieces of Italian Mannerist architecture. Here, a rusticated keystone of almost tasteless proportions thrusts through a delicate pediment. At the same time, wedge-shaped triglyphs (a Doric classical ornament near the roof level, in the frieze) are ‘slipping’ down as if by ‘accident’ (it is quite deliberate). The effect is playful, ironic, willfully perverse.
The Palazzo del Té, Mantua, 1524-1534, by Giulio Romano (Photograph: Marcok, CC-BY-SA)
Decorative keystones can be found in late 17th century architecture in England, and many times in 18th century architecture. The more exaggerated ones lost favour in the mid 19th century, as architects preferred to work in chronologically exact and ‘sober’ styles, including the Gothic. However, by the end of the 19th century the playful spirit in architecture was strong once again, and massive keystones recalling Gibbs and others can be found throughout the work of George Skipper.
