A coffer (or coffering) in architecture, is a sunken panel in the shape of a square, rectangle, or octagon in a ceiling, soffit or vault. A series of these sunken panels were used as decoration for a ceiling or a vault, also called caissons ('boxes"), or lacunaria ("spaces, openings"), so that a coffered ceiling can be called a lacunar ceiling: the strength of the structure is in the framework of the coffers. The stone coffers of the ancient Greeks and Romans are the earliest surviving examples, but a seventh-century BC Etruscan chamber tomb in the necropolis of San Giuliano, which is cut in soft tufa-like stone reproduces a ceiling with beams and cross-beams lying on them, with flat panels filling the lacunae. For centuries, it was thought that wooden coffers were first made by crossing the wooden beams of a ceiling in the Loire Valleychâteaux of the early Renaissance. In 2012, however, archaeologists working under Andrew Wallace-Hadrill at the House of the Telephus in Herculaneum discovered that wooden coffered ceilings were constructed in Roman times. Experimentation with the possible shapes in coffering, which solve problems of mathematical tiling, or tessellation, were a feature of Islamic as well as Renaissance architecture. The more complicated problems of diminishing the scale of the individual coffers were presented by the requirements of curved surfaces of vaults and domes.
A prominent example of Roman coffering, employed to lighten the weight of the dome, can be found in the ceiling of the rotundadome in the Pantheon, Rome.
^Ching, Francis D.K. (1995). A Visual Dictionary of Architecture. New York: John Wiley & Sons, Inc. p. 30. ISBN0-471-28451-3.
^An alternative, in a description of Domitian's audience hall by Statius, noted by Ulrich 2007:156, is laquearia, not a copyist's error, as it appears in Manilius' Astronomica (1.533, quoted by Ulrich).
^An example is the main hieron at Samothrace, where stone ceiling beams of the pronaos carried a coffered ceiling of marble slabs across a span of about 6.15 m (J.J. Coulton, Ancient Greek Architects at Work: Problems of Structure and Design (Cornell University Press) 1982:147.
^Roman wooden coffered ceilings are discussed in Roger Bradley Ulrich, Roman Woodworking, ch. "Roofing and ceilings" (Yale University Press) 2007.