Rustication is an architectural term referring to a decorative treatment of the masonry of a structure's exterior. It is to masonry what pastoralism is to literature, in that it pretends to be getting back to very simple primitive roots while actually being the highly refined product of sophisticated artists and artisans.

It was used as early as Roman times (particularly in the reign of the Emperor Claudius), was particularly popular in the renaissance and baroque periods, and is still in use today (see the interesting image of rusticated cladding being applied to The Getty Center in Los Angeles, California, below).

In its most extreme form, rusticated masonry is artfully carved so that the exposed faces of the stone blocks look like boulders right out of nature. More commonly, the stonework is ashlar (laid in repeating rows of rectangular headers and stretchers), with the rusticated face often chamfered, or bordered by drafted margins--that is, with bevels or smooth framing strips around the edges of the face, like a picture frame. The joints are thus emphasized.

Rustication is often applied by picking (gouging) the surface (picked work), carving it into meandering channels (vermiculate work), leaving pyramidal dimples on the faces of blocks, and, most oddly, by carving the face of a block into the appearance of stalactites or icicles (frost-work)! Sometimes the block faces are left smooth, with the deeply chiselled chamfered joints doing all the work.

Rustication is usually confined to the ground floor of massive buildings (but see the Palazzo Strozzi URL, below), to add to the anchoring feel of that portion in contact with the earth. Sometimes it forms a surround around entries. The masonry is generally fabricated from large stone blocks, and the effect is of massive primitive strength (by emphasizing the sheer bulk of the big blocks).

Below you will find a URL for an image of the astounding vermiculate-rusticated ground-floor of the 1725 Chiswick House, London--see the strongly drafted margins. In addition, it's worth noting the Victorian Richardsonian Romanesque (widespread, USA) which featured intense rustication to emphasize the style's putative getting back to the medieval after the clean lines of colonial, Greek revival, and other earlier 19th-century styles.

If you look long enough you will find brick being made to imitate rusticated stone, and even stucco performing this task. Rustication is not restricted to facades; it often appears in bands on columns (the effect is strange, to say the least: see the Pevsner site, below).

At times in the distant past, real boulders were more or less dressed so as to be fit into a wall; this is called "cyclopean" work in its rougher form, and "polygonal" masonry in a slightly more refined form. The Greeks and Romans both used these styles, as did the Incas. This stuff is the real thing.


URLs.

http://www.bluffton.edu/~sullivanm/england/london/chiswick/frontdet2.jpg (Chiswick House, London.)
http://www.ontarioarchitecture.com/rustication.htm (A site describing rustication, with images.)
http://www.nycjpg.com/prenycjpg/Archives/20020629.htm (Rusticated masonry in the American Museum of Natural History.)
http://www.lookingatbuildings.org.uk/img_hr/inigojonesrustic.jpg (Rusticated gateway by the great English architect Inigo Jones.)
http://www.usc.edu/dept/architecture/slide/ghirardo/CD1/020-CD1.jpg (Renaissance rustication, Palazzo Medici-Riccadri, Florence.)
http://www.usc.edu/dept/architecture/slide/ghirardo/CD1/022-CD1.jpg (Renaissance rustication, Palazzo Strozzi, Florence.)
http://www.lookingatbuildings.org.uk/default.asp?Document=1.C.1.2.1,1 (Rustication from the Pevsner Architectural Guide.)
http://academic.reed.edu/getty/cafeteria/Caf-Res-21.jpg (Rusticated travertine cladding being applied at the Getty Center.)
http://www.lookingatbuildings.org.uk/default.asp?Document=1.C.1.2.1,3&Image=1050&gst= (Frost-work rustication.)
http://xoomer.virgilio.it/thpappa/macart/image/viaggi/Estero/peru/84macchu3.jpg (Macchu Picchu polygonal masonry.)