Truth to materials is a tenet of modern architecture (as opposed to postmodern architecture), which holds that any material should be used where it is most appropriate and its nature should not be hidden. Concrete, therefore, should not be painted and the means of its construction should be celebrated – by, for instance, not sanding away marks left by timber shuttering (béton brut). As another example, porcelain is infrequently used in the exterior facing of buildings, but one modern example is the Dakin Building, Brisbane, California; therefore, the porcelain panels of this building have been left uncovered by any paint or preservative.
http://en.wikipedia.org/wiki/Truth_to_materials

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For a critical discussion of the term's history see :


Günter Bandmann: Der Wandel der Materialbewertung in der Kunsttheorie des 19. Jahrhunderts, in: Helmut Koopmann, J. Adolf Schmoll gen. Eisenwerth (Ed.): Beiträge zur Theorie der Künste im 19. Jahrhundert, 2 Vol., Vol. 1, Frankfurt/M. 1971, p. 129-157.


Materialstil, Materialstimmung, Materialgerechtigkeit, in: Dietmar Rübel, Monika Wagner, Vera Wolff (Ed.): Materialästhetik. Quellentexte zu Kunst, Design und Architektur, Berlin 2005 ( ISBN 3-496-01335-4) This principle was also followed by the Chicago school of architecture and Louis Sullivan.

[also from the above link]

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http://books.google.co.uk/books?id=o1tZpr06WPsC&pg=PA373&lpg...

Book Description: Ostfildern-Ruit, Hatje Cantz, 2004**, 2004. Bound, grey cardboard wrappers, pictorial dustjacket, photo - frontispice, 210 x 275mm., 223pp., richely illustrated in color and b/w. Egon Eiermann was one of Germany's greatest twentieth-century architects. His works, which enjoy recognition worldwide, are characterized by transparency, truth to materials, high artistic standards, and stringent design quality down to the smallest detail. His pavilions fot the Federal Republic at the 1958 Brussels World's Fair established his international reputation while making him an architectural ambassador of the young German democracy. Using predominantly original photographs, the present publication offers a vivid portrait of Eiermann's architecture, while distinguished authorities on his work explore his philosophy and personality in a series of essays. Book is new. Bookseller Inventory # 16875
http://www.abebooks.co.uk/search/sortby/3/an/Annemarie Jaegg...

Since the mid-19th century, first in architecture, then in sculpture, and finally in painting, artists began to realize the expressive, symbolic and style-shaping characteristics of materials, calling for ‘Truth to Materials’ or ‘Materialgerechtigkeit’ in aesthetics.[17] Initial movements in architecture criticized the painting of buildings as ‘lying’, because layers of paint would hide the natural building materials. At the dawn of ‘artificial’ building materials, like concrete, materials came into the focus of the aesthetic natural-artificial debate in architecture, which was basically a debate between romanticism and futurism. Late 19th-century sculptors started trying out new or rediscovering old materials, beyond the obligatory bronze and marble. Degas’ wax statues of young girls were early modest, though scandalous, examples because the flesh-like colored wax provided the statues with a peculiarly sensual flair.[18] Much more radical, regarding both the variety of materials and the dominance of ‘matter over form’, were the objets trouvées or the arbitrarily assembled pieces of garbage of the Dadaists.
http://zdch.amu.edu.pl:8000/FMPro?-db=kursyv.fp5&-format=iss...

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Note added at 1 hr (2009-06-12 10:52:11 GMT)
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Anytime, sats! Glad to help.