Contributions by: Lucas Simonds
Crazing refers to the formation of a network of fine cracks on the surface of glazed ceramics caused by tension between the ceramic body and the glaze. This process can come about in a number of ways. When a glazed ceramic cools after firing crazing can occur if the glaze contracts at a faster rate than the ceramic body (US Bureau of Standards 1928:557). Alternatively, a similar process can occur later in the life of a ceramic if it is exposed to changes in temperature drastic enough to cause significant expansion and contraction of both the ceramic body and the glaze (Cronyn 1990:146). Lastly, crazing can appear as a result of the expansion of the ceramic body alone which produces tension in a rigid glaze (US Bureau of Standards 1928:557).
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Discussion[edit | edit source]
In the context of conservation, the effects of crazing depend on the nature of the ceramic body. Crazing on any type of ceramic will produce aesthetic damage. Not only is the fine network of cracks itself aesthetically unpleasing, but dirt cannot be removed from the cracks, and crazed ceramics may suffer from organic staining (Department of the Interior 2004:511;Durbin 2005:46). If the ceramic body is non-porous, then the damage caused by crazing is generally limited to those aesthetic factors described above (Cronyn 1990:146). Porous ceramic bodies, however, can be exposed to numerous environmental factors through the cracks in the glaze caused by crazing, leading to pieces of the glaze flaking off and the possible deterioration of the ceramic body itself (Cronyn 1990:146).
Treatment of ceramics presenting crazing does not differ from the treatment of ceramics generally, consisting mainly of the application of a consolidant, but care must be taken during soaking treatments to prevent flaking of the glaze (Cronyn 1990:151,152). In the case of ceramic tiles, in particular, a treatment consisting of a mineral densifying agent and sub-surface sealant has been successfully applied to mitigate the possible deterioration caused by crazing (Department of the Interior 2004:514).
References[edit | edit source]
Cronyn, J.M. 1990. Elements of Archaeological Conservation. New York, New York: Routledge
Department of the Interior. 2004. The Preservation of Historic Architecture: The U.S. Government’s Official Guidelines for Preserving Historic Homes. Guildford, Connecticut: Globe Pequot
Durbin, Lesley. 2005. Architectural Tiles: Conservation and Restoration. New York, New York: Routledge
U.S. Bureau of Standards. 1928. Crazing of Glazes Caused by Permanent Increases in Size of Ceramic Bodies. Journal of the Franklin Institute 205(4):556-558