300px|thumb|upright=1.4|Craquelure in the [[Mona Lisa, with a typical "Italian" pattern of small rectangular blocks]]

thumb|Age craquelure in pottery

Craquelure (; ) is a fine pattern of dense cracking formed on the surface of materials. It can be a result of drying, shock, aging, intentional patterning, or a combination of all four. The term is most often used to refer to tempera or oil paintings, but it can also develop in old ivory carvings or painted miniatures on an ivory backing. Recently, analysis of craquelure has been proposed as a way to authenticate art.

In ceramics, craquelure in ceramic glazes, where it is often a desired effect, is called "crackle"; it is a characteristic of Chinese Ge ware in particular. This is usually differentiated from crazing, which is a glaze defect in firing, or the result of aging or damage.

In painted surfaces

thumb|upright=1.3|Typical French craquelure in a portrait from c. 1750, larger and less regular patterns, with curving cracks

Painting systems are composed of complex layers with unique mechanical properties that depend on the type of drying oil or paint medium used and the presence of paint additives, such as organic solvents, surfactants, and plasticizers. Understanding the mechanism of craquelure formation in paint and the resulting crack morphology provides information about the methods and materials used by the artists.

Characterization of craquelure morphology

There are seven key features used to describe craquelure morphology:

  1. Local and global direction of cracks
  2. Relationship to weave or grain direction of support
  3. Crack shape
  4. Crack spacing
  5. Crack thickness
  6. Termination of cracks
  7. Organization of crack network

These seven criteria have been used to identify "styles" of craquelure, which relate crack patterns to various historic schools of art. This links the crack patterns with specific time periods, locations, and painting styles.

  • Italian paintings on panel (1300–1500): cracks oriented perpendicular to wood grain with jagged lines and distinct secondary networks of thin cracks Craquelure formed during the drying process appears within days of painting and is characterized by shallow cracks that are localized to the topmost layers of paint. This localization results from capillary forces, which constrain drying stresses to the free surface of the painting. Variations in RH cause highly non-uniform tensile strains across the gesso surface, and when the material contracts upon drying, it fractures. Craquelure formed during gesso drying are particularly noticeable.

Similarly, wood supports respond significantly to changes in RH. Wood grains tend to swell perpendicular to the grain axis when they are exposed to moisture. As a wet ground layer is applied to the surface of a wood support, the wood in contact with the layer swells while the back of the panel remains unchanged. This can contribute to cupping, in which the panel of wood starts to bow transverse to the wood grain. The increased strains on the convex side of the cupped wood panel causes further fracture in the ground layer as it dries. Craquelure is almost impossible to accurately reproduce artificially in a particular pattern, although there are some methods such as baking or finishing of a painting by which this is attempted. These methods, however, generally achieve cracks that are uniform in appearance, while genuine craquelure has cracks with irregular patterns. Additionally, zinc white paints using linoleic acid-based binders are more successful at producing craquelure than paints with other binders. These deliberate glazing effects are usually known as "crackle", with crackle[d] glaze or "crackle porcelain" being common terms. It is typically distinguished from crazing, which is accidental craquelure arising as a glaze defect, although in some cases, experts have difficulty in deciding whether milder effects are deliberate or not. Some may also only have developed with age. Leading Chinese wares of the Song and Yuan dynasties with deliberate crackle glazes are Guan ware and Ge ware; in Ru ware, the milder crackle may be accidental, though the majority of pieces have it.

Ge ware can have a type of double crackle, known as "gold thread and iron wire", where there are two patterns, one with wide and large crackle and the other with a finer network. Each set of cracks has had the effect heightened by applying a coloured stain, in different colours. There are multiple layers of glaze, and the wider crackle develops first, with the finer one developing inside those sections. The crackle may take some time to appear after firing and is probably mainly caused by rapid cooling and perhaps low silica in the glaze.

Modern applications

thumb|Craquelure in fossilized hyena tooth

Acrylic craquelure

thumb|250px|Pair of Chinese crackled glaze jars with French [[ormolu mounts, both 18th century]]

The modern decor industry has used the technique of craquelure to create various objects and materials such as glass, ceramics, iron. This was made possible by the use of marketing kits that react with the colors used in decorative acrylic colors. The extent of craquelure produced varies according to the percentage of reagent and time of use. To highlight the cracks, glitter powder—usually available in copper, bronze and gold—is used. Mixing different brands of ready-made products to mimic craquelure results in various sizes and patterns of cracks.

See also

  • Crazing
  • Patina
  • Mudcrack

References

Sources

  • Bucklow, Spike, "A Stylometric Analysis of Craquelure", Computers and the Humanities, Vol. 31, No. 6 (1997/1998), pp. 503–521, Springer, JSTOR
  • Vainker, S.J., Chinese Pottery and Porcelain, 1991, British Museum Press, 9780714114705
  • Ward, Gerald W.R. (ed), Grove Encyclopedia Of Materials and Techniques in Art, 2008, Oxford University Press, , 9780195313918, google books
  • The classification of craquelure , Research at the Hamilton Kerr Institute, University of Cambridge