Microscopic fissures, developed from age, in the varnish layer, distorting light refraction which affects the appearance of haze. In extreme instances the varnish layer becomes opaque.? <br><br>A network of cracks which sometimes forms in ceramic glazes; crackle. It may be desirable or not, depending on the artist&#39;s wishes. It is caused by the glaze and clay body contracting at different rates as they cool after firing.Example: China, Canton, Footed Bowl, late 18th, early 19th century, crackle-craze underglaze, off-white and blue, 2 1/2 x 5 3/4 inches, height of foot 1/2 inch. Enlargement of the inverted view. This bowl was made expressly for export. See invert.A similar pattern in the surface of oil paints is known as crackle instead of crazing. Crackle in oil paintings is less likely when the painter follows the rule of fat over lean ? when oil colors will be applied in layers (coats), the first layer should be leanest (least oil) followed by layers with progressively more fat (more oil.) Following this principle results in a work less likely to crack in aging. Similarly, in order to encourage crazing, later coats should have less oil than earlier ones.