A crackle glaze or varnish is a finish which creates a crackled effect. Many people use crackle glazes for antiquing, because peeling, crackled paint has an old-fashioned look which goes especially well with restored furniture. Crackle glazes can be used on a wide range of things including pottery, furniture, picture frames, and even walls. A number of craft stores sell materials for crackle glazing, and you can also accomplish the effect with a few supplies you probably already have around the house.
When a crackle glaze is made, one color is laid down as a base coat and allowed to dry before a crackling medium is painted over it. Once this medium dries, another coat of color is applied, and cracks and peels start to appear almost immediately. To seal a crackle glaze, a clear varnish or polyurethane sealer can be used. This sealer also halts the crackling process so that the second layer of color doesn't flake off entirely.
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Since the underlying color shows through, many people try to use coordinating colors when they apply a crackle glaze. Contrasting colors can also work quite nicely, like a dark under layer and a lighter over-layer. If you aren't sure about how two colors will look together, test them on a small area of the piece to make sure that they look good once they dry. You can also use a medium like gold, silver, or copper paint for one of the layers for a more rich look.
If you cannot track down a crackling medium, you can also use white glue. White glue acts as a layer of resist, preventing the second layer of paint from fully adhering. This will create a more mottled effect; you can also use wax as a resist for a crackle finish. If you are working with pottery, use a glaze which has been designed to crack, and follow the directions closely.
The crackle glaze effect is sometimes called craqueleure, and in fine art, it is sometimes used as a tool to date artworks and establish their provenance. Forgers spend lifetimes mastering the art of a properly crackled final layer of paint in the hopes that their works will pass muster by art inspectors; a dedicated forger may infuse a craqueleure with dust and particulates from an old piece of art from the right region in case an art inspector decides to chemically analyze the finish.