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.
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.