To enhance glaze fit, alter the clay body to allow for more expansion and, as a result, greater contraction, which compresses glazes and prevents crazing (i.e., increase silica for high temperature bodies, talc at low fire). You may also minimize the glaze's growth by adjusting it. There are several approaches that may be used. Change the amount of alkalinity in the glaze formulation or use less soda than usual. Reduce the amount of flux added during the glaze firing process.
Crazing is the surface deterioration of ceramic ware due to exposure to heat and moisture. The most common form of crazing is chipping, but surfaces can also develop ripples or cracks. Crazing is usually an indication that your pot has been exposed to extreme temperatures while wet, which can cause damage to its surface. As ceramics are porous materials, they tend to absorb water when exposed to atmospheric conditions. If the water cannot escape through the walls of the vessel, it will cause the surface to crack or break down.
The best way to prevent crazing is to avoid exposing your pots to extreme temperatures and keep them moist not wet. This will help preserve the life of your craft and provide protection against further damage. Also, consider adding silica to your clay if you want to reduce the chance of crazing occurring.
Glazing a work is not required, although it can improve the burned clay on both an aesthetic and practical level. Glazing seals the artwork, making it stain resistant and food safe (some glazes are not food safe, but I usually stay away from those:). Glazed pieces are also easier to clean. If you do choose to glaze your piece, there are many options when it comes to quality and price. You can find cheap imports at craft stores, but they will likely be made of plastic instead of ceramic. Or you can spend more money for hand-made pieces by artists who use real glass or stone.
The first thing you need to decide is what kind of material you would like to use for your glaze. There are two types of materials used for glazing: liquid and powder. Liquid glazes are applied with a brush or spray bottle and allow for more control over how much goo you put on your piece. They also dry faster than powder glazes. Powder glazes are just that, powder, which must be mixed with water before use. Because powder glazes are less controlling, you may want to add some color to them to create different effects. For example, red powder glaze would make something that looks like burnt brick come out of the kiln.
Once you have decided what type of material you would like to use, you need to think about what effect you are looking for.
Because water will evaporate from the clay during the burning process, you must ensure that it is as dry as possible before applying the glaze. Because leather-hard objects still have a high water content, some potters advocate allowing the clay to cure to a bone-dry level before glazing. This prevents cracking due to shrinkage.
Others fire their pieces fully cured, with just the final coating of color added after the piece is fired. This allows for more control over the color and reduces the amount of dust created by curing in air.
Still others use a combination of methods. They may burn part of their pieces, then finish them with a coat of paint or stain before firing them. This allows them to use less expensive materials for their glazes if they plan to sell their work unpainted.
The choice is yours. Just make sure that the piece you are working with is completely dried out before you start applying colors or textures. If not, you might end up with a really hard piece of clay that's also wet inside. That can be difficult or impossible to work with.
Have fun experimenting with different techniques! And don't forget to allow time for your projects to fire.