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Most pottery is fired twice (or in some cases 3 or more time!). The first firing is called the bisque, then there is a second firing for the glaze. This is the way you probably learned, and they way you probably do it. But it is possible to fire only once. In fact, ancient pottery, such as Chun Pottery, was often only fired once. This is called “Single Firing” or “Once-Fired Ware”.
There are many advantages to doing two firings.
Glazes are easy to apply. You don’t have to worry about the piece absorbing too much glaze and coming apart.
If you apply your glaze poorly, before firing, you can wash it off.
You can more easily do decorative techniques where you apply a "remove glaze" (for example, to wipe off the high spots of a textured surface.)
If your piece is not dry it can “explode” in the kiln. Without glaze on the pieces, this doesn’t hurt anything (except maybe neighboring pieces.) But if that piece were covered with glaze, the pieces would stick all over the kiln.
Organics have a chance to burn off in the bisque firing, so they don’t affect the glazes.
The advantages of single firing are:
Less power used
Some people feel they get a better clay-glaze interface, or they prefer the process of making the piece all at once instead of in stages where focus is first on form, then on decoration.
Not all glazes can be used for the Single Firing method. The glaze should have a high clay content. Beyond that, a lot of testing is required to find a combination of clay and glaze that works consistently for you.
To summarize, there is a good reason why most people use two firings. You will have more loss with single firing. In addition to pieces cracking, you tend to get more glaze problems like crawling and pinholing.) For these reasons, most people believe that in the long run, single firing is not any more efficient. However, it is possible to single firing, and some potters have found ways to do it successfully.
Perhaps there will be a resurgence of interest in Single Firing as fuel costs continue to rise. But it certainly would not be recommended for a one-of-a-kind piece that you spent months on.