Hi Monica! depends on the glazes and the clay bodies you are using. If you are using low fire glazes, like a cone 4-6 I don't see why it would be a problem to do greenware at the same time since there are plenty that fire cone 4-6. Just make sure your greenware & glazes are compatible.
I have not mixed items in a kiln myself, so there may be some other reason I'm unaware of.

Karen

If I was mixing I'd probably load the glazed pieces first, then greenware on top. That way if there was any fumes or moisture from the greenware it would rise and not effect the glaze.

Karen

Thanks Karen for your remarks. What surprised me was when I was told it might explode! I could understand that greenware should be fired to a higher cone to be used for food. I understand that the fumes could affect color, but explode? I once fired gold accidently on a non-overglaze compatible glaze. It sure looked funny, but never exploded. It is my understanding that if something is poured wrong and has an air bubble inside it will explode, but never glaze and greenware. By the way, your idea of having the glaze on the bottom because of fumes is excellent. My kiln is so small (11 x 14") that loading is a challenge. Thanks, MonicaTX

I always thought exploding was due to excess miosture in the clay. That an excessively high temp for the greenware could "solidify" the outside too soon while trapping moisture inside the piece- especially if it is thick. Heat would cause the moisture to turn to steam and the force would "explode" the piece. I have had this happen with some pieces my students did in art class. It can break other pieces in the kiln, too.
Your kiln is petite!...sounds like a jewelry kiln size. I'd like to get one that size for silver Artclay and glass fusing.

Karen

What they mean is that if the greenware has air bubbles or moisture in it, it can explode. If a piece of greenware does explode, sometimes pieces of it can stick to the glazed pieces. Also, sometimes the fumes from the greenware can effect the color of the glazes, especially reds and oranges.

Cones 4-6 are high fire. Cones 06-04 etc., are low fire. Probably just an oversite.

Cindy

Thanks Cindy!...I should have had the 0's.

Another reason not to mix greenware and glazed items is that a glost firing can be stepped up much faster than a bisque firing. When firing such a mixed batch, the needs of the greenware will have to be the prime concern as there are many important changes taking place.
Also, as others have said, be VERY sure that there is no residual moisture in the greenware (and I am using the word greenware in the sense of "raw unfired clay").