I realize that my friend Chris is the expert on ceramics, and that this post could go on the Friends of the Fremont blog, but I thought that due to my blog's emphasis on production and experimental methods that this topic might have a better fit here.
I just got done grading the paper of one of my students, Eric Ritter, who experimented on adding temper to clay. He added temper to wet clay and then he added temper to dry powdered clay. He found that he had better results with temper to clay ratios when he added temper to powdered clay. He also found that adding water to powdered clay enabled him to determine the exact consistency needed to form his vessel.
Eric suggested that mixtures of clay powder and temper may have been easier to transport and trade than wet clay or a pre-made ceramic vessel. I'm not sure how Eric's idea could be tested or if it would show up in any kind of ethnographic record, but I suppose that it could be possible.
If a group of people lived in an area that lacked raw materials, would they just buy pots from those that had already made them or would they prefer to buy the "just add water" mixes?
I would think that if a group without ceramic resources would also lack the knowledge of how to make ceramics and just buy pre-made pots, but who knows?