What do you call it when somebody spends the whole day making pottery?

Kiln time or Kiln day, most likely. I guess you can also call this day "productive", if we go to a general answer.

Explanation:

A Kiln is a furnace or oven for burning, baking, or drying, especially one for calcining lime or firing pottery. It's not unusual for people in the pottery business to refer to the time that they work with a kiln as "Kiln Time".

expository writing is one of the most common types of writing.