(Nanaimo, BC, Canada)
Question: What is the ideal temperate and humidity to store clay?
Also, are the bags that the clay comes in okay for storage? (in other words, is the plastic thick enough?)
Answer: Nicola, great question.
The simplified answer to your question would be, store your clay in a 40-75 degree room.
Hotter temperatures will encourage the clay to dry out faster even with the thick plastic bag, and freezing temperatures create a really strange texture in the clay requiring you to wedge or knead it really well before using it.
The plastic bags do work really well especially if they are well sealed. I do find, however, that if I leave clay for years at a time it will very slowly dry out and turn hard.
The moisture will also tend to migrate toward the bottom of the bag creating uneven clay (dry at the top and soggy at the bottom). This can be prevented by simply flipping the bag every 2 months.
You can also re-hydrate your clay pretty easily by simply pouring a cup or so of water into the bag and letting it sit for a week or more. Flipping the bag every couple of days makes it go faster.
I also like the technique of removing the entire block of clay from the bag, wrapping it in a wet dish towel and re-inserting it into the bag. This helps to distribute the moisture more evenly around the clay.
Once you play with clay enough, you can tell just by poking the bag whether it is too soft or too hard. If it's too hard, add water and wait, and it will get better.
Hope that helps,