I have some clay that I used some 10 years ago. Now it is hard and dry. Can I reuse it by adding water?
You can rehydrate even bone dry clay. There are many videos online for how to do this. A good one is Rehydrating Hard Clay the Even Easier Way.
There are several methods, but an easy one involves just preparing the block of clay and then leaving it alone for three or four days while it rehydrates. You add an appropriate amount of water to a plastic bag containing the clay, and immerse that bag in a large container of water to push the water in the bag against the whole block of clay so it isn't just pooled at the bottom of the bag. Then tie the bag shut to seal it and let it sit for three or four days to get evenly rehydrated.
If you have an odd amount of clay, err on the side of less water. If the clay doesn't get sufficiently pliable, add a little more water and repeat the process. It's easier to get to the right consistency this way than starting with too much water, although I've seen people just putting the block of clay in a tub of water about 2/3 as deep as the clay, and then flipping the clay every few hours.
1Be sure there are no contaminants mixed in with the clay, though. I once had dried out clay growing a kind of algae after rehydrating it.– Joachim ♦Sep 26, 2021 at 15:05