How can you harden clay?

it depends what clay you are using, air dry clay drys by its self if you leave it on the window sill it will dry, for normal clay you bake it in a special clay oven.

by not leaving the substance in a moist environment and by putting it into a stove and leaving it their for about 2-3 days on a very high temperature.

You don't. It will harden somewhat on the outside and gets a crusty feel, but it never seems to harden completely. I think the reason is that it has an oil in the base so it stays moist and soft. If...

Any temperature but for best results in a hot spot or freezing spot. Make sure you don't freeze the clay, burn it or crack it. Certain clays have to be at certain temperatures. All clay s different.

Clay hardens because the water that makes it maleable and soft evaporates. Like mud.