i heard that there is a way to make normal clay harden. is this true, if so can someone tell me how to do this.

you bake it

how hard will it become. will it become hard enough to play with, or will it be brittle and break apart easily

I've done a couple things with clay, and all it's really good for after it's been fired (technical term for baking it) is display. It grinds to dust pretty easy, and isn't exactly light. I would not recommend using it to build something to play with.

depends on how thick the clay is, how long you cook it, and at what temperature it's cooked at. Regardless of that, if you bake it it will become less pliable and more brittle, the curing is what'll factor how brittle it can/will become. Just google stuff about making pottery, it'll give you the most information and it will apply directly to custom apps.

 

Ideally, the best thing to do is to make the clay object and bake/cure it, then create a mold from it and inject the mold with ABS plastic or similar to make the 'useable' item. But you'll always want to factor in costs before you start any such endeavours. I for one, cannot afford it.