how do you harden air dried brittle clay ?

Answer 1

It needs to be fired. This takes a higher temperature than any domestic oven is capable of (the temperature actually depends on the clay type).

One of the most fun ways of doing it is to use the sawdust technique. You need a metal container with a lid such as a metal trash can/dustbin or an old oil drum, plenty of dry sawdust and (of course) your pots, air dried. You can also use a hole dug in the ground or a kiln made of loose bricks.
http://www.firedworks.com/aboutart/aa_sawdust_pottery.html

Fill the container with several inches of sawdust and then begin to include your pots in the sawdust, filling them and placing them more or less towards the centre of the container, with plenty of sawdust round them but don't pack it down too hard. The air included with the sawdust is essential for burning. Continue until your container is almost full. Now light a small fire on the top of the sawdust - hot coals from a barbecue will also work very well. The ideal is to get the sawdust smouldering. Once it is well alight, put the lid on so that it will keep smouldering overnight (and possibly for 24 hours or so).

It's a chancy process and some pots inevitably crack while other firings may not work, but that is one of the joys of primitive firing - you can never predict absolutely what will come out!

Answer 2

You need to fire it in a kiln.

If it is crafting/modeling clay, some of those can be done in an oven, but it will tell you on the outside of the package.

Answer 3

No, there is not any reason to "air dry" until now baking. in actuality, letting it take a seat around is plenty greater possibly to produce problems, because of the fact the solvent holds the products heavily and aids of their melting mutually to form the solid %plastic of the result, and a few of it gets lost (rather slowly) if left in the air.

Answer 4

water it more,.