when casting a mould, you should use wood for the smoother works. Sure, itll burn if you leave it in the air, but thats why you stick it in tight compact earth -just after its rained is a good idea- with just the pouring hole and the top of the wood sticking out/level with the ground. You can easly stick a few bricks or a iron plate over the top -make sure you dont over fill it- to cut out the oxygen supply. The downisde is you need to wood work and dig a hole, the upside is this is one of the easyest ways to forge simple metal tools using natural only materials for a decent amount of quality and little after work.
other than that clay or sand should do the job. ide suggest going with clay if you have a spot to dig it up or can buy some rougth stuff for cheep, since its easy to mold and wont catch fire, sand if your willing to muck about and arnt too botherd about quality. A combination of clay and wood -wooden mold, clay oxygen seal/coating- would be the most ideal without knowing what your doing professionaly.
The key thing is to make the mould out of something that wont burn fast or melt. wood wont burn to a degree that would ruin the mold without the oxygen that is displaced out by the molten metal, and will keep a rigid shape even in extream heat. you stick it in the ground to make sure the mould holds together by the pressure of the earth around it and to dissapait the heat. It is afterall, molten metal.
Good luck. If you want to do anything intricate youll need to learn how to build a small home made forge to heat it up untill its flexable enought to be bent round a hard surface - an anvil classicaly-