Eraser clay is a plastic (PVC) that is very moldable. That is why it does not air dry and doesn't need to be fired like normal clay. Plastic erasers are the most modern eraser type and typically the best performing as well. So in order to make eraser clay you need to be able to manufacture plastic. Or just buy it at the store and use your creativity on the shapes. :) There are three other types of erasers but they involve harvesting rubber and/or pumice stone. I've heard you can also use old bread as an eraser, but I've never tried it personally.