The Extra Firm is stiff with a little bit of give. Makes sharper impressions in clay. Use to blend and model soft clays. The Flat Chisel shape can be The Extra Firm is stiff with a little bit of give. Makes sharper impressions in clay. Used for texturing and modeling, and it's especially good for The Extra Firm is stiff with a little bit of give. Makes sharper impressions in clay. Use in filling seams.