Which type of test is used to detect gun powder residues on forearms?

Paraffin test or dermal nitrate test is used to detect gun powder residues (nitrates and nitries). In this test melted paraffin is brushed on the surface of the hand and then wax is removed and inner surface of wax cast is treated with diphenylamine or diphenylbenzidine reagent. If residue is present then a blue colour will develop.