Using sodium rhodizonate solution to test for Gunshot residue (GSR), also known as cartridge discharge residue (CDR), or firearm discharge residue (FDR). When it is activated the sodium rhodizonate reacts with lead nitrate to change color to a bright red. Gunshot residue is deposited on the hands and clothes of someone who discharges a firearm. It is principally composed of burnt and unburnt particles from the explosive primer and the propellant. This residue is deposited in the vicinity of a firearm being discharged. This image shows a field of view 6.5 cm wide.