Paraffin Used for Gunshot Residue

The paraffin is just a wax that can melt at a reasonably low temperature. And then it can be applied to somebody's hands, allowed to harden. That wax can be peeled off and then tested with a chemical which reacts with nitrates. The problem with the paraffin test is that where there's quite a few other types of substances just in our environment that also have nitrates such as fertilizer. But gunshot residue is so prone to contamination.