Infrared photography will usually reveal even the most minute traces of powder residue. With shotguns, the general rule is that a one inch spread equals one yard, so a ten inch patter equals ten yards, for example. Distant shots are the most problematic. The suspect's hands were coated with paraffin or wax, and then the paraffin was tested with diphenylamine. If the wax turned blue, it indicated nitrates were present.
Nitrates are the most common unburnt part of gunpowder residue .
Unfortunately, the test fell out of popularity over the years because urine, tobacco, fertilizer, cosmetics, and other substances on the hands also yielded a blue color. Modern tests for hand residue look for primer substances such as barium on the thumbs and bullet-alloy substances such as antimony on the fingers. Another test for nitrates is called the Griess test which is designed to assist with measurement of distances and angles. The examiner presses a photographic paper onto surfaces near the target, and then sprays the paper with sodium rhodizonate and a pink to violet color appears if nitrates are present. Yet another test, called the Harrison-Gilroy test, doesn't check for nitrates at all, but for antimony, barium, and lead using the same paper color change indicator.