The wad of a shotgun blast is important to recover because it has factory markings. By analyzing the presence or absence of powder residue thrown out forward towards the target, the analyst can determine muzzle to target distance , not an unimportant consideration with cases of self-defense pleas and alleged suicides. Distances and angles can also be determined by analyzing the halo, tattooing, or spotting around a bullet hole in a target's garments. 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.