An ambrotype is a photograph created on a glass plate that has been coated and sensitized in a chemical bath. Once exposed, the image is fixed, washed and developed. When the glass plate negative is turned around and placed against a black background, a positive image results.
McDonnell used the technique to great effect in Vigilantes, a series documenting Mexico’s Auto Defensa—bands of vigilantes who protect communities against the violence of the cartels. In some communities, they have all but replaced the ineffective policing of federal forces.