Vance, a trapper boy, was 15 years old when this photo was taken. He was paid 75 cents a day for 10 hours of work. His job was to open and shut this door. Because of the intense darkness in the mine, the writing on the door was not visible until plate was developed
Mass production is the answer.
https://www.motherjones.com/environment/2015/10/kids-coal-mines-lewis-hines-photos/