Prior to the late Middle Ages, “death masks” were simply sculpted forms that were supposed to resemble a person in life - for example, the Pharaohs of Ancient Egypt.
From the late Middle Ages to the early 1900s, death masks were a true-form wax or plaster impression of a face after death. Some famous people had plaster forms created of their faces prior to death.
Because of this postmortem (and occasionally antemortem - “around death”) process, many death masks do not appear especially akin to the faces they represent - a good example of this is Abraham Lincoln’s death mask; he appears nearly bald, but this was simply due to his hair being slicked back with grease prior to plastering.
two livings work on one death