López used the wet-plate collodion process, making and developing his plates in a portable darkroom. The plates were sensitive to blue light only; his darkroom was an orange tent. This was the first time photography had covered South American warfare and his images became iconic. The firm did send a photographer to cover the Siege of Paysandú the year before, but he arrived after the fighting was over. He captured images of the ruined town and corpses in a street.