The intense fires created around Hiroshima by the bomb carried large quantities of ash into the atmosphere. The ash had the effect of ‘seeding’ the clouds and the result was a ‘black rain’ which fell 1-2 hours after the explosion. This rain, which almost had the consistency of tar, was a combination of the ash, radioactive fallout, and water. In at least some cases, severe radiation burns resulted.
The material on display is a section of wall from a building far enough away from the blast to have remained standing, but close enough to have sustained damage. The rain penetrated the damaged roof and ran down the walls leaving the tracks seen on the plaster.
Provided by Oak Ridge National Laboratory courtesy of David Fields.