In the NG Pictorial Volume VI, pg. 133, there is a "matching" image #60, photo caption reads "Como, August 2, 1936".
It appears to me to be a problem with a safety valve either not seating fully (out of adjustment), allowing steam to leak continuously; or, perhaps over-firing, keeping the pop valves open, or even perhaps a terminally lazy fireman, overfilling the boiler on a regular basis, ore even a combination of everything above? There seems to have been a lot of liquid hot water involved to make such a relatively even coating of the dome.
My Dad told me of a "lazy Frenchman" fireman. When my Dad was a hostler, he had to climb in the firebox of the rotary and bust up the clinkers to get them through the fire door. Apparently the NP kept that lazy fireman around, as I found a huge klinker over 150 miles away- apparently the product of a similar firing technique! The rotary was never used in our area.