It appears the ash from the No. 5 fuel oil is ending up at the bottom of the furnace in the form of glass.
 
I would recommend that the user do a metal analysis of the ash in No. 5 fuel oil to determine how much of the ash in fuel oil is ending up in the glass.
 
The high amount of silica and alumina in the alumina is most probably from the furnace refractory.  Loss of the furnace refractory usually ends up in the glass residue at the bottom.

Normal analysis of No. 5 fuel oil is:
 
Carbon – 88.7%

Hydrogen – 10.7%

Sulfur –  0.57%

Ash – 0.02%

Moisture – 0.4%

The ash present in the No. 5 fuel will cause fouling in the boiler. The extent of the fouling will depend on the quantity and composition of the ash in the fuel oil and the design of the boiler.