Recovery Boiler House HO ScaleDetails: Walthers Cornerstone Recovery Boiler House Superior Paper Kit 11 3 4 x 8 x 17" 29. 8 x 20. 3 x 43. 1cm HO Scale Walthers Cornerstone #: 933 3901 Brown stock, the fiber water solution produced in the Kraft mill, is concentrated in the recovery boiler via evaporation into a substance called black liquor. As the level of concentration increases, raw rosin is created, skimmed off the top, mixed with other chemicals and heated to produce tall oil.