Ministry Of Defence Building L153 (Group D Incororating Mills)

Gunpowder incorporating mills, built between 1867 and 1868, with a conversion for cordite incorporation around 1898-9, and mid-20th century additions. The building is constructed of English bond yellow brick, with hipped slate roofs to the Engine House and Boiler House. The cross wings have brick walls separating bays, which extend as coped gables breaking through the felt and slate roofs; the side walls were originally wood and canvas but have been replaced with brick in the mid-20th century.

The building follows a T-plan, consisting of a central Engine House with a four-bay Boiler House at its east end, and three-bay cross wings on its north and south sides, used for gunpowder incorporating mills. It is a single-storey structure, with a taller Engine House in the centre. The Engine House and Boiler House feature a chamfered brick plinth and a brick dentil cornice. The Boiler House has four bays, articulated by brick pilasters, originally containing segmental brick arches over windows with blind rectangular panels below. Most of the original sash windows have been replaced with larger mid-20th century windows, although the westernmost bays on both elevations remain blind. The east doorway has been widened and replaced in the 20th century. The Engine House possesses semi-circular arched windows with keystones at the upper part of each elevation (glazed on the north and south sides, originally blind to the east and west), with mid-20th century windows inserted below.

The Boiler House interior has nine trusses with wrought-iron tension rods, king rods, and decorative cast-iron compression members. The Engine House housed a steam engine that powered the edge runners in the incorporating mills. The cross wings have composite timber and iron roofs, designed to be easily replaced to minimize risk from explosions; a drenching apparatus was positioned above each pair of runners. Original gearing for the incorporating mills remains in a brick chamber below ground level. Cast-iron I-section girders support the partition walls in the cross wings, and blocked openings in the gables show evidence of shafting for machinery, likely installed for cordite manufacture. An open verandah previously extended along the west front. A coal yard, containing a chimney in the centre of its wall, was located on the south side of the Boiler House, and the building was served by a tramline to the west.

The incorporation process involved combining saltpetre, sulphur, and charcoal to create gunpowder. Drive shafts and machinery transmitting power to the mills were routed underneath the building to mitigate explosion risk. Each incorporating mill consisted of a pair of large, heavy edge runners of iron, which revolved on a large circular bed. The design is closely modeled on the earlier (1861) Group C Incorporating Mills, Building L157.