Ministry Of Defence Building L149 (Group E Incorporating Mills)

This building, Ministry of Defence Building L149 (Group E Incorporating Mills), is a significant example of 19th-century industrial architecture located within the Royal Gunpowder Factory. The core of the structure was originally built around 1869 as "Pellet Powder Buildings", comprising an Accumulator House, an Engine House, and a Boiler House. It was extensively converted and expanded between 1877 and 1878 to incorporate mills, and further altered between 1878 and 1895 and around 1904.

The building is constructed of English bond yellow brick with red brick dressings, and has slate roofs. The original plan was roughly square, but this was altered with the demolition of the coal yard walls and the addition of a new Boiler House to the rear of the Engine House, a new coal yard, and three-bay cross wings for the incorporating mills to the north and south sides. A Pump House was added between 1878 and 1895, extended roughly 1904, to the north side of the Boiler House.

The central Engine House is taller than the rest of the building and features a brick dentil cornice and pilasters. The Accumulator House has a datestone reading 1887-8, likely indicating a rebuilding, and features a semi-circular arched doorway, paired semi-circular arched windows to each elevation, and a false machicolation pattern of bricks above the cornice. The Boiler House, built between 1877 and 1878, has four bays articulated by pilasters, with the three eastern bays on the north and south elevations each containing a pair of semi-circular arched windows with red brick detailing. The eastern end has a rubbed red brick semi-circular arch. The cross wings each have three bays, with the original, less substantial walling replaced at a later date. The Pump House, situated on the north elevation of the Boiler House, exhibits semi-circular headed windows to its north side, and square windows to its east and west sides.

Inside the Boiler House are trusses supported by wrought-iron tension rods, decorative cast-iron compression members, and cast-iron ventilators. Cast-iron columns on the west side of the cross wings originally formed part of an open verandah with ends of corrugated iron. The cross wings feature composite timber and iron roofs, designed to be easily replaced in the event of an explosion, with a drenching apparatus positioned above each pair of runners which would also have been activated. The original gearing for the incorporating mills survives in a chamber below ground level, mirroring the processes used in building L157. The accumulator provided hydraulic power to press gunpowder into pellet forms, improving performance and vital for the large-bore guns being developed at the time.