Ministry Of Defence Building L157 (Group C Incorporating Mills)

Royal Gunpowder Factory, Gunpowder Incorporating Mills

This building dates to 1861 and was converted for cordite incorporation around 1898–1899. It is built in English bond yellow brick with hipped slate roofs to the Engine House and Boiler House. The cross wings have brick walls separating the bays, which continue as coped gables breaking through the mid-20th century replacement walling. Originally, the side walls were wood and canvas.

The building follows a T-plan with a central Engine House, a Boiler House at the east end, and cross wings on the north and south sides housing gunpowder incorporating mills. The wings were originally 2 bays each but were extended by the addition of a third bay soon after construction. The building is one storey, with the Engine House rising taller at the centre. A chamfered brick plinth and brick dentil cornice articulate the Engine House and Boiler House.

The Boiler House measures 4 by 1 bays. The bays are articulated by brick pilasters, each originally with segmental brick arches over windows and blind rectangular panels below. The window sills were lowered and double doors inserted to the easternmost bay in the north elevation around 1906–1907. The windows were sashes except in the westernmost bay on both elevations, which were blind. A wide doorway to the east has a keystone to a gauged brick semi-circular arch with fanlight. The Engine House has semi-circular arched windows with keystones to the upper part of each elevation; these were glazed to the north and south, and originally blind but glazed 1906–1907 to the east and west. Windows to the lower part of the north and south elevations were inserted in 1906–1907.

The Boiler House interior contains 9 trusses with wrought-iron tension rods, king rods, and decorative cast-iron compression members. The Engine House originally carried a steam engine to drive the edge runners in the incorporating mills. The cross wings have composite timber and iron roofs of relatively insubstantial fabric, designed to be easily replaced should an explosion occur. A drenching apparatus, erected over each pair of runners, would have been activated in such circumstances. The original gearing for the incorporating mills survives in a chamber below ground level. The cross-wing partition walls have I-section girders and blocked openings in gables belonging to shafting for machinery probably installed for cordite manufacture. An open verandah once ran along the west front; cast-iron columns from this survive. On the south side of the Boiler House was a coal yard containing a chimney at the centre of its west wall. The building was served by a tramline to the west.

Incorporation was an extremely important process in gunpowder manufacture, involving the grinding and combination of saltpetre, sulphur and charcoal to form gunpowder. Waltham had pioneered many important developments in the process. This building is the first steam-powered incorporating mill to have survived on the site and most probably in the world. The drive shaft and machinery transmitting power to the mills were passed underneath the building to minimise the risk of explosion. Each incorporating mill consisted of a pair of large heavy edge runners of iron, which revolved on a large circular bed. This was the first and most complete incorporating mill to be built in T-shaped plan form comprising a central Engine House, rear Boiler House and cross wings, thus providing a model for later incorporating mills on the site; building L153 is an almost identical copy.