According to Harris, the mirror in the glass drawing room at Northumberland House ‘represents a considerable advance from the Kenwood library mirror of 1769-70. Not only are the ornaments much more intricate and refined, but the glass as a whole is now fully assimilated into the wall decoration, and can no longer be considered furniture.’ As such, the mirrors from this room were removed along with the wall panels prior to demolition in 1874. The pier glass was executed in accordance with Adam’s design, albeit with alterations to the sides. The executed overmantel mirror frame represents a middle-way between the two variant schemes shown in the extant drawings (SM Adam volume 20/17 and 22/55).

Adam’s first design for the chimneypiece in 1773 (Adam volume 22/55) was unexecuted. It is Etruscan in style and appears to have inspired Adam’s design a year later for the chimneypiece in Lady Derby’s dressing room at 23 Grosvenor Square (SM Adam volume 23/51). The executed chimneypiece in the glass drawing room at Northumberland House follows Adam’s 1774 working drawing exactly (SM Adam volume 18/53). The chimneypiece was removed from Northumberland House prior to demolition in 1874, and survives in the private green drawing room at Syon.