I have found two such box voussoirs in the Rochester Museum, each about 9 in. by 6 in. on the face and 5 in. on the soffit (Fig. [4]). The surfaces are roughly scored across with parallel lines forming an Χ. These two tiles together show an obvious curvature; they came from a villa at Darenth. In the Guildhall Museum I have also found a box voussoir which is almost identical with those at Rochester. It is thus described: “74, Flue (?) tile, red brick, the front decorated with incised cross lines; in the centre both front and back is a circular perforation: 9½ in. long, 6¾ in. high, 6½ in. wide.” The longest dimension is not in the direction of the tube, and the height is greater at one end than the other, so that the wedge form is quite apparent. The small holes in both the larger sides were doubtless to give better hold to the mortar in which they were set (Fig. [5]). Roach Smith recorded what must have been broken parts of similar voussoirs as found in Thames Street in 1848 (Journ. Brit. Archæol. Assoc., vol. iv.), but here they seem to have been used as waste material in building the little piers of hypocausts. Roman builders also constructed vaults of pipes and pots set in mortar concrete as were our box voussoirs, but I know of no British examples. Vaults of wide span seem to have covered large chambers in the Basilica at Verulam (see Victoria County History). The method of using the box voussoirs has been well explained from the Silchester examples by the late Mr. Fox in Archæologia (cf. Fig. [6]). A fragment at Westminster Abbey is either part of a voussoir or of a short flue tile (Fig.[7]).
Some notes made at Bath further explain the interesting methods of building vaults with box voussoirs. There are several such voussoirs in the ruins of the Great Bath, 12 in. to 13 in. deep by 6 in. and 6½ in.; 6¾ in. and 7½ in.; 8¼ in. and 10 in.; 8 in. and 11 in. at the top and bottom. Fig. [a]9] is a sketch of the third; it is scored on the face. The notches cut in the sides take the place of the holes in the London examples, and doubtless were for the mortar to get a better key; Fig. [a]10] is from a vault of this construction which was further strengthened by a series of curved tiles set in the outer concrete mass, which was 6 in. thick; Fig. [a]11] shows the ridge of such a vault—this may be an imagination of my own. One of the fragments showed six or eight flat tiles set longitudinally crossing the lines of the box-tiles (Fig. [12]). The ridge termination (Fig. [16]) is also from Bath.
Fig. 9.
Fig. 10.
Some large voussoir box-tiles from Gaul are shown in the British Museum, No. 394, in the section of Greek and Roman life.
Fig. 11.
