The fullery on Mercury Street, like that just described, had been made over from a private house, built in the pre-Roman period. Among other changes, the columns of the large peristyle were replaced by massive pillars of masonry supporting a gallery above for the drying of clothes. At the rear are four square basins, the two larger of which are more than seven feet across; the water passed from one to the other as in the basins of Primus's fullery. In the corner near the last basin are six rectangular niches for treading vats, separated by a low wall, the purpose of which is clear from Fig. 225. There is a vaulted room at the right of the peristyle, with a cistern curb, a large basin of masonry, and a stone table. Here a substance was found which the excavators supposed to be soap, but which was doubtless fuller's earth, like that found in the establishment on Stabian Street. * * * * * There were naturally fewer tanners than fullers; and so far only one tannery has been discovered. That is a large establishment, however, filling almost an entire block near the Stabian Gate (Ins. I. v), excavated in 1873. Like the two larger fulleries, it occupied a building designed for a house. The appliances of the craft are found in only a small part of the structure; they relate to two processes,--the preparation of the fluids used for tanning, and the manipulation of the hides. The mixture for the tan vats was prepared in a tank under a colonnade opening on the garden. It could be drawn off through two holes in the side into a smaller basin below, or conducted by means of a gutter running along the wall to three large earthen vessels. [Illustration: Fig. 229.--Plan of the vat room of the tannery.] The vats, fifteen in number, are in a room formerly used as an atrium (Fig. 229). They are about 5 feet in diameter, and from 4 to about 5½ feet deep; they were built of masonry, and plastered; two holes were made in the side of each to serve as a convenience in climbing in and out. Between adjacent pairs of pits was an oblong basin about twenty inches deep, lined with wood. On either side of this was a large earthen jar, sunk in the earth; a small, round hole between the basin and each jar seems to mark the place of a pipe tile, connected with the former at the bottom. The large pits were for ordinary tanning; the oblong basins were probably used in making fine leather (_aluta_), a process in which alum was the principal agent, the chemicals being placed in the jars on either side, and supplied to the basins through the pipe tiles. In the same building four tools were found, similar to those used by tanners at the present time. One was a knife, of bronze, with a charred wooden handle on the back of the blade; two were scraping irons, with a handle on each end; and there was another iron tool with a crescent-shaped blade.