SACRARIO (piscina, lavacrum). — A small underground cistern covered by a movable stone, placed in a corner of a sacred place or the sacristy or behind the altar, which communicates externally with a drainpipe or ends in an uncemented pit for water absorption.
It serves to receive the water from ablutions, baptismal water after the administration of the Sacrament, the water in which purificators and corporals have been washed, and the sacred ashes from things burned (e.g., from the cotton used in sacred unctions). All this is done out of reverence for things once blessed or consecrated (Durandus, Rat., I. IV, cap. 55). For the East, there is a testimony from the 6th century in the following canonical resolution given by Giovanni Bar Cursos, bishop of Tell-Mahré (519-38): «Aique ablutionis rerum sacrarum in locum decentem, in fossam profundam proiiciantur et occultentur» (text cited by Hanssens, op. cit. in bibl., III, p. 533).
In medieval churches, beside the altar, on the Epistle side, there was very often in the wall a niche called piscina, artistically made with a basin equipped at the base with an opening for the free flow of water; it served for the ablution of the priest’s hands after Holy Communion (beginning in the 14th century, the ablution was drunk); it is prescribed, for example, by the Synod of Würzburg of 1298, with 3, and by the Cistercian General Chapter of 1601.
With the term s., in the Middle Ages, were also designated the church, the presbytery, the wall cupboard, or the place for keeping the piscina containing the Blessed Eucharist, the sacristy or the “pastorhips” of the Eastern rite (cf. Didascalia et Constitutiones Apostolorum, II, 57; VII, 17; I, 17) and sometimes the book called Ordinarius.