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Piscina at St Nicholas church, Walcot
Wikimedia Commons

A piscina is a small basin usually built in the wall on the south side of the altar in a Christian church, used for washing the priest’s hands and communion vessels at Mass; the plumbing that takes the water to the earth is called a sacrarium. There may sometimes be two piscinas, side by side, one for washing the priest’s hands and the other for the sacred vessels.[1]

Piscinas were introduced into churches by Pope Leo IV in the 9th century, and most surviving examples are medieval, some big enough to double as a font. The washing of communion vessels is now done in the sacristyRoom in Christian churches for the storage of liturgical vestments, sacred vessels and parish records..[2]