They were originally named for the baptismal font. Piscinae seem at first to have been mere cups or small basins, supported on perforated stems, placed close to the wall, and afterwards to have been recessed therein and covered with niche heads, which often contained shelves to serve as ambries. They were rare in England until the 13th century, after which there is scarcely an altar without one. They frequently take the form of a double niche, with a shaft between the arched heads, which are often filled with elaborate tracing.
Ordinarily the sacrarium is used in cleaning the vessels used during the course of the Mass (see Ablution in Christianity). Cleaning the vessels in this basin ensures that any remaining consecrated particles are returned directly to the Earth. If the consecrated Hosts become unusable, the priest disposes of the hosts by placing them in the sacrarium. This is accomplished by breaking the hosts up into small pieces and washing them into the basin - which returns the consecrated hosts to the ground.
At times the sacrarium has been used for disposal of other items, such as old baptismal water, holy oils, and leftover ashes from Ash Wednesday. In the past, consecrated wine was also poured down the sacrarium. In modern church practice, however, any wine that is left over after communion is consumed either by the priest or by those who assist in the distribution of the Eucharist (the Eucharistic Ministers).
In the Roman Catholic Church, pouring the consecrated wine, the Blood of Christ, or the Host down a sacrarium is never permitted. In accordance with what is laid down by the canons, “one who throws away the consecrated species or takes them away or keeps them for a sacrilegious purpose, incurs a latae sententiae excommunication reserved to the Apostolic See; a cleric, moreover, may be punished by another penalty, not excluding dismissal from the clerical state.” This applies to any action that is voluntarily and gravely disrespectful of the sacred species. Anyone, therefore, who acts contrary to these norms, for example casting the sacred species into the sacrarium or in an unworthy place or on the ground, incurs the penalties laid down.