The altar cavity is a small square or oblong chamber in the body of the altar. This cavity is called, in the language of the Church, the sepulchrum.
In it are placed, according to the Pontificale Romanum (De Eccles. Consecratione) the relics of two canonized martyrs, although the Cong. Sac. Rit. (16 February 1906) decided that if the relic of only one martyr is placed in it the consecration is valid, to these may be properly added the relics of other saints, especially of those in whose honour the church of the altar is consecrated. These relics must be actual portions of the saints' bodies, not simply of their garments or of other objects which they may have used or touched; the relics must, moreover be authenticated.
If the altar is a fixed or immovable altar, the relics are placed in a reliquary of lead, silver, or gold, which should be large enough to contain, besides the relics, three grains of incense and a small piece of parchment on which is written an attest of the consecration. This parchment is usually enclosed in a crystal vessel or small vial, to prevent its decomposition. The size of the cavity varies to suit the size of the reliquary. If it is a portable altar the relics and the grains of incense are placed immediately, i.e. without a reliquary, into the cavity. This cavity must be hewn in the natural stone of the altar. Hence, unless the altar be a single block of stone, a block of natural stone is inserted for the purpose in the support. The location of the cavity in a fixed altar is
- either at the front or back of the altar, midway between its table and foot;
- in the table (mensa) at its centre, near the front edge;
- in the centre, on the top of the base or support if the latter be a solid mass.
If the first or the second location is selected, a slab or cover of stone, to fit exactly upon the opening, and for this reason somewhat bevelled at the corners, must be provided. The cover should have a cross engraved on the upper and nether sides. If the third location is chosen the table (mensa) itself serves as the cover. In a portable altar the cavity is usually made on the top of the stone near the front edge, although it may be made in the centre of the stone.
Read more about this topic: Altar (Catholicism)
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