PIETRA SACRA (petra sacra, tabula sacra; gestatorium, itinerarius, viaticum). — A square or rectangular slab (25 × 30 cm) consecrated by the bishop and containing a recess in which relics of holy martyrs are placed; it serves as an altar for celebrating Holy Mass wherever there is no fixed consecrated altar.
It is called a portable or gestatory altar because it can be carried from one place to another, being particularly suited to travel (altare itinerarium, altare viaticum, lapis itinerarius); it is also called a superaltar because it is set into a movable or fixed unconsecrated altar. The CIC, can. 1197, 1, 2, prescribes that it be a single, intact natural stone, sufficiently large to contain the Host and most of the chalice. It began to be used after Constantine, when the altar in the church, until then movable, became fixed and it was no longer permitted to celebrate Holy Mass outside a consecrated altar. The first certain information in the West dates from the sixth century (from a letter of 511 addressed to two English priests who were going as missionaries to Britain); but it may already have been in use, since the letter treats it as something well known and necessary for celebrating Holy Mass during journeys. Then St. Bede relates that two St. Ewalds, missionaries among the Saxons, took with them a board to serve in place of an altar.
During the first millennium, stone or wooden boards were used (e.g., the oak board covered with silver plates found in the tomb of St. Cuthbert at Durban, Ireland); often the p. s. was inserted into the wooden board or enclosed in a wooden frame or box; sometimes it was very small, made of porphyry, onyx, rock crystal, or even slate; the relics were placed between the stone and the surrounding wood or in the wooden box. These latter forms are no longer permitted today (decree aut. 3162 of 31 August 1867). In the Middle Ages, the rectangular form was preferred, sometimes also oval or round; from the fifteenth century onward, the square form. Its dimensions range from 48.5 × 36.7 to 12.5 × 7.5 cm (the smallest p. s. is that of Egbert’s Portable at Trier and measures 2.4 × 2.4 cm). St. Carlo Borromeo prescribes 36 × 29 cm; the code merely specifies a size sufficient to contain the Host with most of the foot of the chalice. In the consecration of a fixed altar, relics to be enclosed in the sepulchre were always required; for portables, however, in the Middle Ages up to the thirteenth century, there were no precise regulations. As signs of consecration, small crosses were inserted from the fifteenth century onward.
Outside a church or a public or semipublic oratory, only those who possess the privilege may celebrate Holy Mass on a portable altar.