Catholic Encyclopedia (1913)/Massa Candida
Under the date 24 August, the "Martyrologium Romanum" records this commemoration:
At Carthage, of three hundred holy martyrs in the time of Valerian and Gallienus. Among other torments, the governor ordering a limekiln to be lighted and live coals with incense to be set near by, said to these confessors of the Faith: "Choose whether you will offer incense to Jupiter or be thrown down into lime." And they, armed with faith, confessing Christ, the Son of God, with one swift impulse hurled themselves into the fire, where in the fumes of the burning lime, they were reduced to a powder. Hence this band of blessed ones in white raiment have been held worthy of the name, White Mass.
The date of this event may be placed between A.D. 253, when Gallienus was associated with his father in the imperial office and A.D. 260 when Valerian was entrapped and made prisoner by Sapor, King of Persia. As to the exact place, St. Augustine [Ser. cccvi (al. cxii), 2] calls these martyrs the "White Mass of Utica", indicating that there they were specially commemorated. Utica was only 25 miles from the city of Carthage, which was the capital of a thickly populated district, and the three hundred may have been brought from Utica to be judged by the procurator (Galerius Maximus).
The fame of the Massa Candida has been perpetuated chiefly through two early references to them: that of St. Augustine, and that of the poet Prudentius (q.v.). The latter, in the thirteenth hymn of his peri stephanon collection, has a dozen lines describing "the pit dug in the midst of the plain, filled nearly to the brim with lime that emitted choking vapours", how the "stones vomit fire, and the snowy dust burns." After telling how they faced this ordeal, he concludes: "Whiteness [candor] possesses their bodies; purity [candor] bears their minds [or, souls] to heaven. Hence it [the "head-long swarm" to which the poet has referred in a preceding line] has merited to be forever called the Massa Candida." Both St. Augustine and Prudentius were at the height of their activity before the end of the fourth century. Moreover, St. Augustine was a native and a resident of this same Province of Africa, while Prudentius was a Spaniard. It is natural to suppose that the glorious tale of the three hundred of Carthage had become familiar to both writers through a fresh and vivid tradition — no older than the traditions of the Civil War now  are in, say, the American South. It is not even probable that either of them originated the metaphor under which the martyrs of the limekiln have been known to later generation: the name Massa Candida had, most likely been long in use among the faithful of Africa and Spain. As Christians, they would have been reminded of Apocalypse 7:13-14, by every commemoration of a martyrdom; as Romans — at least in language and habit of thought — they were aware that candidates (candidati) for office were said to have been so called in Republican Rome from the custom of whitening the toga with chalk or lime (calx) when canvassing for votes. Given the Apocalyptic image and the Latin etymology (candor — candidus — candidatus; cf. in the "Te Deum", "Candidatus martyrum exercitus") , it was almost inevitable that this united body of witnesses for Christ, together winning their heavenly white raiment in the incandescent lime, which reduced their bodies to a homogeneous mass, should, by the peculiar form of their agony, have suggested this name to the African and Spanish Christians.
(For the casuistry of the self-destruction of the Massa Candida, see SUICIDE.)