We don't have the names of the scores of martyrs -- Saint Augustine says 153 of them but the Roman Martyrology says 300 -- who were collectively called the Massa Candida. We know, however, of the gruesome way they were killed during the reign of the Emperors Valerian and Gallienus.
Today's martyrs were arrested in Utica, not far from Carthage in North Africa. The procurator, Galerius Maximus, ordered an altar to Jupiter to be set up near a blazing lime kiln. He then told the prisoners that they could either burn a little incense on the altar or be burned in the kiln. With the resolution for which saints and large groups are noted, the Christians moved forward and flung themselves into the pit.
A word about lime kilns. Their purpose is to bake quicklime out of limestone. Thus, layers of burning material and stone are built over a grate through which the quicklime seeps onto a chamber below. They operate best at a temperature of 900 to 1000 degrees. It definitely takes some serious resolution to propel oneself into the pit -- especially as you heard the brief screams of those who preceded you over the the roar of the furnace.
They became the Massa Candida because their remains were reduced in the flames. The only thing left after the fire is the white mass of lime at the bottom of the kiln. The Catholic Encyclopedia offers a nice connection to Revelation 7 as well.
And now a word about Valerian and Gallienus. Valerian was captured by the Persian king Shapur II and for a while he was used as a stepping-stool. Every time Shapur wanted to mount his horse, Valerian knelt on the ground so the king could step on his back to climb up. After this ceased to be novel, Shapur had the former emperor killed, stuffed, and mounted. Gallienus was assassinated by his troops during a rebellion.