The baptistery is pictured at the Basilica of St. John Lateran in Rome Dec. 9, 2020. Vatican restorers are tackling one of the oldest mosaics in Christianity, dated to around 450 AD, in the vestibule of the baptistery. CNS photo/Paul Haring
The baptistery is pictured at the Basilica of St. John Lateran in Rome Dec. 9, 2020. Vatican restorers are tackling one of the oldest mosaics in Christianity, dated to around 450 AD, in the vestibule of the baptistery. CNS photo/Paul Haring
VATICAN CITY – Even though the Baptisms the Pope celebrates most years in the Sistine Chapel are better known, the most important place to be baptized in Rome for the past nearly 1,600 years has been the baptistery of St. John Lateran.

The ancient baptistery was built in 440 A.D. just behind the Basilica of St. John Lateran, which was the first Christian basilica founded in Rome by Emperor Constantine in 324 A.D.

For generations, it had been the only baptistery in Rome, and, according to the baptistery website, there has never been a year since it was built that a baptism has not been celebrated within its walls.

Almost as old as the fifth-century structure itself, there is a half-dome of mosaics from about 450 A.D. decorating a side chapel, and experts believe they are among the oldest mosaics in Christianity.

"But there's a problem here. A restoration problem. So the Patrons of the Arts in the Vatican Museums have stepped forward once again" to dedicate the funds needed to restore the badly damaged and important piece of Christian art and history, Mary Angela Schroth, the project's coordinator, told Catholic News Service.

Most often, water, time and grime are an artwork's worst enemy, but sometimes the damage is caused by past restorers. That is what mosaicist Roberto Cassio and his small team from the Vatican Museums found when they set up scaffolding and got a close look at mosaics in the chapel's small apse and on a side wall, Schroth said.

For example, a huge metal clamp juts out from the side wall, jerry rigging into place a large panel of stone mosaics embedded in cement – one of the worst mediums to use for supporting mosaics, Cassio told CNS.

Cement is too heavy, too rigid and "irreversible" when future repair or removal is needed, he said. Also, soluble salts in the cement eventually form "crystals" that damage the mosaic surface.

He said they will remove the cement supports and use a lighter mortar, more in line with current conservation methods.

One restorer in early December was cleaning the marble fragments on the side wall after the cracks and grout work were repaired. Some pieces of dark green serpentine from Greece, red porphyry from Egypt and white palombino from Italy are different shades, Cassio said, since early artisans often culled their materials from assorted Roman ruins and monuments.

Schroth said the mosaics are one of the few examples of "opus sectile" – the technique of cutting and inlaying stone to form pictures or patterns – to still be in the same place where they were created, rather than having been moved to a museum.

"It is a miracle it has survived to our day," Cassio said.

While the cleaning and repairing of the stone mosaic on the side wall will be fairly routine, the work on the glass mosaics in the apse will be more innovative, Schroth said.

The mosaics in this chapel, dedicated to Sts. Cyprian and Justina, are opaque tiles of glass, which offer mosaicists many more colors to choose from than the limited palette of stones.

"It isn't colored glass," however, Cassio said.

There is a different "recipe" for each color using different minerals and colorants so that when the glass is forged in a furnace, the additives react to the heat, making a unique color, he said.

What made early Rome stand apart from other cities with its mosaic work was the desire that images look more like paintings, he said, so early artisans kept coming up with "new recipes" for new colors, which numbered some 20,000 hues in the 1700s, resulting in "spectacular creations."

Today there are more than 26,000 colors available, said Cassio, a third-generation mosaicist, who started helping his family's business when he was 10.