St. Peter’s Basilica is arguably the most famous Christian worship space in the world. It is a massive church in the style of the Italian High Renaissance located in Vatican City, an independent microstate enclaved within the city of Rome, Italy. A church that truly takes one’s breath away both in scale and splendour, it is regarded as the burial site of St. Peter, head of the twelve Apostles of Jesus and first Pope. Throughout 2026, Catholic pilgrims from across the world will journey to Rome to celebrate the 400th anniversary of its dedication.
While the basilica contains countless treasures, antiquities, and art, all insured for billions of euros, one particularly mysterious feature of this sprawling edifice are its “painted” altarpieces: twenty-four huge images mounted behind the church’s altars. More than 50,000 people visit the Church on any given day, each one craning their necks to admire these towering works of art. They transport the visitor to a higher realm of artistic appreciation, and provide one of the biggest shocks for visitors.
Upon closer inspection, they will notice not brush strokes on these artworks, but thousands of small edges marking tiny coloured tiles. These are not actually paintings at all, but mosaics of durable glass-ceramic tiles. From a distance, they look just like paintings, but are actually mosaic replicas of famous Renaissance and Baroque paintings. They were all painstakingly pieced together by master artists over roughly a century using 28,000 different colours of tiny tesserae (small pieces). Meanwhile, the famous originals were taken to other churches or museums to better preserve them. So, what are the secrets behind the mosaic altarpieces of St. Peter’s?
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Exterior of St. Peter’s Basilica (by author).
Going to Pieces
St. Peter's Basilica was built between 1506 and 1626. It is the world's largest church. Measuring in at 220 meters (720 feet) long, 150 meters (490 feet) wide, 137 meters (448 feet) tall, and covering 2.3 hectares (5.7 acres), it can hold up to 50,000 worshipers. There are hundreds of religious images and sculptures that decorate the eleven domes, curved ceiling arches, and altarpieces.
Before the current St. Peter’s was built, the First St. Peter’s Church stood in its place from the 4th to 16th centuries CE. Built by Constantine on the location of the ancient Roman Circus of Nero and Caligula (1st century CE), it witnessed the coronation on Christmas Day 800 of Emperor Charlemagne. After eleven centuries of use and various attempts to restore it, the building was ultimately deemed unsalvageable, and in the beginning of the 16th century, the project of replacing it was begun by Pope Julius II.
The Nave of St. Peter’s Basilica, painting by Giovanni Paolo Pannini, 1731. (Public Domain)
He commissioned the famous Renaissance architect Donato Bramante to replace it with a church so magnificent it would dwarf the monuments of ancient Rome. As the project began to stall, Pope Paul III asked a 71-year old Michelangelo to take over in 1546. The great artist agreed, designing the central dome, drum, and much more. Thirty years later, Michelangelo had died, and it was finally time to decorate the interior of the huge new church. Pope Gregory XIII decided to use mosaic on the vaults and domes, and to paint the altarpieces on canvas.
Why mosaic? Well, it was felt that it best represented the decoration used by the early Christians. Mosaic was meant to show continuity with history and did so in the boldest fashion, with an immense inscription in Greek and Latin, in mosaic letters on a background of gold that runs round the whole church.
Pina Baglioni explains:
“The intention of Pope Gregory was to link back to the mosaic tradition of the ancient paleo-Christian Roman Basilicas, overflowing with mosaics.”
Elizabeth Lev, Professor of Art and Architecture at Duquesne University, explains:
“long before frescoes adorned the vault of the Sistine Chapel, mosaic was the greatest artistic medium of Christianity. The mosaics of St. Peter's, often so perfectly executed as to pass for paintings by most visitors, shimmer all over the church, enhancing its transcendental quality.”
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The Altar of the Navicella (or little ship) derives its name from the Gospel narrative of Jesus walking on the water. He is helping Peter, who is sinking as he tries to meet Jesus. This mosaic was made in 1727 after a 1628 painting by Giovanni Lanfranco. (By author).
According to Paolo Di Buono, head of the Vatican’s Mosaic Studio:
“the painter Girolamo Muziano and the Venetian mosaicist Marcello Provenzale first decorated the dome of the Gregorian Chapel of the Vatican Basilica (1578-1580), and following that success, mosaic decoration was applied to Michelangelo’s large dome (1603-1612) and then to all the domes of the Basilica. Many Venetian mosaic masters were brought to Rome, where a school was formed to teach the artists.”
These Venetian masters had created their city’s 11th-century St. Mark’s Basilica, one of the best-known examples of Byzantine architecture and mosaics.
Less than a century after the church had been consecrated in 1626, the oil paintings were beginning to deteriorate inside the damp environment, full of candle and incense smoke. The moisture problem was mostly a result of the high amount of drying mortar inside the cavernous building. “While the domes were being decorated, already by the end of the 17th century, work began on the reproduction in mosaic of the Basilica’s painted altarpieces, which were deteriorating due to humidity,” Paolo di Buono says. The technique of mosaic was chosen to replicate the oil paintings, so that the altarpieces could survive for centuries with minimal upkeep.
For example, some of the greatest works of art of the Renaissance and Baroque periods hang in mosaic form in St. Peter’s, including Raphael’s Transfiguration of Christ, showing Christ turning to light and floating in the clouds above Elijah, Moses, and his disciples. This was his last painting, and he was still working on it when he died at only 37 on April 6th, 1520. Others finished it for him.
Detailed views of the Altar of the Transfiguration in Saint Peter’s Basilica. One of the most beautiful mosaics in St. Peter’s, this altarpiece is a reproduction of Raphael’s ‘deathbed’ painting, now in the nearby Vatican Museums. A team of six artists took nine years to execute the mosaic, finishing in 1767. The detail on the right shows the healing of the demon-possessed youth (by author).
The original oil painting of Transfiguration of Jesus by Raphael, now in the Vatican Museum Collection. (Artist Hideout/Public Domain)
According to Frank DiFrederico, “The idea for a mosaic copy after Raphael's Transfiguration goes back to 1744, when the Congregazione della Reverenda Fabbrica asked permission to copy the painting, then in the church of S. Pietro in Montorio. It was executed in mosaic from 1759 to 1767 by Giovanni Francesco Fiani and five other artists. The cartoon (template) had been painted by Stefano Pozzi, and the final acceptance was made in 1768.” Amazingly, the mosaic copy is double the size of the original painting, which hangs in the adjacent Vatican Museums.
Another example is The Punishment of the Couple Ananias and Sapphira, from the original painting by Cristoforo Roncalli in 1604. It depicts the punishment of the titular couple who had lied to St. Peter about the price of a field they had sold in order to keep some of the money (Acts 5:1-11). Sapphira falls to the ground in front of Peter, while in the background the dead body of her husband is being carried away. The vivid mosaic was created between 1725 and 1727, and continues to remind priests that they are called to give everything to God and not lie. The original panting now hangs in the Santa Maria degli Angeli Basilica in the old Baths of Diocletian in Rome.
Four detailed images of the mosaic altarpiece from the Altar of the Lie. This altarpiece is a mosaic work representing The Punishment of the Couple Ananias and Saphira, from the original painting by Cristoforo Roncalli (known as Il Pomarancio) of 1604. After lying to St Peter, Saphira fell to the ground before the Apostle; in the background two young men carry the dead body of her husband, Ananias (Acts 5:1-11). Painting by Roncalli, 1599-1604; mosaic created from 1725-1727 (by author).
So who are the men and women responsible for these amazing works of art? They work in what is called the “Vatican Mosaic Studio”, a small building with a hidden workshop that creates and restores the Papal mosaic masterpieces. Although you cannot visit it without a special tour, I could fortunately see the small building from the top of the dome of the Basilica. From this unassuming building a few doors down from the Pope’s private residence, has sprung and continues to spring some of the greatest works of art in the world.
Looking down from the top of St. Peter’s Dome towards the Vatican Mosaic Studio (the building circled in red, left, by author); Map of the Vatican showing the studio circled in red, right. (Map by Thoroe)
In the Studio
According to Paolo Di Buono, who heads the prestigious 500-year-old institution:
“The history of the Vatican Mosaic Studio dates from the beginning of the sixteenth century, when Pope Gregory XIII Boncompagni decided to entrust the painter Girolamo Muziano with the mosaic decoration of the dome of the Gregorian Chapel of the Vatican Basilica (1578-1580). Some of the most important painters of the time, such as Giuseppe Cesari known as Cavalier d’Arpino, worked on the preparation of the pictorial models for the mosaicists.”
Fascinatingly, a literal carpet of mosaic tiles were applied to the domes and vaults using a special stucco made of linseed oil. This stucco is another secret of the mosaics. It was developed by the Vatican artists in the sixteenth century, and its recipe has been carefully preserved and guarded for over four centuries so that it is still used today by the mosaicists of the Vatican Studio.
The first mosaic work of art in St. Peter’s was actually created for the First Church. Fragments of it were kept for installation in the new building. Called “The Navicella (“little ship”) of St. Peter”, it was created between 1298 and 1300 by the great Proto-Renaissance artist Giotto. This piece, perhaps more than any other, began the strong mosaic tradition of St. Peter’s that would find ultimate expression over four centuries later in the towering altarpieces of the 18th century.
A modern mosaicist at work in his studio, giving an idea of how the artists inside the Vatican Mosaic Studio work. (Jean-Pol Grandmont/CC BY 2.0)
The history of the mosaics of St. Peter’s is the history of technological developments in the craft. The first tiles used were mixtures of colored glass fused with metallic oxides, and they worked fine for the domes, vaults, and other parts of the church. However, in the late 17th century when it came time to replicate the great oil paintings from the church, a serious technical problem emerged amongst the artists, who were called painters in mosaic: the Venetian glazes then in use propagated a sparkling of colored lights that did not make the mosaic resemble a painting. To imitate a painting necessitated the creation of glazes of an infinitely extended chromatic scale, exactly like what could be achieved with a paintbrush.
The Vatican therefore set about achieving exactly this, by promoting research into new techniques and building its own kiln. The push to develop new techniques paid off in 1727, when Pietro Paolo Cristofari and Alessio Mattioli indeed discovered a means of producing opaque glazes with an extensive range of tints (such as crimson, which they could produce in sixty-eight different shades) and a new type of paste on a base of metallic limes called “peel”.
These glass-metal-oxide mixtures (pastes) were formed into opaque tiles called smalti which could then be broken into pieces with a small hammer. Their opacity negated the earlier unwanted sparkling effects, and along with the range of new tints, insured stellar results in transforming an oil painting into a mosaic. These discoveries prompted Pope Benedict XIII to permanently establish the “Vatican Mosaic Studio” in 1727. The studio maintains and restores the more than 10,000m squared of mosaics in Saint Peter’s Basilica. It also prepares gifts for the Pope to give to heads of State, and other pieces for commercial sale. Prices can exceed $230,000 for a single piece.
Lindsay Lambert gives us a rare peek inside the secretive space:
“Inside the almost silent studio, a handful of artists, some of them wearing lab coat–style smocks over their clothes, sit at individual workstations. Each nook is equipped with an easel and is littered with stacks of books and wooden trays filled with piles of tiny, colored glass tiles, which are made on site in nearly 28,000 different colors. To create the mosaics, each artist cuts his or her own tiles and uses tweezers to place each glass piece into a plaster base—the same manual method that mosaic artists have used for several centuries. Later, at the rear of the space, an artist pauses from his work on a mosaic of the Roman Coliseum. He says that mosaics he’s made have gone on to be gifted to several heads of state, the late President George H.W. Bush among them. A narrow wooden shelf directly below his canvas is littered with several thin sticks of uncut glass in amber and chocolate brown. A wooden tray filled with similar sticks in varying shades of green perches nearby. All are signs of the painstaking task at hand. More complex mosaics can take five months or more to make, he says—time well spent to keep this centuries-old art form alive.”
Elizabeth Lev says:
“Knowledge gained by experience abounds in the Vatican Mosaic Studio after 500 years of preserving ancient techniques and developing new ones.”
Paolo di Buono also notes that:
“Mosaic is a slow technique. In painting you see the results immediately; in our art you have to wait. A mosaic altarpiece can take two to three years to make. A master of this art must have great manual dexterity as well as a sensitive understanding of color. One must be in part a painter in order to be a good mosaicist. Today there is a renewed interest in mosaic. Most of the masters in the studio are young men and women and more are studying the art.”
A montage of views of the interior of St. Peter’s Basilica. Mosaics of various saints, the Virgin Mary, and Christ the Redeemer adorn the interior domes (by author).
A Brave New Basilica
While the Basilica is firmly rooted in its illustrious past, it has nonetheless warmly embraced the future. The Studio continues to carefully maintain the Vatican mosaics, with their most recent restoration of the Basilica’s altarpieces occurring from 2010 to 2012.
Di Buono explains that process:
“The most common problem we face is the “detachment” of sections of a mosaic, that (luckily) remain in situ and that need to be consolidated. The consolidation technique used has been derived from fresco restoration and uses special glues that allow the damaged portions to be reattached to their bases. The process involves making small punctures between the tesserae and then using syringes to fill the puncture holes with glue. Once the glue has dried, we continue upward and repeat the process on another portion, until, finally, we have completely consolidated the whole mosaic. A delicate final cleaning of the surface, with ammonia salts, gives the tesserae back their brilliance.”
Four mosaic altarpiece paintings, from left to right: 1) Altarpiece of St. Jerome; the original canvas altarpiece of the Last Communion of St Jerome is now in the Vatican Picture Gallery. 2) Altarpiece of St. Basil; showing the saint celebrating Mass in the presence of the Arian Emperor Valens. 3) Altarpiece of the Navicella; showing the Gospel narrative of Jesus walking on the water. 4) Altarpiece of the Presentation of the Virgin Mary in the Temple by her parents (by author).
Other technologies will enable people from around the world to study these amazing mosaics up close and personal. According to Michael Belfiore at National Geographic, “Immersive technologies are making it more accessible, both to in-person visitors and people all over the world. Even viewers at home can log in to a new interface to view a high-fidelity digital twin built using hundreds of thousands of photos through a process called photogrammetry and artificial intelligence (AI). Creating the Basilica’s digital twin required drones, specialized cameras, and laser scanning technology to capture more than 400,000 high-resolution images of every corner of the Basilica.”
One final secret remains to tell: there is only one painting in St. Peter’s Basilica that has not been converted into a mosaic. This is the altarpiece oil-on-canvas “Trinity”, painted in 1632 by Pietro Berrettini, aka Pietro da Cortona, in the Blessed Sacrament Chapel. Pietro da Cortona did design mosaics for St. Peter’s between 1652 and 1662, but personally decided against replicating his masterpiece in mosaic (which flattens the brushwork), leaving it in its original glory.
So, as the 21st century kicks off, and St. Peter’s enters its five hundredth year as Christendom’s most holy basilica, there is no sign of a visitor slowdown. The author can personally attest to its continued vitality and inspiration. It is well worthwhile to visit this spectacular church, the largest in the world with its thousands of treasures, at least once in your life, and to see for yourself those amazing “paintings” that aren’t really paintings at all.
Jonathon Perrin is an author and explorer. He has written five books on Amazon. He was fortunate enough to travel around Italy for a month in 2025 exploring everything historical. He is currently writing a book about his Italian adventures. Visit www.jonathonperrin.com for more.
References
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di Buono, Paolo, “The Mosaic Studio”, (https://www.basilicasanpietro.va/en/san-pietro/the-mosaic-studio).
Lambert, Lindsay, “Behind the Scenes at the Vatican’s Hidden Mosaic Studio”, Afar Magazine, May 29, 2019, (https://www.afar.com/magazine/behind-the-scenes-at-the-vaticans-hidden-mosaic-studio).
Lev, Elizabeth, “Mosaics: a Labor of Love; Louis’ Legacy Vatican Studio Adds New Twists to an Ancient Art”, Aug 25, 2005, (https://stpetersbasilica.info/People/LL/Liz%20Lev-Mosaic%20Studio.htm).
“6 Surprising Facts About St Peter’s Basilica”, Walks of Italy, (https://www.walksofitaly.com/blog/art-culture/st-peters-basilica-facts).
“Restoration work on the mosaics of St. Peter’s Basilica: An interview with Paolo Di Buono, head of the Vatican’s Mosaic Studio”, Inside the Vatican Magazine, March 1, 2015, (https://insidethevatican.com/magazine/culture/restoration-work-on-the-mosaics-of-st-peters-basilica/).
Belfiore, Michael, “Immersive technology reveals new views of St. Peter’s Basilica”, In National Geographic, February 7, 2025, (https://www.nationalgeographic.com/history/article/paid-content-immersive-technology-reveals-new-views-of-st-peters-basilica).
Official Website of St. Peter’s Basilica, (https://www.basilicasanpietro.va/en/san-pietro).
Tribe, Shawn, “Envisioning Old St. Peter's: The Interior from the Time of Constantine through the Renaissance”, In Liturgical Arts Journal, September 22, 2022, (https://www.liturgicalartsjournal.com/2022/09/envisioning-old-st-peters-interior-from.html).
Baglioni, Pina, “An art workshop in the heart of the Vatican”, In 30Days Magazine, 2006, (https://www.30giorni.it/articoli_id_10283_l3.htm).
Lubov, Deborah Castellano, “Vatican unveils initiatives to mark 400th anniversary of St. Peter’s Basilica”, In Vatican News, February 16, 2026, (https://www.vaticannews.va/en/vatican-city/news/2026-02/the-400th-anniversary-of-the-dedication-of-st-peter-s-basilica.html).
DiFederico, Frank R., The Mosaics of Saint Peter’s: Decorating the New Basilica, Penn State University Press, 1990.
