This plaque depicts Bartolomeo Alberto Cappelari, Pope Gregory XVI, and was presented as a gift to Antonio Matteucci, secretary and treasurer of the Vatican Mosaic Workshop, in 1840. When a chronological series of Papal portraits were destroyed in 1823 at the Basilica of St Paul, the Vatican Workshop took charge of producing new versions in mosaic, and the tradition has continued ever since. A rectangular version of this portrait by Luigi Moglia can be found at the Hermitage, St Petersburg.

Raffaelle Castellini belonged to an important dynasty of mosaicists active in the Vatican Workshop, where he became a professor at the end of his career.

The term 'micromosaic' is used to describe mosaics made of the smallest glass pieces. Some micromosaics contain more than 5000 pieces per square inch. The earliest attempts at micromosaic revealed visible joins between the pieces (known as tesserae) and a lack of perspective. Later artists such as Antonio Aguatti made huge advances in micromosaic technique, resulting in renderings that were truer to life. Glass micromosaic technique developed in the 18th century, in the Vatican Mosaic Workshop in Rome, where they still undertake restoration work today.

Sir Arthur Gilbert and his wife Rosalinde formed one of the world's great decorative art collections, including silver, mosaics, enamelled portrait miniatures and gold boxes. Arthur Gilbert donated his extraordinary collection to Britain in 1996.