The sixth programme in the series "A History of Britain" explores the lost world of Catholic Britain.

The Swansea Altarpiece is one of very few medieval altarpieces which survive. It consists of a series of pictures carved from alabaster which read from left to right like a strip cartoon. The four smaller panels tell the story of the Virgin. First we have the Annunciation and the Adoration of the Magi, showing Mary as the Mother of God. To the right are the matching scenes of Christ's Assumption into Heaven, where we can see only His feet as He soars out of shot, and the Virgin's own Assumption, combined with her triumphal Coronation by the Trinity. The central panel also shows the Trinity:God the Father, as an old man, the Son on the cross, and the Holy Spirit (the Dove) which has now been lost, leaving only a hole at the top of the Cross where He would have been fixed.

The whole narrative is framed by the two St Johns. The Baptist holds his lamb and wears his traditional camel robe ­ you can see the head and hooves of the unfortunate creature hanging down. Meanwhile St John the Evangelist has a chalice from which emerges a tiny dragon, and his palm. These saints were two of the most popular in pre-Reformation England and with others like St Catherine and St Margaret, would have been as familiar to the viewer as their own family. A Somerset will from the period refers specifically to St John the Evangelist "whom I have always worshipped and loved…". Each saint had symbols which provided a short cut to recognition ­ St Catherine with her wheel for example. They formed a language which every late medieval churchgoer would have understood.

This altarpiece therefore gives us an enticing glimpse of the "lost world of Catholic England" hidden from us by the veil of the Reformation and of the intervening years. This world was one which might be more familiar to Latin-American worshippers than to modern English people. Churches were ablaze with colour and gold highlights glittered in the light of many candles. They were stuffed with devotional images which were adorned on festival days and adored on others. The Swansea Altarpiece retains more of its original colouring than most other surviving alabaster panels, but is still really a shadow of the glorious object it must once have been.

Altarpieces were designed to stand on or just behind the altars in medieval churches. Roger Martyn in the sixteenth century described the altarpiece behind the High Altar at Long Melford as "a goodly mount carved very artificially with the story of Christ's passion all being fair gilt…". Almost every church would also have had side altars dedicated to individual saints or commonly to the Virgin, often in a Lady Chapel named after her, and each of these required some sort of altarpiece. It seems likely that the Swansea Altarpiece stood originally on a Lady Altar, or just possibly on the High Altar of a church dedicated to the Virgin or the Trinity. The panel of the Trinity particularly provides food for thought during the celebration of the Mass, with its graphic representation of Christ's body on the cross and His blood flowing down to be collected by angels in chalices which would not have been dissimilar to those used during the ceremony. Of course Catholic England believed in the real presence of Christ Himself in the Mass, and altarpieces like this one were designed to remind the worshipper of this. They played a key role in the theatre of late medieval religion. Closed or covered during Lent, they would be flung open, or triumphantly unveiled in all their colourful glory at Easter or on other holy days. Although the Swansea Altarpiece is hinged into three sections, enabling it to stand on an altar, the reverse is rough and it is therefore unlikely that it was designed to fold up like many other altarpieces from the same period. It would therefore have been covered with a curtain or veil as necessary.

We do not know who commissioned the Swansea Altarpiece. It may have been a wealthy individual, a group (possibly a guild) or even a religious order. There are no firm indications at all ­ no figures of donors with useful inscriptions, nor coats of arms. Although we might wonder if the presence of two saints called John is a clue, the same pairing appears on another alabaster altarpiece depicting scenes from the Virgin's life, the so-called "Joys of the Virgin". This anonymity is typical of fifteenth century alabaster panels which were churned out in large numbers with very similar designs, and few distinguishing features. In the Victoria and Albert Museum's own collections are other alabaster versions (of the Adoration of the Magi for example), which are almost identical to the scene on the Swansea Altarpiece, even down to the arm of the king who points out the star, and the hunched figure of Joseph, huddled in the corner.

The medieval alabaster industry in England seems to have been based primarily in Nottingham and the panels were clearly popular both at home and abroad. Examples exist as far afield as Iceland, Croatia and Spain. The surviving wooden settings seem to be similar as well, indicating that such altarpieces were exported whole. People tend to think of the medieval world as being intensely local, but actually it was also very international. A common Christendom under the Pope and the universal language of Latin provided a form of European Community long before the twentieth century. Merchants teemed back and forth between England and the Continent importing tapestries, printed books and other luxury goods while exporting lead, tin and of course alabaster panels and individual figures. This is lucky for us, as during the Reformation many of the alabasters from English churches were defaced or completely destroyed. Today they survive all over Europe to tell us something of the splendour of the pre-Reformation church in England.

Alabaster, largely obtained from the Midlands quarries of Derbyshire and Staffordshire, was a wonderful material for carving ­ soft and easily shaped with the most beautiful translucency which shone through the colours and gilt applied to it. The Swansea Altarpiece is one of the finest surviving examples of alabaster carving in the world, and shows us clearly why medieval churches across Europe could not get enough of these beautiful objects.

Eleanor Townsend, Assistant Curator, V&A.