Technically speaking, it is not an altar but an enormous and enormously rich, complex altar frontal -- over three meters long, all of silver with dozens of figures of angels, prophets, saints surrounding twelve scenes from the life of Saint John the Baptist and large central figure of the saint. It took over a century to complete, involving the most renowned goldsmiths of the late fourteenth century through the great renaissance artists Michelozzo, Antonio del Pollaiolo, and Verrochio. To our eyes, one of the greatest differences is how wonderfully many figures stand out against cleaned coloured enamel backgrounds of their niches. But the whole 'altar' is stunning, beautifully displayed and lit.
Visitors to the Museo can see the 'altar' any day, unlike people of the renaissance itself: for centuries, it was only displayed on Florence's greatest annual event, the June 24 celebration of San Giovanni, and occasional other major days.
And, if you do not know the Museo dell'Opera, it is one of the very best in Florence -- which means one of the best in the world, really -- even though over-shadowed in fame by the Uffizi, the Bargello, and the unfortunate Accademia burdened by throngs wishing only to say they have seen 'The David'. Yet the Museo displays a wealth of extraordinary works originally on and in the cathedral of Santa Maria del Fiore and the Baptistery of San Giovanni -- renaissance masterpieces by Donatello, Ghiberti, Luca della Robbia, and the list goes on (yes, Michelangelo too). To boot, it is one of the most simply enjoyable museums in town, with unusually good explanations of many of the works: and due -- with its expansion completed in 2015, we hope -- to become remarkable for displaying great masterpieces in a recreated context evoking their original locations on the cathedral facade and its facing baptistery, and their original meanings for renaissance observers, not simply as museum pieces.
In the meantime, another treat is in store: the great 'Doors of Paradise' by Ghiberti from the Baptistery, under piecemeal restoration for many years, are due to be unveiled on September 8 this year as, once more, a complete portal in the Museo dell'Opera. (Not coincidentally, September 8 is the anniversary of the historic commitment in 1296 (or 1298) to build the new cathedral, on the religious celebration of the birth of Mary.)
Here is the link to the Opera di Santa Maria del Fiore, the museum and more -- http://museo.operaduomo.fi.it/ -- with tantalizing images of some of its glories which inevitably fall far short of their impact 'in person': especially, I think, for sculptures more than paintings, and even more especially for sculptures made for particular locations, fulfilling religious and civic purposes. Remember, the Museo dell'Opera is the immediate neighbour of the cathedral still today maintained by the Opera (offic: see and appreciate them together.