From the fourteenth to the nineteenth century, a painted key usually marks who holds power, over heaven, a city, a prison cell or a household.
A lock or a bolt marks the other side: what someone wants to keep shut, and who is left outside.
Twelve pictures follow, counted down from twelve to one, from Giovanni da Milano's panel of about 1365 to The Order of Release, painted by Millais in 1852 and 1853.
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How These Were Chosen
Each picture had to clear two bars. The first is that a key or a lock does real work in it. Usually the key is painted where everyone can see it, but sometimes it is hidden.
Raphael painted the papal keys into his portrait of Julius II, then covered them with green paint. De Hooch leaves the key to a linen cupboard out of the picture altogether, in the keeping of the woman who runs the house.
The second bar is that historians have written about that key, so its role rests on more than a hunch. A key that exists only in a title did not count.
One figure comes back again and again. Saint Peter, to whom Christ promised the keys of heaven, appears in five of the twelve pictures, and the countdown ends with the moment he receives them.
12. Carlo Crivelli, Saint Peter (1476)
Saint Peter stands on a marble ledge in full papal dress, and a pair of heavy golden keys hangs from his hand on a gilded cord. Carlo Crivelli painted him in 1476, in egg tempera on wood.
The panel was one piece of a great altarpiece made for the high altar of San Domenico, the church of the Dominican friars in Ascoli Piceno, a town in the Marche, in central Italy. At its center sat the Virgin, the special protector of the Dominican order.
Seen up close, Peter's keys are not painted at all. Crivelli carved them out of wood, nailed them to the panel and gilded them, so they jut out from the flat picture like a real object hung on it. They are the one part of the panel that could, in principle, be lifted off.
He raised other details too with pastiglia: gesso, a paste of chalk and glue, built up in relief and then gilded. The tiara, the clasp of the cloak, the rings and the crucifix on the staff all stand out from the surface, and a real twisted cord runs glued along the embroidered border.
Each thing in Peter's hands carries a meaning. The keys are those of the kingdom of heaven, which Christ gave him as a sign of his spiritual power, and in the same hand he grips a book of the Gospels, turned so that we see its gilded edges.
His other hand holds the ferula, the long papal staff, sign of the pope's power on earth.
Why dress Peter as a pope? The popes ruled as Peter's successors, and in the fifteenth century their claim to govern the whole Church was still contested, to the point of civil unrest in the Marche itself.
The friars who ordered the altarpiece stood on the pope's side. The Dominicans ran the Inquisition, the Church's court against heresy, and in the altarpiece Peter stood at the Virgin's right, next to their own protector, as the papacy they supported.
Crivelli adds a gesture of homage. Peter's red shoe pokes out over the edge of the marble ledge into our space, as if offered to be kissed, the way visitors kissed a pope's foot.
The altarpiece did not stay whole. It stood on the high altar until at least 1724, then was taken apart, and in the nineteenth century the Russian collector Anatole Demidoff bought panels from it and from a smaller altarpiece of the same church.
He mounted them in one grand frame for the chapel of his villa near Florence, and that is how the panel came to London. The National Gallery bought the whole ensemble in 1868, and it still carries his name, the Demidoff Altarpiece.
At a Glance
Artist: Carlo Crivelli (about 1430 or 1435 to about 1494)
Painted: 1476
Medium: Egg tempera on wood, probably poplar, with carved and gilded wooden keys
Where to see it: National Gallery, London
About This Painting
How big is Crivelli's Saint Peter?
Crivelli's Saint Peter measures 139 by 40.5 centimeters, about 54 3/4 by 16 inches. Its inventory number at the National Gallery is NG788.3.
Who paid for Crivelli's altarpiece in Ascoli?
The Blessed Costanzo di Meo di Servolo raised the money from the citizens of Ascoli. He had revived the declining Dominican priory, and he appears in the altarpiece as Saint Dominic.
Are the jewels on Saint Peter's tiara original?
No. The cut and polished glass on Saint Peter's tiara is a modern replacement, possibly for polished stones of the kind Crivelli used elsewhere. The silver parts of the tiara have tarnished.
11. Giovanni da Milano, The Apocalyptic Christ (about 1365)
An old man with snow white hair sits holding a blue orb in one hand and a large pair of keys in the other. He looks like God the Father, but he is Christ as the Book of Revelation describes him, the Son of Man who appears at the end of the world.
Giovanni da Milano, a painter documented between 1346 and 1369, painted this panel in egg tempera on wood about 1365. It was a pinnacle, the pointed panel that crowned the top of an altarpiece, and it follows the text of Revelation closely.
His keys look like the pair Saint Peter holds in so many pictures, but they open a different place. Christ is not holding the keys of heaven.