Virgins of Venice, by Mary Laven

Jan Morris hears tales of spin, sin and suffering behind Venetian convent walls

"Get thee to a nunnery!" had alternative meanings if you happened to be a young woman in Renaissance Venice. It might confirm that you had discovered in yourself a truly sacred vocation. It might promise you a chance of a more comfortable life. It might mean that your patrician family, unable to pay the crippling marriage dowries that were current then, thought it more economical to wed you to Christ. It might even mean that you were being sent to one of the neo-convents (the Convertite, the Soccorso) whose function was to convert or succour fallen women.

There was plenty of choice, Heaven knows. There were said to be more convents per square foot than any other city in Europe. They were nearly all shut when Napoleon seized Venice, but you may still see the remains of them scattered around the place, generally forlornly transmuted into prisons, barracks or schools.

Mary Laven neatly describes them as part of the Serenissima's "spiritual economy". They could almost be defined as an Estate of the Realm, so powerfully symbolic was their place in the structure of the Republic. Resilient, too, is their reputation for salacious and scandalous goings-on. My guess is that this book's title was wished upon it by its publishers, well aware of the erotic allure that attends any nunneries, especially those of Venice. It is really, though, a skilfully extended doctoral thesis: scholarly, diligent, with frequent moments of fun and only a modicum of libidinous detail – a much more profound exercise than its trashy title might imply.

The ideal of virginity had special force in Venice. The city was famously virginal itself – Wordsworth's "maiden city, bright and free" – and its communities of theoretical female celibates like vestals round the civic hearth. Once a year the Doge "married" the Abbess of Santa Maria delle Vergini, but he certainly did not consummate the union, any more than he slept with his other ritual bride, the Adriatic. The sacred maidens of Venice were hypothetical hostages to the God who had given the city its surrogate divinity, and preserved it involate.

Their nunneries were also tokens of Venetian order. This was a city of segregations. The nobility were tightly bonded in social unity and political power. The gentry frequented their own societies. The working classes knew their patriotic place. The courtesans were catalogued. The Jews were in their ghetto. The innumerable clergy moved through the city on a plane of their own. And the thousands of nuns were immured in their 30-odd convents, many forbidden by law to move outside them, or have any but the most rigidly supervised contact with the world outside.

Laven makes it plain that nunnery conditions did vary from century to century – eddying, as it were, around the affronted fundamentalism of the Counter-Reformation. However, the picture she paints, drawn largely from her researches among archival papers, is startling enough. Severely enclosed though they were, the convents were adept at networking. They varied in social clout, but the most aristocratic, the Roedeans of virginity, still maintained close contact with inmates' influential families, and could exert genuine pressures upon the affairs of state. At some periods their visitors' parlours were glitzy centres of social life, with dances and dinner parties at which the nuns were by no means always required to watch the gaiety through their protective grates.

So the boundaries were "smudged", as Laven puts it, "between the world of sin and these islands of sinlessness". Sometimes defiant nuns ran away. Often they consorted, against all the rules, with the women who clustered around convent doors, and were generically described by the authorities as "whores, bawds and witches". Prostitutes frequently sororised with nuns; Laven calls them "mirror-images" of each other, and in 1509 it was alleged that 15 convents were themselves really no more than whorehouses.

Men risked awful penalties by penetrating the virginal enclosures – sometimes just to flash their members, sometimes for a bit of kiss and cuddle, sometimes to go the whole hog in a nun's cell – and conniving with them was just as dangerous. In 1614 two nuns at the convent of San Zaccaria bashed a hole in the wall to enable their lovers to enter, and a married couple who gave them a hand felt the full scorching disapproval of the Virgin City: the husband got six years in the galleys, the wife was flogged from San Marco to Rialto and told that if she ever entered a nunnery again her nose and ears would be cut off.

Do not suppose, though, that Virgins of Venice is all sex and sensation. It is not that kind of book at all, and Mary Laven is not that kind of writer. On the contrary, it is essentially a work of analytic pathos and compassion, and of all its characters the one who will live longest in my memory is "a man named Santo who was found in the parlour passing a rose to a nun through the grate". Much true love and affection lay wasted behind those grim stone walls, and who can doubt that many a good woman, never to appear in the archives, lived out her life there in virgin Godliness?

Jan Morris's 'Trieste and the Meaning of Nowhere' is now in Faber paperback