The medal as a medium of art was rediscovered during the Renaissance. Many of the rulers of Italy’s city states were mercenaries, soldiers of fortune who had achieved wealth and power on the battlefield. Educated in the fashionable new schools of the humanists, inspired by the poet Petrarch’s belief in “the pure radiance of the past”, they longed for dynastic and political legitimacy and sought to claim it by attaching themselves to the myths and legends of classical antiquity. The Caesars of old had been depicted in profile on their coins and medals, so that was how the Italian soldier-rulers of the fifteenth century decided that they should be portrayed too. Piero della Francesca painted the thin-lipped despot of quattrocento Rimini, Sigismondo Malatesta, in profile as if he were a Roman emperor on an intaglio gem or medal. He adopted the same formula in his famous portrait of Federigo Montefeltro, ruler of Urbino, posing him in a red pillar-box hat above the rolling hills of his Umbrian fiefdom.

The court artist of the d’Este in Ferrara, Antonio Pisanello, went one step further and revived the antique medium of the portrait medal itself. In his hands, the portrait medal became a powerful tool of dynastic propaganda. On one side of the medal, the ruler or his bride would be depicted in the standard profile pose. For the other side, Pisanello would invent some subtle device, an allegorical pattern, rather like the emblems of heraldry, meant to symbolise the powers and virtues of the man or woman to whom the medal was dedicated – a rock for steadfastness, an eagle for far-sighted calculation, a unicorn for chastity.

The British Museum has a strong collection of medals by Pisanello. So it might seem surprising that none should have been included in the enthralling exhibition of medallic art, from the Renaissance to the present day, currently on display in the cool dark rooms of the Prints and Drawings Department on the museum’s fifth floor. But there is a good reason for the omission. Pisanello’s medals, like many after them, were designed to honour those for whom they were struck, whereas the new exhibition explores the subversive underbelly of medallic art. Its title is “Medals of Dishonour” and it takes as its theme a rich and often wonderfully bizarre counter-tradition of medals made to insult, denigrate and disparage. The brainchild of a contemporary artist and medal-maker, Felicity Powell, it is one of the most original and compelling shows of the year.

The exhibition takes its title from the Medals of Dishonour created during the late 1930s by David Smith, who would later become the principal sculptor of the American Abstract Expressionist movement. Two have been included in the present show, although they are so large and so complex in their imagery that they might more aptly be described as plaques or small bas-relief sculptures. One of them, Private Law and Order Leagues, is a scabrous assault on the murderous racism of the Ku Klux Klan; the other, Cooperation of the Clergy, is an attack on priests and ministers who distort the teachings of their faith to foment war. In bronze landscapes of Surreal strangeness, atrocities unfold.

Even more touching, albeit simpler, is a rather earlier medal designed by a German Jew named Artnold Zadikow and simply entitled War. Created in 1915, as a protest against the horrors of the trenches, the medal shows an emaciated figure of death sitting astride a phallic cannon. Having fought for Germany in the Great War, Zadikow would eventually be betrayed and by the grinning spectre of Nazi evil. He met his death in Theresienstadt concentration camp in 1943.  

The origins of the anti-heroic medal are shown to lie far earlier than the twentieth century. The exhibition opens with a cornucopia of medallic satire. The gleeful celebration of an enemy’s weakness is a common theme. In 1588, a Dutch medallist called Gerard van Biljaer learned to his great joy that the Spanish Armada had been defeated. He cast a medal showing ships sinking and skeletal figures engulfed by waves, as the rulers of Catholic christendom sit in council, gnashing their teeth. English victory meant nothing to him; it was the defeat of Spain, chief foe of the United Provinces, that he wished to trumpet.

Medals of misrule form a distinct category, a form of counter-coinage to set against the actual coinage of a kingdom or domain. These might show Cromwell as a devil, Louis XIV as a vomiting, defecating buffoon, or Louis-Napoleon Bonaparte as a scarab. As early as the start of the eighteenth century, the essayist Joseph Addison had been struck by the contrast between the coins of ancient Rome and “those of a modern make ... often charged with Irony and Satire. Our Kings no sooner fall out, but their mints make war upon another, and their malice appears on their medals.”

The anti-medal really comes into its own in the early modern period, propelled by the humanitarian energies and ideals of the Enlightenment. John Gregory Hancock’s The Uncharitable Monopoliser, created in Birmingham in 1800, is a masterpiece of early social protest, a brilliantly emetic cartoon on a tiny disc of pewter. Outraged by the activities of  speculators in grain, whose hoarding of the commodity had caused widespread famine in the Midlands, Hancock  struck back with a concisely horrific image of obscene greed – a man, mouth stretched open like the maw of a boa constrictor, attempting to devour the world.

Moving into the twentieth century, then up to the present, “Medals of Dishonour” goes from strength to strength. The range of the exhibits is magnificent, including Marcel Duchamp’s Sink Stopper, which is a Conceptualist assault on the very notion of a medal, cast not to proclaim the virtues of prince or king, but simply to stop a leak in the shower of his holiday home in Spain. The father of English Pop Art, Richard Hamilton, who lent the Duchamp medal, also contributes a medal of his own, specially created for the show. It is the ghoulish Hutton Award, with its horribly grinning likeness of Tony Blair. Other specially created modern medals include Michael Landy’s memorably deadpan, double-sided homage to a repeat Asbo offender, and Langland and Bell’s sinister medallic meditation on the interlinked agencies of power and terror, entitled Virtual World.

Fittingly, the most brilliant reinventor of the medallic tradition in the modern day turns out to be Felicity Powell herself, whose idea the whole exhibition was in the first place. Her own medal, simply entitled Hot Air, is a diatribe on the theme of global warming. On one side, a hydra-headed monster speaks with forked tongue; on the other, a pair of human buttocks surface from water like the back of the whale, farting clouds of foolishness into the sky. Her medal itself reinvents the form itself, a kind satirical spool, on which is wound a ribbon of fatuities spouted by self-interested politicians.

Powell’s medal not only serves as a coda to the British Museum’s exhibition, but as an introduction to a truly breathtaking display of her most recent body of work at the Domobaal Gallery. Here, once more, she takes as her inspiration the gems and intaglios, the coins and medals of the Renaissance past. The result is a series of astonishingly virtuoso, utterly compelling and haunting miniature bas-reliefs on the theme of strange Ovidian metamorphoses – human heads that are turning into outcrops of coral, figures shape-shifting into trees, or octopi, faces wreathed in snakes. Worked, with wondrous subtlety, from white wax on dark mirror glass, each one is a miracle of ingenuity.

Collectively, this is one of the most impressive recent bodies of work created by any artist working today. Singlehandedly, Powell has revived and reanimated an entire Renaissance tradition – a rich and intricate tradition of subtle workmanship and symbolism, that includes not only the work of a medal-maker like Pisanello, but also the curious Mannerist fantasies of later Renaissance art and indeed the delicate traditions of relief sculpture as it was practiced by sculptors from Donatello to Arnolfo di Cambio. All this, she has revived and reanimated, bringing a dead language suddenly and startlingly back to life. It is a remarkable achievement.