A frenzy of white torsos in a Catholic excess

The Baroque style of the 17th century was a truly catholic movement in both senses of "catholic". It swept through Europe, from Italy to Bavaria, Bohemia, Russia, and even the American colonies of Spain and Portugal. As so often in art, it was an apparent return to the "old", to the ancient classical times, now viewed through the Renaissance.

This National Gallery catalogue that accompanied its recent display offers an introduction to one part of this globalising style. It is a partial view of one of its most rich and seminal areas. In Italy, for a variety of geopolitical reasons, the Baroque reached dizzying heights of creativity.

Sir Denis Mahon's gift of 70 paintings and 30 drawings that will go to public collections, along with the scholarly apparatus provided by this catalogue, should give a powerful impetus to Italian Baroque studies in Britain.

Mahon's bequest continues to arouse controversy. He is a dogged opponent of government policy, whether Conservative or Labour, which has encouraged the sale of work from public collections and the introduction of admission charges.

Any institution that decides to sell paintings from its permanent collection would find Mahon's works withdrawn by the National Art Collections Fund, which will retain legal title. The result of Mahon's lifelong work is celebrated in Michael Kitson's essay.

Until the 1930s, the study of the Italian Baroque was the special preserve of pioneer German and Austrian art historians. Important collections in Dresden and other museums provided a rich vein for study in the late 19th century. Art history was under the influence of Hegelian philosophy that enabled scholars such as Alois Riegel to appreciate the Baroque as a style.

In Britain, on the other hand, the 18th-century delight in the Italian Baroque was eclipsed by a high Victorian distaste for its Roman Catholic excesses. John Ruskin's view was that "there is no entirely sincere or great art in the 17th century".

It was only when political events forced art historians to leave Germany and Austria that continental research penetrated Britain. As the Nazis rose to power, high-ranking scholars, many of them Jews, such as Erwin Panofsky, Rudolf Wittkower, Fritz Saxl, Walter Friedlander and Nikolaus Pevsner, took refuge in Britain and the United States.

These teachers provided the young Mahon with his early understanding of the Baroque. A number of British art historians, among them Ellis Waterhouse and Anthony Blunt, shared Mahon's enthusiasm and for the first time put English Baroque studies on the map. This took place in the years after the newly established Courtauld and Warburg Institutes began their first art history courses in London University.

In recent decades Baroque studies in Britain have declined, and there are few students working at postgraduate level. The introductory essay in this publication explains why the Italian Baroque has been so neglected in Britain and Mahon's special role in freeing it from the accretion of late Victorian prejudice and ignorance.

It provides a fascinating account of how the first professional British art historians of the Seicento developed their expertise during the 1930s. Direct contact with the object of study is essential in art connoisseurship, and Mahon's growing personal collection provided such material. While Blunt became an international expert on French Baroque architecture and Poussin, Mahon focused increasingly on Guercino.

Baroque paintings may not be "easy" works to a modern viewer. Complexity and a multiplicity of layers of meaning and technique are intrinsic features, especially in the Bolognese school, which includes Giovanni Francesco Barbieri, Il Guercino, Mahon's favourite.

The Carracci brothers and their Academy, Luca Giordano, Guido Reni and others emerged from deep crisis in art, as the Mannerist school declined in the second half of the 16th century. In Bologna, Agostino and Annibale Carracci, with Ludovico their cousin, found an escape route through a new synthesis of artistic knowledge and practice.

As Charles Dempsey has shown convincingly in his book about the beginnings of the Baroque style, far from being eclectic, the Carracci school made a profound study of optics, natural philosophy, and artistic practice itself. They, and Guercino, with them, developed a new approach to light.

The study and re-creation of light as a medium of illusion, to order and heighten our experience and perception of the world, led to a painterly phenomenon unparalleled even in the Renaissance period.

Guercino's drawings reveal the artist's restless, unceasing exploration of movement, in his search for expressive form, using charcoal crayon, pen and wash and red chalk. Direct observation, continuous and acute study underpin the flamboyant flourishes that give movement and vitality to his work. A sketch of Saint George killing the dragon, for example, has a delicacy of touch combined with tremendous dynamism.

Two landscapes and a head by Salvator Rosa are of exceptionally high quality, revealing the dark and deeply disturbing side of the Baroque soul. Not everything in Mahon's collection is a masterpiece. But some of the less significant work reveals more homely, popular appeal. Its exalted, sometimes pompous ecclesiastical propagandism was counterbalanced by a down-to-earth depiction of life in the streets and among the poor.

Ruskin's latter-day supporters, such as The Sunday Times critic Waldemar Januszczak, fail to grasp the tongue-in-cheek, witty side of the Baroque, an aspect that became heightened in the Rococo period. Deliberate artifice, an orchestration of effects, ranging from the sublime to the ridiculous, is inherent in these great decorators.

In Andrea Sacchi's "Saint Anthony of Padua reviving a Dead Man", two plump cherubs hover over the proceedings, simply as light relief. Guido Reni's "Saint Francis" is consoled by a strangely shaped but entirely delightful floating musician angel. Newtonian gravity is defied, but artistic pleasure is satisfied.

Pompeo Batoni may have giggled to himself as he painted the serious look on his sitter's face as she caressed a lamb. In Johann Liss's "The Fall of Phaeton", the three naked nymphs provide an excuse for showing writhing plump female bodies, rather than a sense of tragedy in the scene. But what a delightful frenzy of white torsos and gesticulating arms against the flow of black drapery.

The catalogue entries focus in detail on questions of attribution, dating and provenance, giving a state-of-the-art account of the history of each object. Mahon and the National Gallery's team of curators and technicians provide the material for a new art historical approach to this most varied period.

It is up to future art historians to interpret and explain its aesthetic value. Discovering the Italian Baroque will no doubt remain the authoritative guide to each of the works for a long time to come.

Corinna Lotz is an art historian and critic.