Show displays magic of father of modern art

Given the popularity of Cézanne today, the ready acceptance and enjoyment of the paintings by the artist considered the father of modern art, it is difficult to appreciate just how odd his art seemed when first seen in the late 19th century.

But when Paul Cézanne's now iconic painting of La Montagne Saint-Victoire was shown at an exhibition in Aix-en-Provence in 1895 it was met with incomprehension. Here was a picture of a mountain framed by pines, branches following the lines of the land, with a patchwork of fields beneath in yellow orange and greens: a tautly structured, somewhat flat looking landscape painted in a strangely uneven fashion.

Rejecting conventions, Cezanne was perhaps the first to break away from the practice of 'finished' paintings - he had used hugely varying amounts of paint, sometimes densely applied and sometimes, as in the lower right corner, deliberately omitted or thinly washed.

It was a scene he painted many times, from many angles. The mountain, lying to the east of Aix-en-Provence where this son of a prosperous banker was born in 1839, was emblematic to him of his home and its landscapes. But his treatment of the painting was less to do with replicating the superficial appearance of a landscape, and more with expressing what he would describe as a "harmony parallel with nature".

Perplexing as it was to most at that 1895 show, one viewer, the young poet Joachim Gasquet, admired it - and said so. Cézanne was so moved by this praise that he signed the picture and gave it to him. It is one of the few works after the 1880s that have his signature. Two years after Cézanne's death in 1906 Gasquet sold the painting for the high price of 12,000 francs.

In 1925 the same picture was bought for considerably more, £14,250, by the textile manufacturer and art collector Samuel Courtauld (1876-1947). By then in the habit of buying one if not more Cézanne paintings a year, Courtauld had first come across the artist's work three years earlier and, in his own words, "felt the magic of Cézanne" ever after.

Always buying by instinct, he assembled most of these pictures between 1923 and 1929, a time when Cézanne was still regarded with great scepticism in the UK. Courtauld's passionate commitment to Cézanne's art helped establish his reputation in this country and from the outset works by Cézanne were at the heart of his collection. The Cézannes were a major part of his gift that founded the Courtauld Institute of Art in 1932, the first academic centre in this country to study the history of art.

Today this collection, the largest and arguably finest group of works by Cézanne in the UK, is on show for the first time in its entirety as the finale of the gallery's 75th anniversary celebrations.

The Courtauld Cézannes presents work from the major periods of his long career, with well over half of the works not typically on display. It includes paintings such as the Montagne Sainte-Victoire and Card Players (1892-5) showing the artist at the height of his powers, and more experimental pieces such as Lac d'Annecy painted by a bored Cézanne on holiday with his wife and son in 1896.

He wrote dismissively of the view in a letter to his now friend Gasquet, saying the conventional beauty of the landscape was "a little like we've been taught to see it in the albums of young lady travellers". In the vibrant oil we see in the exhibition, in its cool greens and blues, one feels the depth of the lake and the depth of Cézanne's determination to transcend the commonplace picturesque.

The exhibition also includes the first Cézanne ever purchased by Courtauld, Still life with Plaster Cast (c1894) a complex painting with radically distorted perspective that prefigured the advent of Cubism, and rarely seen drawings and watercolours.

Among them is the brilliantly beautiful Apples, Bottle and Chairback (c1900-6), a supreme example of Cézanne's mastery of the watercolour medium. Remarkable for its scale, its luminous washes of colour and the web of interlocking graphite and coloured lines that dance to give form to the fruit, the still life provides a perfect example of how in his work Cézanne blurs the distinctions between line and colour, drawing and painting.

Having said that, however, the piëce de résistance of this exhibition just has to be the previously un-exhibited group of nine autographed letters in which Cézanne reflects upon on the principles of his artistic practice.

In the first known of the letters to his protégé Emile Bernard, written on April 15, 1904, in gloriously baroque handwriting, Cézanne famously advises the younger painter to "treat nature in terms of the cylinder, the sphere and the cone". In the last, a moving letter written one month before his death, the "old and sick, pig-headed" man he calls himself is still insisting that art should be based on constant study of nature.

There's a lot packed into this small, rather special exhibition at the Courtauld Gallery, Somerset House, Strand, London, until October 5.