You know you are in for something special when you get a frisson of pleasure the moment you see the first pictures in a show. Guercino is one of the great names of 17th-century Italian art. Though his name is not one of the best known, almost forgotten at one point, he was a brilliant draughtsman — and recognised as such very early in his career. Giovanni Francesco Barbieri (1591–1666), known by his nickname Il Guercino or ‘squinter’ due to an early accident, was born in Cento, near Ferrara. In nearby Bologna his work was praised and influenced by the Carracci family (notably, he absorbed their use of light and shade and naturalism). In Rome the young painter’s style was formalised, classicised, but unrestrained energy shows through in his preparatory drawings. There’s no need to see the finished paintings to enjoy them. They satisfy on their own. Their power lies in their brevity.

It’s a free exhibition. Which is wholly appropriate, for most of the works come from the collection of Sir Denis Mahon CH, CBE, FBA (1910–2011), a distinguished art historian and collector of Italian Baroque art who was also a campaigner for the arts in the UK — including advocating free admission to museums. Few shared Sir Denis’s passion for Guercino. At the time he was buying Guercino drawings (mid 1930s to early 1960s) and championing the importance of Italian Baroque art, the style was almost universally disliked; the taste for it had waned from the late 1800s onwards. It was something Mahon could never understand, but it meant prices were low.

Exhibition curator Catherine Whistler told me that Sir Denis bought a major Guercino painting very cheaply in 1936, for £200 (Elijah Fed by Ravens, now in the National Gallery). And, when bidding for another baroque painting (a Guido Reni, now also in the National Gallery), he found himself up against a dealer who wanted it only for the frame!

The liveliness of Guercino’s hand and mind comes across in the variety of subjects. From births to deaths, from babies to wrestlers, angels, saints, devils, from tavern to torment to tooth extraction, he reveals himself an adventurous, expressive artist.

You can see how he works out his ideas for a picture: rapid pen strokes, often minimal, thinking as he goes, reinforcing decisive features in ink, exploring his subjects on the paper. A trio of drawings shows this very well. Deciding how best to depict the Biblical story of Jael killing the tyrant Sisera, in the first drawing flurries of pen strokes convey the heightened emotions and lavish use of ink picks up on crucial details (nail; open-mouthed onlookers). In the second (above), whirling lines no more than suggest Sisera’s body, and a diagonal composition highlights the moment of discovery.

Other highlights include a drawing (right) of two women whispering: a private moment, drawn in red chalk, smudged or wetted to evoke the soft shadows thrown by candlelight; and a martyred St Lawrence modelled with sparing use of black and red chalk to suggest the embers.

Easily missed is the sketch of the old man lying on his back, legs flailing — or, one leg propped on a chair in Guercino’s studio for a study for St Jerome surprised by the angel. It’s a wonderful moment. As Catherine Whistler says, you are right there with the artist.

Sir Denis Mahon was a regular visitor to the Ashmolean when he was at Christ Church. He placed his collection on loan at the museum in 1984 with the intention that the drawings should be accessible to a wide audience, and should eventually become part of the permanent collection.

A delightful photograph displayed in the exhibition shows the laughing centenarian at home not long before he died last year. n At the Ashmolean Museum until April 15. Free admission.