The little portrait that packs a mighty punch … Study for Head of Lucian Freud (1967) by Francis Bacon. Photograph: Christie's Images Ltd 2014

The slice of genius brought to you by Francis Bacon, Lucian Freud – and Roald Dahl

Study for Head of Lucian Freud, painted by Bacon and owned by Dahl, is expected to fetch millions at Christie's this week. It's worth every penny

Jonathan Jones

Tue 2 July 2014

The really big financial killing at Christie's auction of Postwar and Contemporary Art in London this week will not be made by Tracey Emin's My Bed. Instead it will involve a famous children's author and two remarkable painters, as Francis Bacon's Study for Head of Lucian Freud (1967) also goes under the hammer.

A portrait of the brilliant Freud by his peer – or superior? – Francis Bacon is bound to attract some sensational bids. After all, Bacon's Three Studies of Lucian Freud currently holds the world record as the most expensive painting ever sold at auction after it fetched $142m at Christie's in New York last year. Another Bacon triptych, of his lover George Dyer, sold at Sotheby's this week for £26.7m.

Francis Bacon

But those were triptychs, monumental works in three parts. Bacon's Study for Head of Lucian Freud, painted in 1967, is a much smaller picture. It is expected to sell for up to £12m. Don't be surprised, however, if it goes for a more sensational price.For this is a more exciting painting than Bacon's record-breaking Freud triptych.

Size isn't eveything. In art, a compact work of genius counts for more than sprawl. This little portrait packs a mighty punch.

Study for Head of Lucian Freud is displayed at Christie's in a darkened chapel of its own, isolated from the excitement of Emin's bed. It more than lives up to its quasi-religious setting. This is Bacon at his most radical – not just a distinctive British painter, but the heir of the great European modernists. Freud is unrecognisable under a wild smear of green. Pink and grey lumps of flesh collide with that green in a shocking chaos of a face. What has Bacon done to his friend? It looks more like rivalry, as he literally paints Freud out of the picture.

This painting is almost as hard to read as Picasso's Cubist portrait of Ambroise Vollard. It is a scintillating explosion of artistic energy, squeezed into a small framed rectangle and all the more powerful for that intensity of scale.

What's more, it has a touching story attached. It belonged to the children's author Roald Dahl, who kept it all his life. Dahl and Bacon? Most people probably associate the author of Charlie and the Chocolate Factory more with his regular illustrator Quentin Blake. Yet he was friends with Bacon and at one time owned seven of his works – this is the last to be sold.

It's tempting to imagine Bacon instead of Blake illustrating Dahl's books. Visions of smeared globs of flesh representing The Twits or The Witches come to mind. But actually, it is not surprising that Roald Dahl appreciated Bacon's darkness. There's plenty of savagery in his books – from the monstrous twits to the vile farmers who try to kill Fantastic Mr Fox. And in his stories for adults, Dahl had a pronounced taste for the gothic, not least in his black comic tale Royal Jelly.

The directors of Christie's may well be putting in an order for Royal Jelly from nearby Fortnum and Mason's after they auction this grisly jewel of a Francis Bacon painting.

THE GUARDIAN