I REVISITED Ferrara last November to see a powerful exhibition of the Spanish painter Zurbarán which is now opening in Brussels. (Go to the BOZAR. Don't even ask: just go some time between 29 January and 25 June.)

I reminded myself of a city that is full of bicycles. Good food, yes, a fine cathedral with its late-medieval arcade of shops, a moated castle, terrifyingly high markers of previous floodwaters on the walls, and bicycles everywhere. After the exhibition, a decent lunch, and a return to the fine city art collection, I do not remember seeing any confectionery shops, but there were bicycles.

Giorgio de Chirico, who was born in 1888 to Italian parents in Volos, in Greece, where his father worked as an engineer building the Athens-to-Thessalonika railway, recalled Ferrara rather differently.

He and the Futurist painter Carlo Carrà had met there on service in the First World War, and de Chirico wrote of a "city where one could find sweets and biscuits in exceedingly strange and metaphysical shapes".

We might think that hungry soldierswould have other appetites, but the artistwas used to finding the metaphysical in the quotidian, and once remarked, somewhat harshly of his home city, from which Jason had sailed in search of the Golden Fleece, "Life in the little city of Volos was full of metaphysical and provincial events," betraying his own sense of otherness and estrangement from reality which carried on even when he became friends with Apollinaire in Paris.

De Chirico returned repeatedly to the metaphysical in his art, convinced that "to be really immortal a work of art must go beyond the limits of the human; good sense and logic will be missing from it. In this way it will come close to the dream state and also to the mentality of children."

Whereas his paintings of stark sunlit squares, closed off by tall arcades, often with a steam train chugging its way across the back, which appear like stage sets, are familiar, his sculptures are much less widely known outside Italy.

This is the second exhibition in Canonbury to draw largely on the collections of the Gallery of Modern Art in Bologna, after last year's revelatory showing of Giorgio Morandi's etchings. These are supplemented by a handful of exhibits from Prato, including the undated three-metre-high bronze figure The Metaphysician.

Although there are some drawings and paintings in the two-room exhibition (14 in all, including the wondrous oil painting of Horses at the Seashore, in which two fine animals stand amid ruined columns and look out over the coast of the Aegean), the exhibition is centred on the sculptures.

De Chirico had been interested in sculpture from early on, and had written an essay (1927) on what he regarded as the "phantomatic" appearance of works in a museum which can be glimpsed, rather like seeing people in a room that we first thought was empty. He wrote of statues as being cut off from nature, removed from the beauty of the outside world, as solitary figures in their own right.

The works on display are all late pieces, beginning in 1966 and continuing up to the artist's death in 1988. Much as he often reworked, and repeatedly redated his canvases, de Chirico set about transforming figures from some of his earlier paintings into three-dimensional forms.

So, for instance, the famous 1917 painting of The Disquieting Muses, with its haunting pair of hieratic figures with their egg-like heads that reflect the distorted lozenges in the Ferrara sweet shop, provides a repeated source for inspiration.

But the drapery of the peplon worn by each of the women conceals the sheer violence of the form, and the head transforms the figure into a sort of unknown monster from the queasy depth of a dream. These are sentinels, and their purpose, like that of the chorus of women in Richard Strauss's rendering of the Oedipean tale Elektra, is unlikely to be as bearers of good news.

Perhaps it is only a trick of the eye, but, for me, only those statues that are cast in silvered bronze really work. Here, the light refracted off the surfaces renders the form more transparent and accessible. This is clearly seen in the almost identical 1974 sculptures of Architecture Muse, where the silvered figure breathes as her bronze sister seems flat, unrelenting, and dull.

The more classical equestrian statue of Hippolytus (1969), as a Greek ephebe on horseback, and the timeless figure of a groom with a horse, from the following year, are easier to understand, and therefore more immediately appealing.

The Estorick display has respondent graphics on the walls, which appear almost as sketches for the statues themselves. I am not sure that the artist himself would have approved of this method of explication, but it certainly helped me to understand what he was attempting, even if I remain to be convinced by his work in three dimensions.

Part of the problem for me was that I had just seen the most extraordinarily beguiling and beautiful piece by Lynn Chadwick in a delicious exhibition at Mascalls Gallery at Paddock Wood.

"Dealer's Vaults" (until 22 February) is the latest exhibition that Nathaniel Hepburn has mounted in the gallery in the grounds of this school, and brings together 14 works of 20th-century sculpture from the stockrooms of London art-dealers.

Five of the artists shown, including Reg Butler (1913-81), had featured in the British Pavilion at the Venice Biennale in 1952, and here I was introduced to the work of Peter King (1928-57), with an elm-wood statue of a female figure, which he produced in 1948 (Tomasso Brothers Fine Art). The texture and volume of the woodcarving is exceptional, and the work stands as a fine monument to an exceptional artist who died in a motorcycle accident before he was 30.

But it was the 1971 maquette of two seated figures (Grosvenor Gallery) by Lynn Chadwick - who at 41 became the youngest ever sculptor to be awarded the first prize at the Venice Biennale, in 1956 - which moved me most.

It was one of Bernini's rivals in Rome, Alessandro Algardi (1598-1654), who once wrote to one of his patrons to ask whether his sponsor had any idea at all just what skill it took to make people look at a white marble bust and to see it as real flesh and blood.

I was at once reminded of this with the two polished bronze "faces" of Chadwick's figures, which contrast starkly with the darker, unpolished, and rougher surface of the rest of the work. The male figure has a characteristic rectangle and the female a triangle, and in them we see the details of a face by the act of superimposition to which Algardi alluded.

That this is no mean achievement is made abundantly obvious by de Chirico's use of a similar technique for the monumental figure of The Metaphysician. The bulbous head and bared left shoulder stand out from the rest of the dark-bronze body, to which he had added further polished shapes. Even though there are a couple of etched lines on the head, they do not exert the power of recognition which Chadwick so compellingly achieved.

From Ferrara to Paddock Wood by way of Highbury and Islington (the nearest station to Northumberland Lodge for the Estorick Collection) is no great distance, but Chadwick and de Chirico inhabit very different worlds.

"Giorgio de Chirico: Myth and Mystery" is at the Estorick Collection, 39a Canonbury Square, London N1, until 19 April. Phone 020 7704 9522. www.estorickcollection.com

"Dealers' Vaults: 20th Century Sculpture" is at Mascalls Gallery, Mascalls School, Maidstone Road, Paddock Wood, Kent, until 22 February. Phone 01892 839039.

www.mascallsgallery.org

"Zurbarán: Master of Spain's Golden Age" will be at the BOZAR, Rue Ravenstein 23, Brussels. For information and tickets, phone 00 32 2 507 82 00. www.bozar.be