His current exhibition, The Metal Mirror, is the result of an 18-month collaboration with the curatorial staff at the British Museum's Department of Coins and Medals. Its presentation in conjunction with the department has attracted some controversy, because the coins that Sack has chosen to photograph are precisely those spurned by professional numismatists, art historians and collectors on account of their poor state of preservation. They are mostly ancient, and all of base metal (mostly bronze) which has oxidised over centuries of burial, producing strange encrustations and richly coloured patinas.
Using macro photography and sophisticated lighting, Sack has produced a series of 25 colour prints, up to a metre square. Photographic technique is subsidiary to artistic intent, though the technical quality of these prints - some of them at magnifications of up to 100 times - is superb. So vivid are the colours that many visitors assume that they must have been enhanced by the use of filters or computer manipulation, but this is not the case. The colour fidelity is astounding; the fiery reds, acid greens, indigos and subtle ochres are the entirely natural results of the oxidisation of the metal, viewed in microscopic detail. The images are cropped close so that the edges of the coins - and in most cases the inscriptions - are not visible, and Sack has resisted pressure to label the prints or to display the original coins, on the grounds that such information might inhibit the viewers' imaginative response (although details are given in the catalogue).
The result is to make the viewer look not at coins, but at the images on coins. The ancient die engravers, though often highly skilled, worked at speed and with great economy of line; enlarged to this degree, their deft chisel strokes assume a primal energy reminiscent of cave paintings. Add to this the dents, scratches and corrosion, and the images are transformed into something shadowy and richly evocative. A domed Baroque church looms out of a Venetian fog on a 17th-century Catholic medallion; elephants, horses and mythical beasts prance and caper through clouds or flames; gods and goddesses wrest themselves free of the volcanic magma in which they have been entombed for aeons.
Perhaps the most hauntingly romantic - and one of the most popular with viewers - shows a galley with six oarsmen and a curving prow, moving soundlessly through milky green swirls of mist and water, as if Paul Klee had painted Charon's boat crossing the Styx. The image turns out to be entirely in keeping with the coin's provenance: it was one of many sacrificial offerings thrown into Coventina's Well on Hadrian's Wall. A striking picture of a dark brown palm tree, cropped tight against a background of vibrant red and turquoise, could be a bright "ethnic" bedspread from London's Camden Market; in fact, it's a detail from a small bronze coin - no larger than a thumbnail - struck in Palestine by Jewish insurgents during Simon Bar Kochba's uprising against Rome (AD132-5).
It is the imaginative dimension of these works that has proved provocative in the academic context of the British Museum. In the interest of unprejudiced research, archaeologists have to keep a tight rein on their imaginations. In private, however, few would deny that it is imagination - the sense of wonder evoked by an object that speaks directly to us from the past - which fuelled their initial interest, and which keeps the public interested. Perhaps, then, there is a quiet agenda in this unorthodox move on the part of the Department of Coins and Medals, for Sack's photographs recapture just that sense of wonder, putting the numinous back into numismatics.
The curator Brendan Moore points out a slab of yellow ochre, interrupted by a grey fissure running from the top and some enigmatic orange squiggles. At first I can make out nothing in what appears a purely abstract composition. Eventually the lines resolve into two superimposed profiles. The coin is literally a palimpsest, a worn sestertius of Hadrian overstruck, more than a century later, with the portrait of the rebel emperor Postumus. Hadrian's disdainfully aristocratic features are tilted to about 1 o'clock; a patch of black oxide has adorned the introspective emperor with a cool pair of shades. Across the side of his head is stamped the piratical, raggedy-bearded profile of the later usurper.
I glance away for a moment. When I look back, these ghosts have melted away...
The Metal Mirror continues at the British Museum, Room 69a (Department of Coins and Medals) until 5 December. Admission free