Jeff Koons: The Painter and the Sculptor, Liebieghaus and Schirn Lunsthalle, Frankfurt Kitsch is a German word and so is Koons, an anglicisation of Kuntz or Koentz, the artist being of Pennsylvania Dutch stock. So the fact that Jeff Koons's biggest ever exhibition - an encyclopaedic two-parter, covering his 30-year careers as sculptor and painter - should be in Frankfurt rather than Los Angeles seems doubly appropriate. Apt, too, as the city is the centre of German money-making, and money has always been Koons's special subject. You'll know his work, of course - the 40-foot West Highland Terrier, covered in flowers and nicknamed El Poopy, outside the Guggenheim in Bilbao; the eye-wateringly graphic images of the artist pleasuring his now ex-wife, the porn star Ilona Staller; Michael Jackson and Bubbles. This last, in life-size porcelain with a great deal of gilt, is in the first room of Koons's sculpture show at the venerable Frankfurt Liebieghaus. The Liebieghaus is an historical museum, and the Koons show, dotted through its permanent collection, was curated by an archaeologist with an interest in Greek polychromy. Michael Jackson ... is in a gallery of Egyptian funerary sculpture. Placing a cat among dogs will not make it a dog; indeed, it may put the cat at a disadvantage. Seeing Koons in this context is clearly meant to suggest some continuum between his work and Dynastic tomb art. But does it? Surprisingly, the answer is yes, and in a number of ways. For all its kitsch, Michael Jackson and Bubbles understands the age-old link between money and immortality. Koons did not invent Jackson's taste: Jackson did that for himself. What Koons did was to monumentalise it by blowing it up out of scale - what is now a life-size sculpture started life as a trinket - and bring to it production values of a kind normally associated with Hollywood films. As with Pharonic sarcophagi, fame translates into expense, and expense into life after death. Michael Jackson ... was made in 1988, two decades before the singer's demise. Yet seeing his kohled eyes among those inscribed on Late Period mummy cases tells us less about Jackson (or even Koons) than it does about the undying urge to defy death through art. This likeness could be nothing more than coincidence, but it isn't. As you wander through the Liebieghaus's chronological display, Koons's sculptures become more and more difficult to tell apart from the museum's. His Louis XIV (1980), and Lorenzo Ottoni's Bust of Pope Alexander VIII (circa 1700), shown beside each other, could easily be of the same date. That is a specific resemblance, a likeness Koons means us to see. Much more compelling are the generic echoes of history in his work, spotted by the Liebieghaus's curator. Thus the show's second room sets Koons's Sling Hook - a pair of Oldenburg-y soft sculptures of an inflatable dolphin and lobster in aluminium - alongside a group of Hellenistic marble muses from the 1st century BC. Sling Hook may lift the lilos from low art to high, but Koons has insisted that every flaw and crinkle in the cheap plastic originals be reproduced in his expensive finished product. That same playing-off of the impulse to perfect and a love of imperfection is there, too, in the headless Greek forms, whose marble drapery was carved to include the creases that come of being folded in a linen chest. All this might suggest intelligent curating rather than an intelligent artist. Koons is clever, though, perhaps even brilliant. He may play the hick American, the lover of sweetcorn, soft porn, Lurex, Hanna-Barbera, plastic flowers, gilt, Franklin Mint and the colour pink. But the historicist echoes in his works are not there by accident, nor are they just there to show off. Over at the Schirn Kunsthalle, this intelligence is spelled out in his paintings. Seen away from LA, the seriousness of these becomes clear. Koons's skill lies in his eye for kinships, between the shininess of gold, gold Spandex and glacé cherries, between the commercial artwork of 1980s booze ads and high art, the breasts of the Hellenistic sculpture in Antiquity 1 and those of the models in Triple Elvis. The title of this last is borrowed from Warhol, Koons's magpie eye alighting on the glittering things. There are references to Roy Lichtenstein and Pierre et Gilles, to gestural abstraction, Ed Ruscha, Claes Oldenburg and, naturally, to Jeff Koons. But his use of the antique is most like Picasso's. Born of a time when everything is recorded and nothing forgotten, when high and low art are relative terms, Koons's is an aesthetic of too-much. Everything is on his canvases, layered and overlayered until you want to shout, "Stop!" But he doesn't stop, thank God. To 23 Sep (www.koons-in-frankfurt.de)