Size, of course, is never any indication of value. But what is it that makes such tiny pieces so phenomenally important? And how did they slip from the grasp of emperors regarded, literally, as the sons of heaven, to end up being sold in hotel ballrooms in Hong Kong? The answers to both questions are connected: that works of such flawlessness could have survived the political upheavals of half a millennium (in the case of the chicken cup; the rose cup is about 280 years old) gives them the expensive aura of extreme rarity.

Sotheby's chicken cup, for instance, is one of only three now in private hands. There are two in the Palace Museum in Beijing and eight in Taipei's National Palace Museum. Such cups were made during the reign of Chenghua (1465-1487) who seemed to have had a particular fondness for chickens - he once wrote a poem praising a Song dynasty painting of a hen and her chicks - and who was the first emperor to be interested in using colour on porcelain. The term doucai, which means coloured enamel was applied within a blue-and-white outline, is used to describe the effect.

'Chenghua had been on the throne for a long time when these imperial wares decorated with coloured enamels appeared in about 1480,' says Julian Thompson, co-chairman of Sotheby's Asia and head of its Chinese department. 'There's no real explanation as to what triggered that, but he was a very cultured emperor who was interested in the arts. In fact, in later historical records there are memorials complaining about the incredible costs he incurred in having such pieces made.' The imperial kilns were located at Jingdezhen, in Jiangxi province, which had been recognised as a rich source of kaolin clay since the 14th century. (Even today, about 10 per cent of the town's population is still employed in the ceramics industry.) A whole sub-culture of appreciation sprang out of the industry: Chinese scholars would rank ceramics according to a strict hierarchy and by the 16th century Chenghua doucai cups were already considered the best. That made them, even then, famously expensive.

And yet the curious thing is that, despite their legendary value, no one in the West had even glimpsed these little cups until the 1930s. 'They were totally unknown until then,' Mr Thompson says. 'It seems likely that Pu Yi, the last emperor, started selling them off to dealers who then sold them to Western agents.' The Sotheby's cup was sold privately to Mrs Leopold Dreyfus, a London-based collector, and was shown at the first exhibition in London entirely devoted to the arts of the Ming dynasty, which was held in 1957 (and which Mr Thompson attended as a teenager). This is the first time it has appeared at auction.

The Christie's cup has a slightly more colourful provenance. For a start, it was almost certainly part of the booty from the plunder of the Summer Palace by Anglo-French forces in 1860. 'Unimaginable numbers of things were taken then, maybe millions of pieces,' says Anthony Lin Hua-tien, chairman of Christie's Hong Kong. 'If you think that the Summer Palace was four times - and more - the size of the Imperial City in Beijing, you get some idea of the magnitude.' The other big raid on imperial ware took place during the Boxer Revolution of 1900 'but that wasn't on the same scale, otherwise there'd be nothing left in the imperial collection'.

The famille rose cup was made during the reign of Yongzheng (1723-1735) who was, as Mr Lin, cheerfully puts it, 'an absolute tyrant of a man'. Yongzheng appeared to have been living proof that an artistic soul did not necessarily signal niceness of character: he pressganged all the finest artists and scholars of the time to work on imperial ware, thereby demoting them to the level of artisans.

His edicts must surely have caused a tremble in even the most steady calligrapher's hand. Porcelain from the Jingdezhen kilns was sent to Beijing to be decorated in the imperial workshops and if Yongzheng was not happy with the result, everyone knew about it.

In the second month of 1782, he thundered forth: 'Of the enamel wares that have been lately produced, the designs are vulgar. In addition, the materials used to make them were inferior . . .' The result, however, was the most beautiful porcelain produced by the Qing dynasty.

The mysterious word guyuexuan was coined to describe the work of Yongzheng's 12-year reign. It literally means 'terrace of the ancient moon' but it refers to the painting, poetry and calligraphy which appeared on porcelain. So individual was it that even when pieces were intended to be made as a pair, they were never identical: the reluctant artists who created them did not think like the more mass-market-minded craftsmen down at the Jingdezhen kilns.

The intricate, flawless little cup of bamboos and roses was probably never used. Mr Lin thinks it probably reposed within the imperial study and was only touched by the emperor himself - until, of course, it was purloined by a Western army. That it survived both the rough overland journey from Beijing to the sea, and the ocean journey to Europe, seems miraculous (many pieces must have been obliterated en route). That it subsequently became the property of Barbara Hutton seems a wonderful irony of history.

Hutton, fabulously wealthy and seven times married, was designated a 20th-century empress by the media from the moment she was born, in 1912, until her death in 1979. Her money came from Woolworth's, an empire which became a byword for cheap and cheerful, but her tastes were honed in a wholly different stratosphere of expenditure. The rose famille cup became part of her fine collection of Chinese porcelain and was exhibited in Honolulu in 1956. She sold it, anonymously, at auction in 1971 to Paul and Helen Bernat, collectors who were based in Boston, and it was sold again at auction in Hong Kong in 1988.

Both cups are on view today - the chicken cup is at the Conrad and the famille rose is at the J W Marriott - as are other fine works from the imperial kilns. You may not be able to touch them (the famille rose cup, reports Mr Lin, is 'buttery, oily, unctuous') but they are proof that the essence of mighty empires can be expressed by minute, expensive fragility.