A man who looks after the pottery and porcelain in the British Museum is going to break a prize exhibit _ the 2,000-year-old Portland Vase from ancient Rome. Art dealers estimate the value of the vase at around $21 million. The 10-inch-high, four-pound glass artifact is regarded as one of the finest pieces of early Roman glass. Nigel Williams, the museum's chief conservator of ceramics, said the vase must be broken and reassembled because the glue holding the blue and white cameo-glass pieces together after two earlier breakages has become yellow and extremely brittle. ``It is the most difficult restoration I have attempted in 27 years,'' Williams told reporters at a news conference Wednesday. He said he expects the task to occupy about 40 percent of his time for the rest of this year. He hopes to have the vase back on display by the end of December. It was made by a sculptor named Dioscourides and depicts men and women in a leafy glade _ probably the courtship of Peleus and the sea-goddess Thetis, who became the parents of the mythical Greek warrior, Achilles. ``It is one of the very few surviving Roman cameo vases and has always been the subject of great academic debate as to who is represented on it,'' said museum spokesman Andrew Hamilton. The vase was formed by inserting a gob of cobalt blue glass into a cup of opaque white glass and then blowing both together. The white outer layer was then carved to produce the white cameo relief on a blue background. The vase is believed to have belonged to the Emperor Augustus and was discovered in 1582 in a tomb near Rome. It came to Britain in the 18th century via Sir William Hamilton, the British envoy in Naples and husband of Lady Emma Hamilton, later the mistress of Adm. Horatio Nelson. In 1785, the vase was sold to the wealthy Portland family for roughly the equivalent of $1,875 dollars in today's money _ a vast sum in those days. The Duke of Portland loaned it to the British Museum in 1810. While on display in a showcase at the museum, the vase was deliberately smashed into about 200 pieces on Feb. 7, 1845 by William Lloyd, an Irishman who had been drinking heavily for a week. ``I have no doubt that the man who broke the vase, whoever he is, is mad,'' the duke replied when the museum wrote to him to express regret. The vase was glued together by the museum's restorer, John Doubleday, and put on display again. Doubleday's glue had become so weak and discolored after a century that in 1948 the vase was dismantled and rebuilt by conservator James Axtell, who also incorporated three of the 37 small chips that Doubleday had found too difficult to replace. ``I will be using more advanced techniques than were available to Axtell, including epoxy resin fused by ultraviolet light,'' said Williams. He also intends to replace the 34 glass chips Axtell left out. ``The vase should hold together comfortably for another 200 years after that,'' Williams said.