TWO marks distinguish the postage stamps that were forged by Jean de Sperati in the early 20th century.

The first are minute, repeated signs that seem inevitably to have crept into the creating process―a flaw in the outer green frame of the 1880 Nevis 6d stamp, for example, or the over-long left arm of the “T” in the “TEN” of the 1886 Lagos 10 shilling stamp. In the Australian £2 stamp of 1913 (pictured below) there is a faint break in the horizontal lines of red shading between Tasmania and the mainland

Sotherby's

The second distinguishing feature is Sperati's hand-written signature that now verifies the rarest pieces.

Sperati's forgeries have become so collectible that some, such as the red 1913 Australian £2 stamp and the olive-bistre coloured 1865 96 cent stamp from Hong Kong, are now worth more than the originals he copied. As Herbert Bloch, a former chairman of the Philatelic Foundation's expert committee, told a conference more than 20 years ago, “He [was] the greatest forger, by far, of all times ... No-one even approaches him.”

Portable, easily hidden and eminently tradable, stamps have long been seen as a good hedge against inflation, especially in times of trouble. The last five years, however, have seen a peace-time upsurge, particularly in the middle and top end of the market. This has been driven by the entry of new collectors from mainland China―where there are thousands willing to spend as much as $100,000 on a single stamp, says Richard Ashton of Sotheby's―and by the arrival of six or seven new American buyers in their 40s who are spending upwards of $5m a year on their collections.

Chief among these last is Bill Gross, manager of the world's largest bond fund and one of only two people to have amassed a complete collection of 19th-century American postage stamps. Mr Gross is believed to have spent $100m on his collection since 2001, and is one of the few major American collectors also to buy non-American stamps.

Part of the lure of Sperati's forgeries lies in the dramatic tale of how they were revealed. In 1942, Sperati, a thin-faced Italian living in German-occupied France, sent a small selection of stamps to a dealer in Lisbon seeking his approval. These were intercepted by the French authorities, who charged Sperati with illegally exporting stamps worth more than 60,000 francs. When an expert witness testified that the real value of the stamps was more in the order of 234,000 francs, Sperati felt obliged to admit they were fakes, and that he had been forging stamps for more than three decades.

Sperati's forgeries went undetected because he usually used perfectly genuine, though very common, stamps as the basis for his fakes. Using chemicals, he would bleach out the original design, while retaining the postmark, and then print a new image of a rare stamp on genuine stamp paper. In addition, he was a master at getting the colours right, which made uncovering his forgeries particularly difficult.

After his court debacle, Sperati began to realise that his cheap fakes were becoming sought after, which is when he started to sign them. In 1955 the British Philatelic Association bought up his stock and his printing materials, and brought out a booklet entitled “The Work of Jean de Sperati”, which identified all the Sperati forgeries then known. As more forgeries were identified, a second book was published in 2001. Today, collectors and dealers seek out Sperati's fakes, often pairing them up with the genuine article.

Sir Gawaine Baillie, a British businessman, discovered his passion for stamps when, as a boy, he was sent to America during the second world war. He stayed with his mother's cousin, Joan Payson, and her brother, Jock Whitney, both of whom were already noted collectors. Since his death in 2003 his vast stamp collection has gradually been sold off, so far netting £15.1m ($29.3m) against a pre-sale estimate of £11m. The tenth and final sale includes his holding of Sperati forgeries, probably the finest amassing of the master's work ever to appear at auction.

Sir Gawain Baillie's Sperati forgeries will be included in the final part of Sotheby's sale of the Baillie philatelic collection in London on January 18th

POSTCRIPT: The final sale of Sir Gawain Baillie's stamp collection made £826,422, bringing the total to £15.9m. Lot 565, a 1913 £2 black and rose Die Proof in colour brought £3,720, a record price for any individual Sperati forgery.