They’re the sort of pieces that rarely come on the market: a pair of giltwood sofas with reeded top rails and baluster legs. For ornamentation, there are carved acanthus leaves, lions’ heads and rosettes. Manifestly Neoclassical, they represent the designer’s synthesis of archaeology with modern furniture making.

That designer was Robert Adam (1728–92), the greatest British architect of the 18th century. In 1981, the two sofas slipped through the auction market with no press notice. The price today for the pair is around $1.5 million.

“Adam’s name is almost a synonym for English late 18th-century style,” says Alan Rubin, the director of Pelham Galleries in London. Born into a family of architects, Adam trained and practiced in Edinburgh. As a young man, he had ambitions to open a London office. The best preparation, he decided, was a grand tour of Italy. He arrived in Rome in 1755, and over the next two years, studied ancient ruins in the company of other artists and architects. These sketching parties visited both the celebrated Roman monuments and the recently excavated sites that gave new insights into ancient domestic interiors. At the same time, he mixed with other tourists, the wealthy English “milords,” who would, he hoped, someday hire him to design their houses.

When Adam returned to London in 1758, he was imbued with an imaginative knowledge of Roman antiquities. His achievement was to assimilate what he had seen in Italy into a graceful scheme of surface ornament and painted decoration. He was immediately successful, undertaking 25 projects in the first three years.

For collectors, Adam presents a challenge because he designed comparatively few pieces of furniture. He was expensive to hire, so many of his clients saved on this “extra” by ordering directly from a furniture maker, who was current with the Classical idiom. Adam typically supplied designs for wall furnishings like mirrors and girandoles, but it was customary to go elsewhere for the seating furniture and other moveable pieces.

Another obstacle: Most of the furniture he designed is still in situ in historic houses or has passed into museum collections. Rubin reports that in the past 15 years, he has made only two sales of pieces designed by Adam, both to museums.

There is, however, a lot of “Adam style” furniture on the market. So pervasive was his influence that by the 1770s furniture makers were turning out pieces based on his designs, which were published in two parts, the first between 1773 and 1778 and the second in 1822.

The market for Adam’s work is strong today. A circa-1770 sideboard with fluted tapering and block feet, a simplified version of one he designed in the late 1760s, sold at auction in London last November for $38,860. Further proof of his influence is the window seat with scrolled arms, which was inspired by the Roman reclining couch. It was a popular seating form during the late 18th century, and examples come on the market often. One sumptuous pair, with fluted columnar legs and upholstered in an Adam-style silk-damask, failed to sell in London five years ago at a low ebb in the furniture market; the estimate was $38,750 to $62,000.