Over the centuries, painters and sculptors have exchanged works with one another, usually as a mark of mutual respect and friendship. Henry Moore and Ben Nicholson did just such a swap early in their careers and it proved to be an even deal between two of the most significant British artists of the last century. In 1931, Moore handed his close friend an exquisite sculpture of a girl’s head, carved from a block of white alabaster, and Nicholson gave him an enigmatic still-life painting. Each of the pieces had been created by the pioneering artists in 1929, long before their artworks were to change hands for millions of pounds. The swap had nothing to do with money but, when Nicholson was short of funds in the 1950s, he had to part with his Moore. It went to an American, Thomas Adler of Cincinnati, in whose family it has remained until now. Today that sculpture, Head, is considered so important that it is expected to fetch between £2m and £3m at Sotheby’s in London, where it was displayed this weekend – the first time it has been seen in public since 1952. It will be offered by the auction house on 21 November, the star lot of its modern British and Irish art sale. “Art historians and collectors consider Moore’s carvings from the 1920s and 1930s to be among his most supreme achievements,” said André Zlattinger, head of modern British art at Sotheby’s. “What’s amazing is all those works are unique, carved by the artist, whereas obviously later on he worked in bronze. I haven’t seen a head like this ever appear on the market in alabaster in this kind of amazing condition. It’s really beautiful.” Moore, a miner’s son, was a pioneer in post-war Modernism who is best known for his monumental bronzes, with mother and child and the reclining figure among recurring subjects inspired by the human body and natural forms. Nicholson, the son of the artist William Nicholson, created geometric paintings and reliefs that are among the most influential abstracts in British art. Early Nicholsons from the 1920s and 1930s are among his most significant works and are so highly sought-after that they sell for up to £4m. Zlattinger said the two artists “weren’t interested in money” – just close friends who were “passionate about their work”. Both were at the heart of avant-garde artistic circles based around Hampstead, north London. Sculptor Barbara Hepworth, Nicholson’s future wife, was among them. Head, carved shortly after Moore’s marriage in 1929 to Irina Radetsky, is considered a stunning example of his early talent at a seminal moment in his career. The auction catalogue notes that, through the sculpture’s material and form, it creates “an impression – as opposed to a full portrait – of a girl’s head, one that transforms and fluctuates when viewed from different angles”. Moore had particularly responded to carving in alabaster in his early years due to its softness when worked with a chisel. Sotheby’s research established that Nicholson had given Moore his Still Life – Jug and Playing Cards. Moore held on to it, having written to his friend in 1931: “We find new things to enjoy in it every day.” Nicholson also showed his appreciation of Moore’s sculpture, sending him a letter with a drawing of it alongside his collection of mugs, hairbrushes and a glass jar. But, following Moore’s death in 1986, the Nicholson painting was eventually sold and is now thought to be in a private collection. It was exhibited in 1993 by the Tate in London. Adler had never wanted to part with the Moore sculpture, turning down repeated offers from a dealer, his daughter Margot recalled: “I always knew how much he cared for his Head, imagining how much peace, loveliness, wonder she kept alive in his life. The way light might travel through her alabaster, as if signalling a vibrancy inside her ‘soul’, made her very special.” Sebastiano Barassi, head of collections and programmes at the Henry Moore Foundation in Hertfordshire, suggested that Moore’s sculpture may in turn have inspired Nicholson to create his 1933 linocut of a female head in profile, which is thought to be a portrait of Hepworth: “So I think there’s a closeness that is visual as well as personal.”