In the course of producing his distinctively surreal paintings, Joan Miró made many experimental prints.
Usually, these proofs would be destroyed once the artist had completed the final piece of artwork. But this week, a rare set of 15 Miró prints will go on sale.
The set of workings has never before been seen in public. Since 1968, it has resided with the current owner, the wife of a Spanish diplomat to whom Miró presented the prints. Most of the other drafts in existence are housed in museums, donated by the artist so that scholars could examine his working method.
What is unusual about the series leading to the finished work Oiseau Solaire, L'Oiseau Lunaire, Etincelles, is that it has until now remained in private hands.
The set begins with simple black china ink brushstrokes, and demonstrates how the artist experimented with different colours - orange, blue and green - before finally settling on a dark grey background with coloured circles.
At auction on Thursday, the etchings, which date from 1967, are expected to fetch a modest £15,000 - but their real value resides in the unique insight they give into the Spanish painter's creative process.
Alexander Hayter, specialist at Bloomsbury Auctions, said: "They are very personal works to the artist. They are the stages the artist has gone through to make his finished image and because of that they are usually destroyed. But because each of these is so beautiful in its own right, he kept them and gave them to the present owner.
"They are from the late 1960s - three quarters of the way through his output. The bold, expressive brush strokes are almost like Japanese calligraphy and they are very colourful.
"They start from a simple series of marks and then through stages the image is built up and becomes more and more complex. If you look at them in order, it's looking at the artist's thought process, because he added things and also took them away."
Mr Hayter said the 15 prints were being sold as a set, because it would be a "shame" to split them up.
Miró was born in 1893 in Barcelona. He studied at the Barcelona School of Fine Arts and the Academia Galia, and his early work was inspired by the Cubists and the Fauves, as well as Catalan folk art. He is renowned for his use of bright colours and surrealist, dreamlike shapes on massive canvases. A museum in Barcelona has been devoted entirely to his work.
Also on sale at Thursday's auction is one of Roy Lichtenstein's earliest prints - his first pop art work - and a rare Picasso etching of his lover, Dora Maar.