Selsey born Keith Vaughan is best known as a Neo Romantic artist whose friendship circle included painters Graham Sutherland and John Minton.

In the 1950s he discovered the Russian émigré painter Nicolas de Stael and began to explore the reconciliation of figuration and abstraction, and balanced male nudes in abstract landscapes.

But, says Jenny Lund, curator of Keith Vaughan: A Volatile Medium, his later gouache works reveal his experimental side. They are instinctive, and reflect the artist’s mood and feeling.

“It is interesting how he experimented with that medium. How he would mix different media together working with gouache, rather than letting the medium work on its own.

“For example, he would add vinegar so that the pigment became unstable and started to bubble up to create its own texture. It was something uncontrolled.”

The gouache works are part of a new exhibition covering the final 15 years of the artist’s life.

Gouache, a form of opaque watercolour paint prepared with gum, had always been an important medium for Vaughan. Its portability meant he could make quick sketches during World War II, when, as a conscious objector, he was stationed at Ashton Gifford near Codford in Wiltshire.

“He started out as a graphic artist and a commercial artist and in his later years he became freer in his expression,” explains Lund.

“We can see it in his gouaches. He works quicker. He works in series and would do many simultaneously. They have spontaneity and freedom his earlier works don’t have.”

Exhibitions featuring these works are rare.

“We wanted to put something together which would rectify that.”

She believes the gouaches have been overlooked, in part, because the subject matter of his earlier works – namely war – is clearer. Also, Vaughan’s Neo Romantic works are often discussed because of their links to other well-known artists from the time - John Piper and Robert Colquhoun as much as Sutherland and Minton.

A donation from Dr Ronald Lande sparked Lund to put the show together with a former University of Sussex postgraduate student, Dr Darren Clarke.

He bequeathed Vaughan’s gouache, Two Interlinked Figures (1965), to the museum in 2012 via the Contemporary Art Society.

That followed another donation in 2008 of a pencil drawing named The Back View Of A Sailor.

The exhibition also features drawings, photographs and extracts from Vaughan’s diary and traces his life from a retrospective exhibition held at the Whitechapel Gallery in 1962 to his death in 1977.

Vaughan was a keen diarist. But the entries became more melancholic as he neared the end of his life and eventually committed suicide. The last thing he did was to write in his diaries, as his drugs overdose kicked in.

His journal was inspired by André Gide. It reveals the tension in his life and work, between intellectual Puritanism and unrepressed sensuality, especially as a gay man troubled by his sexuality The show sets the diaries beside early photographs from the 1930s.

“He is extremely honest and frank. We do a see man going through a dark period and his relationship with his partner was very problematic and that all comes through. His struggles and his self–criticism are visible. He is very hard on himself.”

Keith Vaughan: A Volatile Medium is at Brighton Museum and Gallery, Pavilion Gardens, until November 9.