NEW YORK (REUTERS).- A rare Andy Warhol self portrait in stark white and red, which had been in private hands for over 30 years, will go on sale at Christie’s in London next month, anchoring the auctioneer’s post-war and contemporary art offering. The unusually large canvas, 6 feet by 6 feet (1.8 m by 1.8m) features the pensive artist staring straight at the viewer but with half his face swallowed in a shadow of blood red paint. One of 11 self-portraits in this 1967 series, it is the only one restricted to two colors. Christie’s estimates a gavel price of three to 5 million pounds ($8 million) for the painting. “This is one of the missing pieces of the Holy Grail,” said Amy Cappellazzo, Christie’s co-head of contemporary and post-war art. It was put up for sale by the estate of the painting’s purchaser, who is now deceased, she added. Warhol was at the height of celebrity when he first exhibited the painting along with the 1