Possible Clues:

• Catalan surrealist Joan
• 'Triptych Bleu I, II, III,' e.g
• Spanish guy Joan, whose work, like this crossword, is also 18-Across
• Friend and influencer of Calder
• Artist who advocated the 'assassination of painting'
• Surrealist from Barcelona
• 'Dog Barking at the Moon' painter Joan
• Artist Joan from Barcelona
• Catalan painter and sculptor
• 'People at Night, Guided by the Phosphorescent Tracks of Snails' painter Joan