Sir Terry Frost, R.B. and W. Spiral for A., 1991, oil paint and acrylic paint on canvas, 1905 x 1905 mm, Tate.
This is a good-size painting, so the sheer thrill factor of standing in front of a great big fuck-off spiral is somewhat diminished on a computer screen. Nevertheless, you can see how the image works, with the Op Art disorientation of the spiral form working against the impreciseness of the free line Frost's brush leads us down. If it were any looser, it wouldn't be a spiral at all. It would just be an uncoiling mess. But Frost knows exactly how far to push it.