I use Rohrer & Klingner Verdigris Ink and am quite happy with it. Prompted by this article I looked up its pH value, which is close to neutral with 7.98, while

I use Rohrer & Klingner Verdigris Ink and am quite happy with it. Prompted by this article I looked up its pH value, which is close to neutral with 7.98, while many other colors are more off-center.