Which Reformation theologians used the analogy of water and ink for sin? On page 98 of Miroslav Volf's Free of Charge: Giving and Forgiving, he says:


  But even if all of us are sinners from head to toe, none of us is a
  sinner through and through, with nothing good remaining in us. As
  sinners, we are still God's good creatures. To illustrate the
  relationship between being a good creature and being a sinner,
  Reformation theologians used the analogy of water and ink. Water is
  the good creation, ink is sin, and the sinner is a glass of water with
  a few drops of ink. All the water in the glass is tainted, but it's
  still mostly water, not ink. Analogously, all our good deeds are
  marred by sin, but they are still mostly good deeds, not crimes
  masquerading as merits. Now apply this to gifts. We give gifts. None
  of them are pure. Yet with all their impurities, many of them are
  still genuine gifts, not just hidden ways of loving ourselves.


There is no footnote. Who is Volf most likely referring to? What is the original context(s)?
Expanding the search a bit to account for polluting agents other than ink, the closest I can find in Calvin is in his commentary on Isaiah 1, referring to the line in v. 22 that he translates as "Thy silver is become dross, thy wine mixed with water". 

Even here though, the sense is not the same as what Volf is going for; Calvin says that the image is one of hypocrisy: 


  The comparisons here employed are exceedingly well adapted to this end, for dross bears some resemblance to gold; and in like manner, the color of wine mixed with water resembles that of pure wine; and yet both are very far from having that purity of which they make an outward show. In like manner hypocrites, by their hypocrisy, may be said to assume a false color of silver, though they are of no more value than dross.