This is obviously a special manifestation of the floating effect. It can easily occur in mixtures of white pigment and carbon black due to its low density and variety of surface states. This applies particularly when the components of the pigment paste are not adequately matched to each other. The cause can lie in a pigment specific instability of the carbon black/white pigment mixture which can be eliminated with better dispersion.
The success of such attempts is recognisable in that in the worst case cofloucculation would occur but the uniform grey shade would be retained. If this way method is not successful, an additive may help. Suitable products are finely dispersed extenders based on aluminium oxide, silicon dioxide or calcium carbonate, principally as rheological control agents. Altematives are provided by surfactant-like additives, either ionogenic (anionic,cationic, amphoteric) or neutral (mono, bifunctional) substances. These should solve the problem.