Pardon if I'm posting in the wrong section - I'm an artist, not a chemist, and am not quite sure where to post this question.
The modern process of synthesizing ultramarine blue pigment involves sulphur. From what I understand, if the pigment's pH is 6.5 or greater, the color is stable. If its pH falls below 6, the pigment begins to decompose and let off sulphur fumes. If ultramarine blue pigment dispersed in water (known as a pigment paste or dispersion) begins to smell like rotten eggs, what substance could be added to counter this? Or, once the color starts emitting sulphuric fumes, does that indicate it's already breaking down, the color is unstable and should be thrown out?