Dave, I don't find pigment concentration a factor in staining.... try it
yourself.... double the pigment in a mix and see if you get more staining.
>
> Logic would say then, that the more you dilute, the less staining it
> would be, but I am sure you have read, heard, or experienced that this
> is not the case. The reason is when you add dichromate, you also dilute
> the thickness or viscosity of the gum. At some point the gum becomes not
> viscous enough to hold the pigment in suspention (think about trying to
> hold sand in suspension in water), so the pigment starts to settle to
> the bottom, so more pigment touch the surface of the paper, and you
> start to get *more* staining. You cannot see this with your eye because
> they still look the same, and watercolorists don't have to worry about
> this because they don't have to deal with lifting the pigment later as
> gummists do.
I think it's the viscosity rather than the amount of pigment touching the
paper... or maybe both, but, but you're right, *logic* doesn't do it...
> This is the reason why I also think that
> the gum-pigment ratio test without dichromate means nothing practically.
Or say it means nothing absolutely..;- ) The action of the light is in
there, too.
Judy