Why are some printmaking inks more opaque than others? I’m currently working on a commission that won’t be unveiled until after it is delivered to its new collector. Without giving away the details, I found myself needing to shift from a middle blue tone into much darker greens. Enter the yellow opaque ink…
Opacity vs transparency : not all inks are the same
After using Gamblin’s relief inks for a while, I intuitively know that yellow, napthol scarlet and titanium white will be the most opaque. Other inks like quinacridone red and phthalo blue will be very transparent. For me opacity means that it will cover up the color underneath more completely, and combine with it less. Even with opaque ink straight from the can, you will never get the color on the paper that you see on the glass. They aren’t completely opaque.
I wanted to answer the question of why some inks were more opaque than others. It was remarkably difficult to get such an explanation without jumping off into a discussion of physics for which I was not qualified.
Getting to the why of opacity and transparency
Two on-line sources got me closer to an explanation of why inks vary in opacity. Why Some Paints are Transparent and Others Opaque, a page from Natural Pigments on-line store, has the most complete explanation. The Anatomy of Paint: Pigment and Binder from Essential Vermeer 2.0 has a discussion with a bit less physics.
The answer has three variables
Relative Refractive Indicies
Remember that all pigments whether naturally gathered or industrially created all start out as a powder. The powder is combined with a binder to allow the pigment to be applied to a surface. With printmaking, burnt plate oil (a heated version of linseed oil) is the binder.
Both pigments and binders have a rating for their ability to refract — or scatter — light. The more similar the number, the more likely the created ink will be transparent — or not scattering the light. Linseed oil has an index of 1.479 so pigments that have a very similar index will allow the most light to pass through, appearing transparent. Other pigments like zinc oxide, have an index of 2.00, and therefore appear opaque — not allowing any of the light through.
In my previous post on gold ink, I noted that my favorite one — Charbonnel’s Etching Ink Gold — had almost noticeable particles. It is very opaque, and this makes sense because the light is being refracted off the (larger) gold pigment particles, and can’t make its way down through the ink layers to the white paper.
Distribution of Pigment
This variable is obvious to me, as I liberally mix transparent base into my relief inks to increase the transparency of any color. Even still, I can never achieve the transparency of a phthalo blue with the yellow or white inks. This is probably because of the above two variables.
I’m not sure I completely understand refractive indicies, but I do get a kick out of imagining the light rays trying mightily to penetrate my layer of ink and make it to the white paper.
Now back to my commission and to some new work. Science class is concluded.