Wednesday, May 21, 2014

Getting the most color for your ink, part 2

In the first part of this series, I made the point that mushy dots are efficient dots. The logic is pretty simple. Higher dot gains means more color for the same tone value. I went on to give some lame explanation, and gave a link to a TAGA paper that went way above my head.

I don't know if anyone believed me in that post, but I will press on with yet another absurd statement related to ink mileage. Gloss is good for ink mileage. First, I will explain why this is, and then I will suggest some avenues for research to take advantage of this little appreciated fact. 

The smoother the surface, the better the ink mileage

It was almost two years ago that I posted a blog called Flat Paint is Not Flat. I am pleased to say that over 1,000 people viewed this post, and it would appear that there have been somewhere around 7 people who actually read at least some part of it.

The blog made the point that glossy surfaces make an object appear richer in color. I dramatized the point with a photo like the one below. This photo shows the effect of clear lacquer on black ink on copy paper. The arrow on the right shows the untreated ink. The arrow on the left shows the richer black that you get with the lacquer.

Effect of a coat of clear lacquer on copy paper

What's going on? Why is the left leg of the K slightly gray, whereas the right leg is a full black?

The answer is surface reflection. If you want to sound like a color scientist like me, you call it specular reflection. (And I do want to sound like a color scientist.) About 3% to 5% of the light that hits the surface of the ink will bounce directly off the surface of the ink. Sadly, the interaction will be such a brief fling that the light will leave without even a memory of the tryst. (If you want to sound like a color scientist, you might state that a little differently. You would say something dry and unromantic like "the specular reflection is spectrally non-selective.")

If you hold a glossy catalog at a shallow angle to the light, all you will see is the specular reflection. Note in the photo below that the color of the incoming light and the color of the reflected light are the same.

If you want to sound like a color scientist, you must pretend to get excited
when the "Victoria Secret for Optical Engineers" catalog arrives

If you try this with a newspaper, you may come to the conclusion that there is hardly any specular reflection from uncoated stock. That's not the case, though. The specular reflection behaves like a billiard ball bouncing from the rails of a billiard table. Equal angles and all that. If the surface is smooth, like the Victoria's Secret catalog, then all that specular light from a single light source will reflect at the same angle. 

We called it pool where I grew up
We weren't pretentious

(We are coming to the key point here, so I started a new paragraph.) If the surface is rough, like the uncoated stock of a newspaper, then the specular light will bounce around at all angles. And we will see it. We won't notice it, because it is diffuse, but we will see it nonetheless. It's like adding 4% white light on top of the specular reflection of the actual ink on paper. We don't see that specular reflection on a glossy stock because we involuntarily tilt the VS catalog so that it doesn't interfere with seeing all the sexy plano-convex lenses.

Remember that K in the above picture? The two images below were taken of the same printed sheet with the camera and sheet in the same position. The image on the left was with the illumination coming in at the billiard ball angle to the camera. The area that was lacquered is apparent. The image on the right was taken with the light oriented to minimize the specular reflection. The same amount of light is reflected - it just reflects differently.

Getting a little free ink by getting lacquered up

Practical application

What good is theory if you don't have any practical application? The practical application is this: The smoother the surface, the better the ink mileage. The million dollar question is what you can do to get a smoother surface? I have no idea which of these are cost effective. This is more like a group brain storming session with one person in the group. I invite any interested party to join me.

1. Use a smoother stock.

Well, duh! This is, of course, a balance in cost between paper cost and ink usage. One point to make is that it's not all about "holdout". People generally think that the poor ink mileage with newsprint is because the ink seeps into the paper and pigment hides behind paper fibers. That's certainly part of the explanation, but I think the larger effect is specular reflection. 

2. Add a lacquer overneath the inks. 

I know this is often done, but usually it is added to increase rub resistance. It also improves gloss, and hence improves ink mileage. Coating with silicone is done to allow one magazine to slip against the one below it, but it also increases gloss.

3. Apply MgF to the surface.

Ok, this is a little tongue in cheek, but maybe there is something there? In optics, magnesium fluoride is used as an anti-reflective coating. It is optically softer than glass (it has a lower index of refraction). Like a ball bounces better on concrete than on water, MgF will reduce the surface reflectance of glass from 5% down to 1%. This is actually a different effect that the dictum of "make it smoother". I don't know if there is something like MgF that is practical.

4. Dry the ink slower, and at a lower temperature.

In commercial web offset, it's all about running the press faster. Less press time means more impressions per hour and more income per hour. But, think about what the poor ink has to go through to make 3,000 FPM happen. Ink is about one-third oil, and the oven evaporates this oil. I don't have any data on this, but it sure seems to me that such quick evaporation has to disrupt any sort of smooth surface. 

Maybe bringing the dryer temperature down to "just right" will increase ink mileage? Maybe running the press just a tad slower will increase ink mileage all around? Or maybe not. Unfortunately, conventional ink keys keep us from accurately measuring  ink mileage. The proliferation of digital inking systems might make this sort of research possible in the future.

5. Use EB or UV curing.

If you buy into the idea that the evaporation of oil from the ink disrupts a surface that would otherwise be smooth, how about using an ink that doesn't have a carrier that needs to be evaporated? In electron beam ink curing, a stream of high energy electrons is used to polymerize the ink. There is nothing that evaporates, so in addition to the drop in VOCs, the surface should be inherently smoother. The same reasoning goes for ultraviolet cured ink. Both inks are more expensive than traditional inks, but maybe the economics can be tilted if you figure in the potential ink savings from needing less pigment.

6. Ink polishing.

Uncoated stock is run through a calendaring process to make the surface smoother. These are chrome rolls that run at a different speed than the paper so as to compress the paper. I wonder whether it's possible to use a similar technique to polish the surface of the ink without smearing? 

7. Use more resin? 

I have seen that some inks are naturally glossier than others. I don't know why. Maybe it's the ratio of varnish in the ink? Maybe something else. Another thing I don't know is whether ink chemists think "let's increase the gloss" when they think about improving ink mileage.

Any other suggestions? Can any of these suggestions be turned into anything practical? I know it's unusual for me, but with this blog, I am going to actually admit to ignorance rather than pretend to know everything.


  1. Do you think the glossy finish actually increases contrast? I noticed in your FREE INK! sign that the white paper under the lacquer looks less white. It makes sense, since the specular reflection would diffuse the white light just as much over the uncoated white paper.

    Maybe it would increase contrast more if just the ink was more smooth, without smoothing out the paper.

    Or maybe contrast just doesn't matter as much as boldly colored ink. Our eyes will trick us into thinking anything close to white (and as long as it's the whitest thing in our field of view) is "completely white" anyways.

  2. You're right, the paper did change color of the paper a bit.

    Putting lacquer on the rough surface of the ink definitely increased the *density* of the ink. Putting lacquer on the paper did something different... it made the paper a little less translucent, which is generally not a good thing. If I set that same sheet on a dark backing, the white area with lacquer would really stick out. That makes it kinda hard to see if there is a color shift due to something other than show through.

    For this to make sense in the press room, I think you would want a lacquer plate that didn't put lacquer on white areas.

  3. I think that you are right on Math Guy. A varnish over ink fills in the pockets that are in the paper bringing the color up to the surface. The only thing I think needs to be mentioned is the ink management part if this equation. If you are going to put gloss over color you need to be aware that you are going to have to make sure that the ink being laid down has the affect you are looking for, before it goes to press. That may mean pulling proofs with the gloss over it in the color match process. I have been in many locations where they apply the ink and gloss and get a darker print then they planned on. The other thing to mention, is, if you are planning to use gloss to bring a color density up, do not add it to production that you have already printed. For example, you have a 8 decks and a six color job and you decide to add gloss to that product, you can get shifts in hue that will change the color perception. I you are using an x-rite you will see this shift in hue much easier. Make sure you look at the colors with gloss over them first and adjust ink formulas accordingly.