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.

Getting the most color for your ink, part 1

So, my wife says to me the other day, "Hey Math Guy, you should do a blog about ink mileage - you know, making the most efficient use of pigments." I love it when she calls me Math Guy. It's so much more intimate than John the Math Guy. "Surely you've thunk some thoughts that no one ever thought to thunk before." I don't love it quite as much when she calls me Surely.

But she does have a point. I do have a few thoughts. I have been waiting for an opportunity like this to share them with an eagerly waiting world.

Dot gain is your enemy

I remember years ago hearing a competitive pride in the press room when it came to printing sharp dots. Mushy dots were a sign of a sloppy pressman. Really good pressmen, running on really good presses would produce sharp, crisp dots. Really good pressman would make Felix dots. All pressmen wanted to be Felix. All presses wanted to be Felix presses. All printing inks wanted to be Felix inks. All fountain solutions wanted to be Felix solutions.

Felix halftone dots versus Oscar halftone dots

And so it was the goal to make crisp, Felix dots, and press crews worked at reducing dot gain.When a press crew managed to bring the dot gain down by one point, there would be huge bonuses and wild parties and groupies everywhere. Many of you remember those days. Mick Jagger would show up, and the roll tender would get a call to be on the Carson show. You get it. The whole magilla.

Then those darn pre-press guys started getting involved. Stick-in-the-muds, every one of them. They complained about needing to run a different plate curve for every job. The pre-press folks didn't buy into the whole press room machismo thing. They didn't want to be on late night TV. They just wanted consistency.

And so it came to be that the printing pundits made the rounds, popping in on the Tonight Show and getting little blurbs on the bottom of page 17 of the tabloids. The message went from "Dot gain and Communism are the enemy" to a more subtle one. "Dot gain, just like Kim Kardashian, is inevitable. We can't get rid of either one of them. All we can do is try to control them."

In light of all of this, what I am about to say is heresy. When it comes to ink mileage, dot gain is your friend. Yes, it needs to be controlled and kept consistent, but more is better.

Kim Kardashian proudly displaying high dot gain

Why is there dot gain?

A point to consider: stochastic printing (FM screening) has high dot gain. That means you need to adjust the plates curves to make it print like conventional printing. Get it yet? You need to bring down the 50% when you're printing stochastic. Have you caught my point yet? You need to put less ink on the paper when you are making a halftone. Less ink for the same amount of color.

I think that the fact that stochastic screening requires a different plate curve is common knowledge, but I'm not sure that everyone has connected the halftone dots. Stochastic printing requires less ink. High dot gain means less ink.

But... maybe the "less ink" part is not obvious. Maybe I need to expound on a question that has baffled philosophers of printing science for decades. Why is there dot gain?

Every Phy-Ed major knows why there's air - to fill up volleyballs!

I know of three explanations for why halftone dots come out fuller than one would expect: more ink, more diffusion, and more squish. The physics is probably correct behind all of them, but it is likely that one or two of them are the major factors. 

More ink

The simplest explanation is that the plate simply delivers more ink. The more ink, the more dot gain. As it was explained to me by Herr Gutenberg, it all had to do with ink/water balance. When you put a little more water on the plate, it will crowd out the ink in at the edges of the dot, and there will be less dot gain. If there is a bit less water, the ink will have the upper hand at the edge of the dot, and the dot will grow.

Based on this, the model is this: more dot gain -> more ink -> more color. Simple enough.

This all makes sense, but I have tracked press runs while adjusting water up and down within reasonable limits. I saw a lot of change in dot gain over hours of press time, but little of it was correlated with the amount of water. I don't think that the "ink/water balance at the edge of the dot" theory is the big explanation for dot gain. 

More diffusion

In 1936, Yule and Neilsen came up with the idea that there are two parts to dot gain: physical and optical. They said that the dot on the paper is indeed larger than the dot on the plate, but that there was a second effect. The paper between the dots takes on some of the color of the ink because of light diffusing into the paper.

I won;t go into much detail explaining it here. You can look at my previous blog post for that. I just want to say here that the Yule-Neilsen effect gives you a little extra color for free. That apparent tinting between the dots acts like more ink, more ink that you get for free.

More squish

Noffke and Seymour came up with a little different explanation in 2012 - dot squish. (Some of you may recognize the name Seymour. He has this blog?) A pristine silo of ink is first deposited on the printing plate, and then that nasty old press comes along and presses it flat. Note that there is no change in the volume of each dot, just it's shape. 

Halftone dot transmogrification under imply pressure

But what of the color? Does squishing the dots change the richness of the color? Well, yes. I go into more detail in the blog on the Noffke-Seymour effect, and Pat and I went into excruciatingly painful detail in the TAGA paper. 

(By the way, the call for papers for the 2015 conference is out. The conference is set for gorgeous downtown Albuquerque, March 22 through 25. Email me if you have any questions, or want to discuss an idea. john@johnthemathguy.com)

Does this get you more halftone for your money? Let me motivate the idea a little bit by considering the extreme. Let's take that silo to the extreme. Keep the silo the same volume, but picture it becoming more of a needle - a very tall spire of ink that has a very, very tiny footprint on the paper. Being very tall, the microdensity of that ink is very high. The color at the top of that spire is very rich. But it covers an infinitesimal amount of paper, so the overall reflectance is pretty much the same as the paper. 

That tall narrow spire is the very least efficient use of ink. It is the cleanest, crispest dot possible, but it is absolutely lousy when it comes to ink mileage.

Dot gain is your friend

The first of the three explanation for the cause of dot gain predicts that dot gain is "cost neutral". You put in more ink, and you get just that much more color. The other two explanations predict that dot gain is like getting a little extra pigment in your halftone for free. My own observations are that the first explanation of dot gain is not the major effect.

So, my conclusion is that dot gain is your friend. According to the Yule-Neilsen model, whatever it is on press that causes more spread of light into the paper, like higher line screen or stochastic printing (or perhaps some reformulation of the paper?) can reduce the cost to print a halftone of a given color. 

According to the Noffke-Seymour model, whatever it is on press that causes the halftone dots to spread out more, like decreasing viscosity or increasing pressure, can reduce the cost to print a halftone of a given color. 

Moral of the story -- An efficient halftone dot is a happy halftone dot