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.

Flat paint is not flat

I needed to paint the inside of an ambient light exclusion device for an optical mensuration1 experiment. (To the layman, I was trying to paint the inside of a box black.) I had decided on flat black mainly because it would look cool. Flat black really gives A/B plywood an air of scientificalness.

The trouble was, this was autumn and the temperature in my garage2 was 45°. The warning label on the spray paint can advised me against painting at temperatures below 50°, but I threw caution to the wind. In single-minded devotion to the greater cause of Science, I forged ahead, hoping that I did not get caught by the Spray Paint Police.

I can look back on this today and laugh at my naiveté. I was quite frankly surprised by the results. The flat black paint came out glossy.

How can flat paint become glossy?

Naturally, my first reaction was to get out the microscope to investigate the microstructure of the surface. This is what I saw.

The image at the left is the surface that I had just painted. The one on the right was a surface that looked flat black because I had painted it at the regulation temperature. Clearly, the glossy surface on the left is smoother than the matte surface on the right. Ironically, the surface of “flat” black paint is not flat!

My “flat black” spray paint became glossy because the vehicle (the stuff that carries the pigment) evaporated much slower at 45°. This gave the surface of the paint time to smooth out.

Changing color with colorless water

Seeing these images, I immediately had a flashback to one day when I was five years old. I had found myself with a small plastic bucket filled with water and a paint brush. The young scientist in me bloomed when I painted the wall of our house with water, and found that it turned the house from a chalky light green to a richer green.

Some adult wandered by and asked me what I was doing. When I said that I was painting the house, they frantically went off to find my mom. My little ruse had fooled even them. I probably got spanked before the truth was realized. If only I had known Fresnel’s law at the time, I would have been able to explain the phenomenon to my mother before her hand came down on my precocious little butt.

Fresnel’s Law

Fresnel’s Law3 describes what happens when light goes from one medium to another, in this case from air to the surface of the paint or water. When light goes from air to a surface that is optically “harder” (having a higher index of refraction), some light enters the surface and other light reflects directly from the surface, in billiard ball fashion – with angle of incidence equaling angle of reflectance.

Fresnel’s law predicts what percentage of the light is reflected and what percentage of light enters the second medium. The law takes into account the angle of incidence, the indices of refraction of the two media, and the polarization of the incoming light. All that is fabulously interesting, but all we need to know is that somewhere between two and five percent of the photons act like little billiard balls. We call this specular reflection.

I brought my good friend Smeldon in to demonstrate. He reads his magazine4 with the light behind him, and the magazine tilted slightly away from him. Without even thinking about it, he will orient the magazine so as to avoid the specular reflection. Smeldon could orient the magazine so he sees only the gloss of the magazine, as in the drawing on the right. If he were to do this, the print on the paper would be largely washed out by the specular light.

Smeldon’s magazine was printed on a coated stock. After the paper is formed, it goes through a second process where various additives, like kaolin (a white clay) and calcium carbonate, are affixed to the surface of the paper and then the paper is polished to a glossy finish. The coating makes the surface glossy. Most magazines are printed on coated stock. Newspapers and Sunday paper inserts are printed on uncoated stock.

If Smeldon were to try this same experiment with a newspaper, he would not see the same results. In fact, he would be hard pressed to find a way to orient the newspaper in such a way as to see gloss. (The interested reader will try this before going on. Go ahead. I’ll wait.)

Smeldon is not an accomplished color scientist and applied mathematician like me (and he is not nearly as good-looking as me) but he does have a bit of a scientific bent. His conclusion is that newsprint is an exception to Fresnel’s law. It would appear that newsprint has very little surface reflection.

Holding out on the dark side

Smeldon is inquisitive and intuitive, but in this case, he is wrong. A newspaper has plenty of specular reflection; it’s just hiding in plain sight. I’ll get back to that in a bit.

Anyone who has looked at a magazine and newspaper side by side will notice that the magazine has much richer color. A solid black on newsprint looks almost gray when sitting next to a solid black of a glossy glamour magazine.

Why is this? I have heard many otherwise intelligent people attribute this to “holdout”. As the explanation goes, glossy stock will “hold out” because of its nice smooth and hard surface. That is, it will impede the progress of ink that tries to seep in. Newsprint has no protective coating, so ink will seep into the paper, and some of the cute little pigment particles will hide behind paper fibers.

I am sure this is explanation is true, but hey, flies walk on the ceiling. I am not convinced that this is the predominant effect. I submit the following image as proof. The image is of newsprint. I borrowed my wife’s clear nail polish and painted over the word “FREE”. The black ink with the polish is noticeably darker. I didn’t add pigment. This was clear nail polish. This is not explained by the “ink hiding behind the paper fibers” theory – the cute little pigment particles are still in hiding. Why did it get darker?

Newsprint with splotch of nail polish

The next picture sheds some light on the quandary by shedding light from a different direction. To take the next picture, I left the camera and the newsprint where they were and moved the light over to the specular angle. The splotch of nail polish has suddenly gotten much brighter because the specular reflection has suddenly reappeared.

Same sample, with light at specular angle

Going back to the original pair of microscope images, we saw that the “flat” black ink on the right had a rough texture. Photons hit this rough surface and reflect specularly in all sorts of directions. Contrary to Smeldon’s conclusion, newsprint still follows the magic of Fresnel’s law, it’s just that the specular reflection goes in all directions.

In the first “free estimate” image, it’s not so much that the nail polish made the black ink darker. It’s more that the specular reflection in the other parts of the image made the black ink without the polish look lighter. The nail polish focused the specular reflection so that it all bounced away from the camera.

My conclusion is that smoothness will make a surface appear darker in color. This is not because there is less total light reflected. It is because the specular reflection from a smooth surface generally bounces off in a direction where we don’t normally notice it.

Notes 1)      No, I didn’t spell that word wrong. “Mensuration” means the act of measuring. All good mensurologists know this word and use it all the time. It’s like a secret handshake. 2)      Due to certain previous incidents, I wasn’t allowed to paint in the basement like other respectable mensurologists do. I guess we can see why I don’t have a Nobel Prize. Yet. 3)      In my humble opinion, Fresnel’s Law is way cooler than Snell’s Law. By the way, Snell’s Law was discovered by Ptolemy. I don’t know who discovered Fresnel’s law, but we know from a previous blogpost that it probably wasn’t Fresnel. As for the Fresnel lens? That idea came from Georges Louis Leclerc, and not Augustin-Jean Fresnel. 4)      Magazines were a technology common in the 20th century that was similar to the iPad. While articles could be read on these devices, you could only check the status of a few celebrities, and one magazine could barely hold a few dozen articles. To their credit, they were produced from a once common renewable resource called “trees”. Ask your grampa to tell you about trees.