Hope you had a great weekend! I’m 99% sure on the science of this one, but if I messed up, let me know! — Dan

Let’s say you wanted to paint your room yellow, and just to make it easy, let’s assume the room is currently white. Easy — you just break open a can of yellow paint and get to work. But then, you change your mind and want to make the room red. OK, still not a problem — wait for the yellow to dry and add a coat of red paint. But then you think better of it again and you want a blue wall. Again, not a big issue; you just let the red paint dry and paint over it with a coat or two of blue. That’s how paint works, after all: the topmost coat tends to be the one that’s most visible, as seen in the gif below (which I made via this video.)

Nothing surprising there, as the captions suggest: the yellow is covered by the red which is in turn covered by the blue. That’s just how paint works.

But the stuff above? It’s not actually paint. It’s printing ink, a type of colored pigment used for printmaking. And in this case, the pigment (which I’ll just call “ink” from here on out) has a secret: it’s not just their colors that are different. And that difference makes something cool happen. Let’s take a look at another clip from the same video, and you’ll see what I mean. The yellow ink goes down first, then the red, and then the blue, but only one coat of each. And when it’s done, the yellow still appears to be on top. What’s going on here?

This isn’t magic. It’s science.

The three inks don’t just differ in color — they also have different viscosities, which basically means they vary in “thickness.” Imagine you have two cups, one with water and the other with honey, and go to pour each one. The honey, which has a high viscosity, move very slowly; the water will flow easily. We can do the same thing with ink.

In the second gif above, the printmaker added a lot of linseed oil to the yellow ink, which causes it to become very thin and runny — that is, it has a very low viscosity. The red ink has some, but a lesser amount of linseed oil added to it, so it’s less viscous than the yellow, but still rather “wet.” The blue has no oil added to it, and it’s thick and has a higher viscosity than the other two.

The old adage (and scientific reality) that “oil and water don’t mix” is literally coming into play here. The yellow ink has a lot of oil, the red has less, and the blue has none. This prevents the ink colors from combining. Think of it this way: the thin yellow ink has already created a slippery, wet layer on certain parts of the glass plate beneath it. When the thicker, stickier red ink comes along on the second roller, it can't grab onto that slippery thin yellow layer. The same is true for the blue vis-à-vis the yellow and red layers already applied — the thicker ink only sticks to the bare glass parts of the plate that don't already have the thin yellow ink on them yet.

(It's kind of like this: imagine you painted your hand with something really slippery. Now try to get peanut butter to stick to your slippery hand — it won’t. The peanut butter — basically, the same as a thick ink — will only stick to the dry parts you didn't make slippery.)

And this is more than a neat trick — it’s a huge time-saver for printmakers. Without this technique, multicolor printing would be cumbersome; you’d need to go through the entire printing process for each color. You’d print the blue, let it dry, clean off the plates, add red ink, print again, let that dry, and then repeat for the yellow. With viscosity printing, the printer can do all the colors at once. It’s a game changer.

Plus, it looks like a magic trick.

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Today’s Bonus fact: The word “cliché” has origins in printmaking. Before everything was digital, printers used metal plates to stamp images or text onto paper. When they needed to make many copies of the same image — like an illustration or a decorative element — they created a stereotype plate, a single piece that could be used repeatedly. In French, the sound the molten metal made when poured into the mold to make these plates was said to go “cliché!” — like a sharp, quick click. Over time, it’s likely that the printers began calling the plates themselves clichés. As Merriam-Webster explains, the plates were often used for stereotypes, the association between the two words developed and ultimately stuck.

From the Archives: Why Capital Letters are Called “Upper Case”: It has to do with a printing press.

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And thanks! — Dan