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The Two Types of Erasable Ink and Why One Is Much Cooler Than the Other

What do beer cans, shopping receipts and erasable pens have in common?

Introduced in 1979, the Papermate Erasermate offered something that, up until then, had been science fiction: an erasable pen. While correction fluids and tape were a well-established solution to inky mistakes, covering up errors was not the ideal way to correct them. Erasermate offered a simple alternative—just remove them.

Nearly three decades later, in 2007, Pilot debuted their own erasable pen by the name of FriXion. You’d be forgiven for missing this product launch. Erasable ink seemed like yesterday’s news at that point—literally a decades-old technology. But FriXion pens are a lot more revolutionary than they may seem.

The older Erasermate type of erasable pens are made with ink contained in rubber cement during production. The rubber cement keeps the ink particles from absorbing into the paper but doesn’t prevent them from adsorbing onto it. What results is a mark on paper that is pretty similar to a graphite one made from a pencil. Since it hasn’t soaked into the paper but is just sitting on top of it, it can be rubbed off of the surface.

FriXion pens, meanwhile, rely on an entirely different phenomenon for their erasability. In fact, depending on how you think about it, they’re not really erasable at all.

The ink in FriXion pens is made of a three-component system. Contained in microcapsule particles with a diameter of 2-3 µm (For reference, that’s around 25 times smaller than the width of a human hair) are a leuco dye, a colour developer, and a colour temperature adjusting agent. The specific chemicals aren’t known—they’re proprietary trade secrets—but the general process works like this: At normal room temperatures the leuco dye and colour developer are in contact, and the ink is colourful. As the temperature rises the colour temperature adjusting agent starts to work, and the leuco dye and colour developer separate, rendering the ink colourless!

For FriXion pens, this colour change happens at roughly 60 ˚C. This can be achieved by heating the paper with a hairdryer or some other heat source or by rubbing it with a rubber eraser. Friction can make a surprising amount of heat surprisingly quickly—think about how you can start a fire by rubbing two sticks together.

A compound that changes colour with temperature is called thermochromic, and they have a few very important applications. Notably, receipts are printed on paper coated in such a compound. Cash registers don’t actually contain any ink. Instead, they are called thermal printers and work by selectively heating parts of their printer heads to produce the shape of a letter or number on the thermal receipt paper. Other technologies that use thermochromic dyes include mugs that reveal a design when heated and cold indicators on beer cans, like Coors Light.

The really neat part, in my opinion, is that this colour change reaction is reversible. Hence why one can debate whether FriXion pens truly are “erasable” or not. Lowered to about -10 ˚C (most freezers are around -18 ˚C) the “erased” ink will reappear as if by magic, but we know it’s actually just chemistry.

I am a big fan of erasable FriXion pens and recently experienced their reversible colour-changing phenomenon firsthand. Unfortunately, I left my notebook in the hot summer sun and returned to find it totally blank. Luckily, having researched this topic, I knew a couple of hours in the freezer were all I needed to recover my precious notes.


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