# How do erasable pens work?

I understand the principle behind an eraseable pencil. Graphite is highly slippery, you rub it onto the paper, and an eraser can rub it off again.

I've had friends at school with 'eraseable ink pens'. The ink was 'special' and the eraser was 'special' and it could erase the pen in the same way a regular eraser could erase pencil.

My question is - how do eraseable pens work? (Ie what substances are involved and what are their properties?)

• It's not so special rubber. When I had that pen, I soon found out that it's the heat what causes the magic. I would clean whole pages of text on the electric cooker. Aug 31 '14 at 22:56

Instead of a typical oil/dye ink, erasable pens use a compound closer to rubber cement, which acts like a liquid when a bit of pressure is exerted. 1

A bit of history and a link to patents can be found here

EDIT: Be sure to check out the other answer here about thermochromic ink too.

There are two types of erasable pens: rubber cement and thermochromic pens. Thermochromic pens are much more popular nowadays because rubber cement pens literally harden like cement, and can't be erased after a few hours. Thermochromic pens, however, use heat from friction that makes the ink invisible; not exactly removing it from the paper, but close enough.

One downside to thermochromic ink is that the ink can fade over time and rubbing the paper too much can make it invisible, but you can undo this by placing it somewhere extremely cold (think about $$\pu{−10 °C}$$ (freezer)) and it should reappear.

The ink has three components. One component is a pigment (the color) that reacts to acidity by turning invisible. The second component is a chemical that acts as an acid or a base. The third component reacts to heat by activating the acid/base.

By twiddling these three components, the manufacturers can make an ink that disappears when enough heat is applied and reappears when sufficiently cold. When you erase with the rubber, the friction creates heat which changes the acidity of the ink and makes it invisible. When it is cooled down, the acidity changes back and the ink becomes visible again.

In my experience, some of these inks work better than others.

• I don't think the OP is referring to ink that "becomes visible again" after having been erased? Jan 1 '17 at 20:07
• Some erasable pens like Frixion do indeed have ink that can become visible again after having been erased, by freezing. Frixion says, "The ink will reappear at temperatures under -10°C" here: pilot-frixion.eu/uk Feb 7 '18 at 15:26