Some colour dissolves, some just sits there
Why Does Ink Stay on the Paper?
Ink is colour carried by a liquid — either dissolved as dye or floating as solid pigment specks — and the liquid leaves.
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Step 1 of 5
Ink is colour plus a way to carry it.
Every ink is the same two parts: something coloured, and a liquid to move it onto the paper. The liquid is always meant to leave.
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Step 2 of 5
Dye dissolves. Pigment does not.
Dye molecules come apart in the liquid and go wherever it goes. Pigment is ground-up solid specks that are merely floating, and settle if left.
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Step 3 of 5
Dye soaks into the fibres.
Because it is dissolved, dye travels with the liquid straight into the paper and lodges among the fibres. That is why it is hard to rub off — and why it spreads.
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Step 4 of 5
Pigment sits on the surface and lasts.
The specks are too big to go far into the paper, so they dry on top in a thin film. Sunlight breaks dye molecules apart far faster than it damages a speck.
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Step 5 of 5
A felt pen is a wick with a tank.
The barrel holds ink soaked into a fibrous plug, and the tip draws it out through the gaps between fibres — the same trick as a candle wick.
The ink in the Dead Sea Scrolls is carbon pigment — soot mixed with gum. It is still black after 2,000 years, because a soot speck has nothing left for light to fade.
The short version
Ink is colour carried by a liquid that leaves: dissolved dye soaks into the fibres, while solid pigment dries in a film on top.
Try it yourself
Put a dot from a felt pen near the bottom of a paper strip and stand it in a little water. The colour climbs and splits — most black inks are several dyes mixed.
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