Why does blotting paper absorb ink?

It is mainly a question of the surface of the paper. A very hard, very smoothly glazed paper will scarcely absorb any ink. If we write on such a paper, the ink takes a long time to dry; and what makes the writing is simply a layer of the solid matter left by the ink. This layer lies on the outside of the paper, and can almost be scrapped away.

All other papers absorb ink to some extent. The drying of the ink means that the water of it has evaporated into the air, while the solids that were dissolved in it remain in or on the paper. But a paper of loose texture, with a rough, unfinished surface, like blotting-paper, absorbs ink just as sponge sucks up water. The water of the ink, instead of mainly remaining on the outside of the paper until it dries, runs into the substance of the paper, according to the amount of ink we use. That is why the letters are not sharply defined when we write on blotting paper.

Additional reading:
Blotting paper (Wikipedia)

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