Two sheets can weigh the same and print completely differently. The difference sits in the top layer. An uncoated sheet is paper through and through, while a coated sheet carries a thin mineral layer that changes how ink behaves the moment it touches down.
What the coating is
A coated paper is a base sheet with a layer of mineral pigment, usually clay or calcium carbonate, bound with a latex or starch adhesive. The coating is applied on the paper machine or in a separate pass, then calendered between rolls to control how smooth and glossy it ends up. One coating gives a matte or silk finish, and two or three coatings give the high gloss used on magazine covers and packaging. The mineral layer is thin, measured in microns per side, but it decides almost everything about how the finished job looks.
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