Mixing oil paints Paint is a coloured substance that, when spread over a surface, dries to leave a solid coating. Paint is used for artworks and for protecting and colouring surfaces, such as walls and fences. Varnishes and lacquers are paint-like substances that, when dried, leave a hard, protective layer. Inks—used for writing and printing—are similar to paints, but they do not leave such a thick coating on a surface. Adhesives are substances that stick together surfaces—they are commonly called glues. It is not just glues that make use of “stickiness”—without stickiness, none of these substances would work.
Some substances, such as honey, treacle and resins, are naturally sticky. In these, stickiness is created by attractions between molecules. The hydrogen atoms in these substances attract the molecules in other substances to them, rather like tiny magnets. These are the same forces at work between water molecules, but the molecules that make up sugary substances have many more hydrogen bonds holding them together.
Tar was the first adhesive known to be used by humans. Stone tools stuck together with tar (made by heating birch bark) have been found in Italy, dating back 200,000 years.
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