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Coloured pencils are pigment held together by a binder and encased in wood. The quality depends on the ratio between pigment and binder and the quality of the wood. The artist quality pencils will create more vibrant colours and the most popular wood for pencils is cedar. Most colour pencils will use a wax binder and with these pencils you can add layers and layers of colour to get a depth of colour.
However, the drawback of wax pencils is the potential for wax bloom and build-up. This occurs when the binder starts to evaporate and move to the surface creating a white residue. Although in most cases this can be wiped away. However, Polychromos pencils differ with their oil binder which holds and spreads the pigment. It also reduces the risk of any bloom over time and you’ll notice a difference with these leads.