A binder is any material or substance that holds or draws other materials together to form a cohesive whole. Often materials labeled as binders in different proportions or uses can have their roles reversed with what they are binding. An example would be straw helping to bind clay together as in cob, and clay helping to bind straw together as in a natural insulation.
Liquid binders are added to a dry substance in order to draw it together in such a way that it maintains a uniform consistency. For example, in the Classical World painters used materials like egg, wax, honey, or bitumen as binders to mix with pigment in order to hold the pigment particles together in the formation of paint. Egg-based tempera was especially popular in Europe from the Middle Ages until the early 16th century. However, since that time, the binder of choice for paint has been oil. Today in the artform of sculpture, organic binders are what is traditionally used. An organic binder is a glue made from an animal source or a gum made from a vegetative source. Glue is often made by the boiling of hoofs, bones, or skin of animals and then mixing the hard gelatinous residue with water. Gum-based binders are made from, alternatively, the boiling of plants.
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