A binder is any material or substance that holds or draws other materials together to form a cohesive whole mechanically, chemically, or as an adhesive.
Often materials labeled as binders can have their roles reversed with what they are binding, i.e. fibrous filling materials can also be considered to be binders as they contribute to the strength of composite materials when the matrix material is prone to crack under tension and shear forces.
For example straw is helping to mechanically bind clay together as in the building material cob. Clay on the other hand binds the straw together as an adhesive in cob and in light clay mixtures used to manufacture natural insulation.
In a more narrow sense, binders are substances that transition from dough-like to stone-like state and, thus, bind filler powder and particles added into it.
The transition/binding property is used widely to prepare shaped articles (e.g. pots and vases) or to bind solid pieces (e.g. bricks).
Based on their chemical resistance, binders are classifies by the field of use: air (gypsum, air-cements, magnesia), water (Roman cement, Portland cement), acid-resistant (silicon fluoride cement, quartz cement), and autoclavable (harden at 170 to 300°С i.e. 8-16 atm pressure and, e.g., comprise CaSiO3 materials) .
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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