Bookbinding and the Conservation of Books
A Dictionary of Descriptive Terminology

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1. A phenomenon exhibited by various gels, in which the system displays the mechanical properties of a gel when undisturbed, but becomes a liquid when mechanically agitated and again becomes a gel when allowed to stand. This "reduction in viscosity" is due to a temporary breaking down of an internal structure of a system under shear. The viscosity of thixotropic systems depends on the "shear history," i.e., the extent of previous mechanical agitation to which the material has been subjected. The property is important in paper coating colors because it allows the mixing of color formulation to a viscosity which permits the color to be applied and allows surface leveling due to after flow of the color on the material. 2. A property of adhesive systems which causes them to thin upon isothermal agitation and thicken when allowed to stand. (17 , 309 )

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