Littleton softening point

Softening temperature The Littleton Softening Point is most commonly used. At this temperature the glass hs a viscosity of 10 N s/m. ... 

This relation contains three constants (A, B, which can be calculated from three viscosity points. These are usually provided by the sink point, the Littleton softening point and by determined dilatometrically (refer to Section 5.2). By means of these values and of eqn. (9) it is then possible to interpolate satisfactorily and even to extrapolate, within a narrow range, the entire viscosity curve for current types of glasses. The suitability of relationship (9) may easily be checked by plotting the three values measured on the log tj — l/(T— To) diagram the points should lie along a straight line. 

The Littleton softening point is the temperature at which a glass fibre 0.55 — 0.75 mm in thickness and 235 mm in length is elongated under its own weight at a rate of 1 mm min when suspended n a standard vertical tube furnace heated at a rate of 6—10°C min (Littleton, 1940). 

The softening point is more properly termed the Littleton softening point, after the specific test used to define this reference point. The viscosity of 10 Pa s does not represent the deformation temperature for all objects. This particular reference point is defined in terms of a well-specified test involving a fiber -0.7 mm in diameter, with a length of 24 cm. The softening point is defined as the temperature at which this fiber elongates at a rate of 1 mm min when the top 10 cm of the fiber is heated at a rate of 5 K min. In fact, if the density of the fiber is significantly different from that of a typical soda-lime-silica composition, the viscosity will not be exactly 10 Pa s at this temperature. 

The most widely used viscometers are based on measurements of the rate of elongation of a fiber of known dimensions under a known load. This method can be used for viscosities ranging from 10 to 10 Pa s. This method is also used for the determination of the Littleton softening and annealing reference points. Since the method requires formation of a long fiber for a specimen, it is well suited for many easily-worked commercial glasses, but difficulties in the formation of good fibers from many experimental compositions often limit the use of this method for basic research studies. 

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