Glass melting homogenization

Figure 2.6 Photochromic glass (a) glass melt containing dissolved CuCl and AgCl (b) melt is cast into a homogeneous glass blank (c) heat treatment precipitates crystallites (much exaggerated in size here) in the blank and (d) sodium chloride structure of AgCl containing copper impurities and Frenkel defects.

The homogeneous glass, in which a few percent of alumina is added for better processing, is prepared from a melt at 1300-1500°C (Schnabel 1976) or 1000-1200°C/1450 C (McMillan 1980). It is important for the properties that the melt is as homogeneous as possible. Schnabel (1976,1978) produced glass capillaries or hollow fibers directly from the glass melt at a viscosity of 10 P. The phase separation was carried out by heat treatment between 500-800°C. 

High thermal efficiencies that improve the homogeneity of the glass melt product... 

The melt remains in the furnace for several days. A satisfactory yield during the subsequent drawing process requires that the fining of the glass melt be as good as possible i.e. very homogeneous and free from nonmolten raw materials and gas bubbles. This is achieved by air injection through one or more rows of platinum tubes at the... 

An expression is developed for computing the mean size z, of randomly dispersed agglomerates, from the variance S2, obtained with a certain sample size, R. The author postulates a rate equation for solids mixing, which he feels may also be applicable to the homogenizing process in glass melting. 

Combinations of the intrinsic redox system of glass melts with several polyvalent ionic redox systems, which are often present as trace impurities, are of importance for the transparency of optical glasses [18]. An example of these homogeneous equilibria is the combination with the iron(III)/iron(II) ion system. 

After the batch has been thoroughly mixed, it is melted at high temperature to form a homogeneous liquid melt. For small-scale melting of glass the batch will be placed in a crucible. For glass-melting experiments in the laboratory it is common to use Pt, or a Pt alloy such as Pt-5% Au or Pt-20% Rh crucibles. 

The parent glass melt is cooled down to the processing temperature in the fore hearth, homogenized and fed to the rolling machine via a nozzle. The glass flow is controlled by the size of the nozzle gap and the roller velocity. 

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