The Glass Transition

Here is a partial answer to the question, 'How does a glass (read: glaze) that has been cooled rapidly differ from one that has been cooled slowly?'

When molten glass is cooled it "freezes". Looks like a solid; breaks like a solid; and like a solid supports heavy objects placed upon it.

Glass freezes slowly, gradually, in a range that is called the glass transition region, an interval with lower and upper end points, T_Low, T_High. Below T_Low, the glass is solid; above T_High, is liquid. Within the glass transition region, the glass is neither liquid nor solid, having properties reminiscent of both liquid and solid.

The values of T_Low, and T_High and the size of the interval depend on the rate at which the glass is cooled, the properties of the glass, starting with its structure also depend on the cooling history.

To give a sense of why this might be so we show a diagrams of a two dimensional silica glass (with the SiO2 tetrahedra replaced by triangles). This is followed by a comparison of the fast cooled _vs_ slow cooled versions of that glass. The situation for the glass is analogous to a large unruly mob of theater patrons rapidly filing into an auditorium; many seats will be left unfilled, and inaccessible. This is contrasted with the situation if the group is only allowed to move in slowly, with ushers directing traffic.

With the glass, bonding is in one direction. Once bonds form, they don't break. Two nearby silica units, if they are sufficiently close, and not obstructed by intervening obstacles will attach. In more detail, a corner of a silica unit can be attached to at most one other such unit, and only at one of its corners. So, two silica units will attach firmly if a corner of each encounters a corner of the other. If the glass cools rapidly, like the unfilled seats in the auditorium, many silica vertices will be trapped in interior spaces with no possible connecting vertices available to them. If, however, the glass cools slowly, there will be a chance for a more ordered and regular structure to form.

The more regular structure of the slow cooled glass will be denser than the open shaggy structure of the fast cooled glass, and at any temperature, below T_Low, it will have a lower coefficient of thermal expansion.



Two Dimensional Illustrations



Figure 1

Figure 1 - A Two Dimensional Silica Liquid


Figure 2

Figure 2 - A Fast Cooled Two Dimensional Glass


Figure 3

Figure 3 - A Slow Cooled Two Dimensional Glass


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