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Devitrification is a chemical process in which glass molecules rearrange themselves into a stable structure, usually resulting in a hazy, somewhat opaque appearance. Molten glass that cools before its molecules have had a chance to rearrange into tight bonds remains clear, but most compounds’ natural tendency is towards a more stabilized structure. Devitrification is an important geological concept, but is often also discussed in the realm of glass art. Most artists view devitrified glass as blighted and generally seek to avoid this outcome.
There are many different kinds of glass compounds, and scientists tend to view the term "glass" more broadly than do those in common society or the artistic world. In chemistry, glass can include any range of compounds that adopt a glassy state when heated towards their melting point. Glass typically only appears clear and completely transparent as a result of chemical instability. The natural tendency of most glass and glass-like compounds is to seek a more stable, solid structure.
Clear glass comes into being through a process known as vitrificattion, wherein a solid is heated so hot that it becomes a liquid. During vitrification, the molecules in the solid — sand, sometimes, or other quartz or earth minerals — are rearranged into a state of imbalance. The original crystals dissolve, and their core elements hang against each other in suspension. This often creates the clear glass so familiar in everyday life. The crystals generally try to get back to their crystalline structures, however, and when they do, devitrification occurs.
Examples of devitrified glass and rock are most commonly found in nature at the site of volcanic eruptions. Liquid magma from the core of the earth often causes almost instantaneous vitrification of surrounding rocks and minerals, but as the eruption slows and the temperature once again cools, the solid state returns. Devitrification is almost always very evident in the re-constituted rock’s structure and shape. It is nearly impossible for a solid to convert to liquid, then go back to its solid form untouched. The molecules are the same, but the compounds they form and the chains they create over the course of their transitions gives way to a marbled, striated surface.
Devitrification of quartz and other minerals in nature is often seen as something of beauty, and typically holds high geological and academic significance. The same is not always true in the realm of glass art. Humans have been blowing and kiln-firing glass into all sorts of vessels and creative shapes for centuries. Much of the practice depends on precise temperature controls and manipulation of liquid glass’ exposure to heating and cooling mechanisms.
When glass art projects are fired for too long or at too high of a temperature, portions of their surface can devitrify, which is usually immediately apparent through patches of clouded or muddled surface. It is rare for an entire project to devitrify, but certain edges or sides can change for a range of reasons. Heat is usually at the core, but general glass thickness, any marks left on the surface, and overall composition all play parts.
Artists usually take certain precautions against glass devitrification, including precise timing controls, use of specialized devitrification spray products, and intensive cleaning regimens. Not all structural changes are avoidable, though, which often means that repair is in order. Sometimes reheating or re-firing a devitrified piece will soften out a blemish, but other times more drastic measures like sandblasting or new glass coatings are required. Many artists have self-proclaimed foolproof methods, but there is simply no one-size-fits-all solution. The devitrification of glass is a normal and natural process that is often very difficult to completely avoid.