Devitrification is the process in which glass molecules rearrange themselves into a stable structure, resulting in a hazy and opaque appearance. Clear glass is created through vitrification, but devitrification can occur when the crystals try to return to their crystalline structures. It is common in nature at the site of volcanic eruptions and has high geological significance. In glass art, it is considered a flaw and artists take precautions to avoid it, but repairs may be necessary. There is no one-size-fits-all solution to avoid glass devitrification.
Devitrification is a chemical process in which glass molecules rearrange themselves into a stable structure, which usually has a hazy and somewhat opaque appearance. Molten glass cooling before its molecules have had a chance to rearrange themselves into tight bonds remains clear, but the natural tendency of most compounds is toward a more stabilized structure. Devitrification is an important geological concept, but it is also often discussed in the realm of glass art. Most artists consider devitrified glass to be marred and generally try to avoid this outcome.
There are many different types of glass compounds, and scientists tend to view the term “glass” more broadly than those in mainstream 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 fully transparent due to chemical instability. The natural tendency of most glass and similar compounds is to seek a more stable and solid structure.
Clear glass is created through a process known as vitrification, in which 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 disequilibrium. The original crystals dissolve and their central elements hang against each other in suspension. This often creates the clear glass so familiar in everyday life. Crystals generally try to return 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 earth’s core often causes near instantaneous vitrification of the surrounding rocks and minerals, but as the eruption slows down and the temperature cools again, the solid state returns. Devitrification is almost always very evident in the structure and shape of the reconstituted rock. It is nearly impossible for a solid to convert to a liquid, then return to its solid form intact. The molecules are the same, but the compounds they form and the chains they create during their transitions give way to a marbled, striated surface.
The devitrification of quartz and other minerals in nature is often seen as something beautiful and generally has high geological and academic significance. The same is not always true in the realm of glass art. Humans have been blowing and firing glass into all kinds of vessels and creative shapes for centuries. Much of the practice depends on precise temperature controls and manipulating the liquid glass’s exposure to the heating and cooling mechanisms.
When glass art projects are fired for too long or at too high a temperature, parts of their surface can devitrify, which is usually immediately apparent through patches of dull or hazy surface. It’s rare for an entire project to devitrify, but some edges or sides can change for a variety of reasons. Heat is usually at the core, but the overall thickness of the glass, any marks left on the surface, and the overall composition all play roles.
Artists usually take a few precautions against glass devitrification, including precise timing controls, the use of specialized devitrification spray products, and intensive cleaning regimens. However, not all structural changes are avoidable, which often means repair is in order. Sometimes heating or relighting a devitrified piece will soften a flaw, but other times more drastic measures such as sandblasting or new glass coatings are required. Many artists have self-proclaimed foolproof methods, but there is simply no one-size-fits-all solution. Glass devitrification is a normal and natural process that is often very difficult to avoid altogether.
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