What accounts for the unique structure of glass? Recently, scientists at Germany's Johannes Gutenberg University Mainz used a combination of light scattering and microscopy to explain the origin of glass' amorphous solid state.

Glass isn't a fluid or a crystal. It's an amorphous solid, something in between a liquid and solid. But why doesn't glass crystallize?

Some scientists have hypothesized that portions of solidifying glass freeze their thermal motion and thwart the crystallization process. Others suggest solidifying glass forms particles or clusters that don't fit into a crystalline pattern.

The new research, detailed in the journal Nature Physics, shows the latter hypothesis best explains glass' aversion to crystalline structures.

Experimentation revealed the importance of compacted regions of hard spheres within a melt. Melts can result in the formation of either glass or crystal. Collections of hard spheres, or precursors, can under the right conditions spawn crystals. But they can also prevent crystallization and encourage glass formation.

Researchers found that the higher the concentration of hard spheres, the more quickly crystallization becomes impossible and glass formation ensues.

Scientists used polymer spheres to mimic the presences of hard spheres. Concentrations of polymer spheres above 50 percent encouraged crystal formation, but when concentrations crested 60 percent, the melt was unable to form lattice structures and glass formed.

"In other words, glass results when so many crystallization precursors are formed that they in effect arrest each other," study author Thomas Palberg, a physics professor at Mainz, explained in a news release.

"For us, this means that an unexpected and fascinating link has been found between the two solidification scenarios," Palberg concluded. "Arguably, this was one of the most important missing pieces of the puzzle."