It seems that the expression clear as glass doesn't apply to the nature of glass. There is more than a little disagreement over what glass actually is.
"They’re the thickest and gooiest of liquids and the most disordered and structureless of rigid solids," said Peter Harrowell, a professor of chemistry at the University of Sydney in Australia, speaking of glasses, which can be formed from different raw materials. "They sit right at this really profound sort of puzzle."
Philip W. Anderson, a Nobel Prize -winning physicist at Princeton, wrote in 1995: "The deepest and most interesting unsolved problem in solid state theory is probably the theory of the nature of glass and the glass transition."
He added, "This could be the next breakthrough in the coming decade."
Thirteen years later, scientists still disagree, with some vehemence, about the nature of glass.
Peter G. Wolynes, a professor of chemistry at the University of California, San Diego, thinks he essentially solved the glass problem two decades ago based on ideas of what glass would look like if cooled infinitely slowly. "I think we have a very good constructive theory of that these days," Dr. Wolynes said. "Many people tell me this is very contentious. I disagree violently with them."
(I love that quote from Wolynes.) It is a fascinating article. One of the little blurbs that intrigued me was that glass becomes more stable over a (very) long period of time. Sort of like how concrete cures.