1. Correct.

2. Not sure exactly what this is asking. Glasses are often defined as (supercooled) liquids cooled until their viscosity exceeds 10^13 poise (= 10^12 Pa s).

3. Yes, typically crystallization is observed upon heating a glass above Tg.

You may find some classic articles of interest if you have not yet seen them, for example:

Debenedetti PG, Stillinger FH. Supercooled liquids and the glass transition. Nature. 2001; 410(6825): 259-267.

The glassy state is a non-crystallized solid material state with so called "nearby/close order" and viscosity in the range of (traditionally) 10^13.6 Ps. The glass transition in its "classical" way with sudden and visible change of other properties (e.g., a DSC peak) does NOT always occur. For example, the very high molecular weight molecules such as polysaccarides or proteins can go WITHOUT any DSC peak. The major criterion for the glass is its viscosity.

NOTE though that IT DEPENDS on the relation between the relaxation time (Tr) of the material and the time of observation/reaction To.

Example 1: old window glasses of the EARLY Medieval cathedrals "flows" like water: time of observation is very long that makes it comparable with the Tr of the glass.

Example 2: if an airplane crashes onto water surface from a high attitude, the water behaves like an asphalt so the airplane disintegrates into small pieces. Here, To is very short and comparable with the small relaxation time of water.

A supercooled LIQUID has no direct connection with a glass as its VISCOSITY is low. It may go to the glassy state or it may not, but there is NO direct connection, there are two different processes: one is a thermodynamical parameter (supercooled), the other one is a material physical property (very viscous non-crystal glassy state). They are connected thru the phase transition diagram (the third dimension would be for example, concentration of the glass forming material in a water solution, the molecular weight-percent of a mixture, etc).

Glassy is a designation of order. It is used even for polymers, metals, and semiconductors. It is not unique to the material called (window) glass. The designation glassy means short range order with no long range order. The designation is in the group of terms with amorphous (no order at any scale) and crystalline (perfect order over long length scales).

Old window glass does not flow. That is a myth. https://www.cmog.org/article/does-glass-flow

The viscosity of a material is not a criteria to designate it as glassy or as a glass. Viscosity concerns flow, not (molecular level) ordering. Viscosity is a designation of where a material sits in the transition of solid or liquid state.

I belong to the school that think that the glass is determined by its viscosity. Amorphous vs. crystalline, all that short/long range and "levels of order" things is the molecular level. The glassy state (of course not only window glasses, that would be silly!) is determined by its material quality - viscosity. As I said, you can have a perfect glass that does not show any transition characteristics except for the highly elevated viscosity near its "Tg". Biopolymers for example. Looks like my opponent has never worked with such materials (not surprisingly, he put a key word "even"!). I am as a preservation scientist deal with polymeric glasses w/o any peaks on DSC all the time.

@I Katokov: I defer to one modest clarification. Viscosity may be used in some circles to characterize the presence of a glassy state. It is however only an indirect or implicit measure. The true measure behind the definition of a glassy state is the type and degree of order at the molecular level. For a reference that is counter to the use only of viscosity, liquids are also glassy materials (while gases are amorphous). A glass transition temperature Tg occurs at a second order phase transition from a viscous (glassy) liquid to a (glassy) solid.

@Weiner: As I said, and many think like me:

1. Type and degree of order are NOT definitions of glassy, but rather a molecular definition of an amorphous state in SOME cases.

2. As I've also said, many high molecular solutions are glasses but they don't have have any glass transition points whatsoever, that is why I put "Tg" in "". That is why the very word "vitrification" in the preservation size is exactly to put a material in a very viscous state so accordingly to the Einsten-Stokses' law (a.k.a. Einstein-Smoluchowski equation), the characteristic time of the reactions in this glass are millions and even billions years ago. It is in my papers. Has NOTHING TO DO with the "second order phase transition". That point of view is outdated/applied to some of the cases.

The glass in cathedrals has definitely not exhibited any flow since installation. That is a myth that was disproven within the inorganic glass community decades ago. The viscosity of these glasses extrapolated to room temperature would be so great that such flow would require longer than the age of the earth.

In medieval times, glass panes were made by spinning a blob on the end of a rod dipped into the melt. This results in material which is thinner at the edges than in the middle. Panes cut from this large circle are thus wedge shaped when viewed on edge. The accepted way to install them was to place them with the thick edge down. So they look like flow had occurred. Occasionally, we find pieces which were installed with the thin edge down. That does not mean that the glass flowed up! Check with Arun Varshneya if you want to learn more about this.

I saw it in an old Frankish cathedral with my own eyes!

I will oversimplify the answer of your question with water and glass example. I hope this will answer your question.

Super-cooling leads to a super-cooled liquid in case of water (meta-stable state). It has a high tendency to solidify by nucleation but can not crystallize until it has the right conditions.

Glass is an amorphous material (solid and meta-stable). You can solidify anything (under controlled conditions) using a critical cooling rate! Therefore, you can obtain a glass of any material but it will in time crystallize it may take hundreds of years. For example those glasses you see in old chapels you may find small nucleation and growth areas.

An understanding of TTT graphs would be useful to grasp the idea.

Here are some literature:

Article A Kinetic Treatment of Glass Formation

https://www.intechopen.com/books/supercooling

Thanks Dilek for "popularization" of my posts ;-)

IK

We have attempted to distinguish amorphous and glassy states ― and the corresponding material substances: Article Aspects of the non-crystalline state

Glassy state is ONE of many types of amorphous state and "TO COMPARE" them as like comparing brain and ... frontal lobe.

IK

P.S. Gosh, so many unqualified folks here!!

I think glassy state is that state of supercooled liquid where viscosity reaches 10^13.5 poise which corresponds to the viscosity of solids. Supercooled liquid and glassy states differ in viscosity and not much differ in molecular dimensions.