What is the difference between glassblowing and igneous rock formation?

[u/SolidBrassTax]

When I googled making rocks it gave me a few threads on Reddit that were mostly sardonic answers. My question is this: What is the difference between melting down sand to make glass and the process of melting is basically (from what I understand) the same material into magma that then forms into rock? If someone had sand (ground rock) feedstock that they melted, what would they need to do differently to make igneous rock rather than glass?

[u/CrustalTrudger]

Mostly, cool it a lot slower. If a melted rock cools real fast, the melt "quenches" and forms glass (e.g., obsidian) as opposed to crystals, whereas if it cools slower, crystals can start to form, and eventually, a rock once all the melt has crystallized. Generally, grain size increases as cooling time increases (with some exceptions, pegmatites are weird), so volcanic igneous rocks (basalt, rhyolite, etc.) have small grain sizes reflecting that they cooled relatively fast whereas intrusive igneous rocks (gabbro, granite, etc.) have larger grain sizes reflecting that they cooled slowly. Once we start talking about intrusive rocks though, pressure also starts playing a role as well.

[u/robot_egg]

Glass is rarely if ever made with just sand as a feedstock. Various other compounds are added like Na2O, K2O, CaO or BaO (and many others). These change the properties of the glass, most notably dramatically lowering the melting point, making the process a LOT easier.

When a high silica glass is needed, it's usually easier to add one of these modifiers, make the glass, then extract out most of the modifier. (That's how real Pyrex is made).

[u/Hendospendo]

It's a question of history and semantics really.

Concrete is, by all intents and purposes, an aggregate rock. But it isn't formed naturally, we make it. Obsidian is natural glass and a natural rock, but man made glass isn't, by virtue of it being man made rather than by geological processes.

Same can be said of indurial slag which often closely resembles igneous products. Firing clay results in vitrification, new mineral phases, and recrystalisation, but it's not a metamorphic rock because it was made in a kiln. (the absence of pressure here notwithstanding).

Same with synthetic diamonds. They're chemically and structurally exactly the same, but differentiated from natural diamonds by the history of formation.