a network modifier
If the network formers are the builders, the network modifiers are the bolt-cutters. A modifier is an oxide, typically of a large, low-charge cation such as sodium (Na2O), potassium (K2O), calcium (CaO), or magnesium (MgO), that does not join the glass skeleton. Instead its oxygen barges into the network and snips one of the connecting bonds, while the metal cation settles into a nearby cavity, held only loosely by electrostatic attraction. Every modifier oxide breaks bridges, and by breaking them it loosens the whole structure.
The chemistry is precise and worth walking through. Pure silica is a fully connected network in which every oxygen bridges two silicons. Add one unit of Na2O and its single oxygen inserts into a Si-O-Si bridge, converting that one bridging oxygen into two non-bridging oxygens, each now bonded to only one silicon and carrying a negative charge. The two sodium ions sit beside them to balance the charge. So each Na2O added creates two non-bridging oxygens and cuts the network a little more. The more you add, the more chopped-up and less connected the network becomes.
Why do this on purpose? Because pure silica melts near 1700 degrees C and is fiendishly viscous, far too stiff to melt in a bottle furnace or blow into a window. Adding soda as a flux slashes the melting temperature and viscosity, making the glass cheap and workable, which is exactly what soda-lime glass is built on. But there is a price: a heavily modified glass has more non-bridging oxygens, so it expands more on heating (poorer thermal-shock resistance) and is chemically less durable. Pure soda glass, water glass, actually dissolves in water, which is why lime is added back as a stabilizer to restore durability.
Think of pure silica as a chain-link fence with every wire tied. Each spoonful of soda ash is a pair of wire-cutters that snips a few links: the fence goes floppy and easy to fold, which is why the melt pours and blows. Add too much and the fence falls apart, dissolving in water like the sodium-silicate water glass used in cheap adhesives. Lime tightens what is left back up.
A modifier snips network bonds to make the melt fluid; too much destroys durability, so lime is added to stabilize it.
A modifier is a trade-off, not free performance: it buys low melting temperature and easy forming at the cost of higher thermal expansion and lower chemical durability. Cheap, workable window glass is deliberately weakened silica.