a network modifier
You have built a rigid jungle gym (the network former's job), but it is so strong and stiff that you can barely bend or reshape it — pure silica has to be melted white-hot before it will flow. What if you deliberately snipped a few bars here and there? The frame would loosen, sag more easily, become workable. A network modifier is the chemical scissors that does this: an oxide added to a glass whose job is to CUT bonds in the continuous random network, loosening it so the glass melts and flows at far more convenient temperatures. Soda (Na2O) and lime (CaO) in ordinary window glass are the everyday modifiers.
The mechanism is precise and worth walking through. Take soda, Na2O, which supplies sodium ions and extra oxide ions. When it dissolves into the silica network, each added oxide ion breaks one silicon-oxygen-silicon bridge: an oxygen that was BRIDGING two silicons becomes two NON-BRIDGING oxygens, each bonded to only one silicon and carrying a negative charge. The sodium ions (Na+) do not join the network; they sit in the holes nearby, parked next to the non-bridging oxygens to balance their charge. So one unit of a modifier like Na2O converts two bridging oxygens into two non-bridging oxygens — it depolymerises the network, snipping the connected framework into shorter, less linked pieces. Modifiers are typically large, low-charge cations of the alkali and alkaline-earth metals (Na+, K+, Ca2+, Mg2+): too big and weakly bonded to build a network themselves, they instead nestle in it and cut it.
This is why modifiers are the glassmaker's main control knob. More modifier means more broken bonds, a looser network, lower melting and softening temperatures, and easier forming — which is exactly why soda-lime glass can be blown and pressed while pure silica cannot. But there is a trade-off, and honesty demands stating it: the very bonds you cut are what made the glass tough and chemically durable, so a heavily modified glass is softer and more easily attacked by water. Pure soda-water glass (water glass) is actually soluble; lime is added back partly to restore durability. The art of glass composition is balancing enough modifier to work the glass against enough network to keep it strong.
Add soda (Na2O) to molten silica and the softening point plummets from over 1700 degrees Celsius toward around 700 to 800 degrees, because each Na2O unit snaps a bridging Si-O-Si link into two non-bridging oxygens and parks a Na+ ion beside them. That is the whole reason ordinary glass can be melted in a modest furnace and blown by hand, while pure silica demands special equipment.
Modifiers cut the network: each unit turns bridging oxygens into charge-balanced non-bridging ones.
A modifier does not simply dissolve inertly in the glass — it chemically breaks network bonds, and every bond it cuts trades workability for a little lost strength and durability. This is why glass composition is always a compromise, never a free lunch.