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Why are silica-based glass networks more viscous than borate-based glass networks?

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Silica-based glass networks are generally more viscous than borate-based networks due to the fundamental differences in their atomic structures and the role of boric oxide as a flux.1

In silica glass, the network is composed of a three-dimensional arrangement of $S i O_{4}$ tetrahedra connected at their corners. This structure creates a highly stable, rigid network that requires significant energy to deform, resulting in high viscosity and high softening temperatures.21

Conversely, boric oxide ( $B_{2} O_{3}$ ) acts as a flux when added to silica, which lowers the working temperature and reduces the overall viscosity of the glass melt. Borate structures are inherently more complex and flexible, ranging from simple trigonal planar $B O_{3}^{3 -}$ units to more intricate chains and rings of three- and four-coordinated boron atoms. Because these structures are less rigid than the continuous, corner-sharing tetrahedral network of pure silica, they facilitate easier flow at lower temperatures. Consequently, the addition of boron to a glass composition effectively disrupts the high-viscosity silica network, making the material easier to process.13