Well, how is the glass made?
The strongest types of glass (in terms of materials alone) are borosilicate, aluminosilicate, and fused silica. That means either intentionally introducing sources of aluminum (read: alum, which was known to the ancients) or boron (borax, which was traded on the Silk Road), or finding extremely high-purity silica, or finding some way to remove any existing impurities.
The strongest types of glass in terms of technique are annealed, laminated, and heat-tempered, and chemically-tempered. Tempered is clearly the best option here, and both types have plausible places where magical materials could be useful. That is, heat-tempered glass must be heated to a very high temperature (essentially making it partially molten again) before rapidly cooling the exterior but letting the interior cool slowly. That sounds like two magical materials: one to heat it up, another to cool it down at the right rate. With chemical tempering, the glass is immersed in a molten salt bath (or two: either potassium nitrate alone, or first sodium nitrate and then potassium), which causes an ion exchange in the surface of the glass; the larger potassium ions push against the already-fixed positions of the silicon and oxygen atoms, putting the surface under compression and the interior under tension just like with heat-tempered glass, but for different reasons. For this method, the two salts you have to acquire (and then get to a molten state) would be excellent picks for magical (or "magical"—meaning IRL mundane but not actually known to the medieval chemistry presumed by D&D.)
So, as a brief list of things you could consider:
High-purity silica crystals or sand
Chemical additives to add strength
Fuel or magical heat sources (doubly so if going for heat tempering)
Material for the mold
Salts that can be melted for chemical tempering
Magical materials that are associated with strength (e.g. diamond, ironwood, mithril or adamantine powder, petrified wood, dragon bone powder, etc.)
