You touch a single, nonmagical object made primarily of glass and magically transmute it into glassteel—a substance that retains the transparency and appearance of glass while gaining the durability of steel.   The object to be transformed must:

Be primarily composed of nonmagical glass (e.g. leaded glass would be acceptable, but the lead would still be only as strong as lead)

Be no larger than a 5-foot-by-5-foot panel, or equivalent volume (DM discretion)

Weigh no more than 200 pounds

Not be part of a creature (but it may be an object to be worn or carried)

Once transformed, the object becomes as strong and resilient as steel while remaining clear or translucent. The transformed object is as heavy as the glass object had been.   Properties of Glassteel:

AC 19 (as steel)

Hit points as steel of equivalent size and thickness

Resistance to bludgeoning, piercing, and slashing damage from nonmagical attacks

Transparency retained; optical flaws remain

Nonmagical after completion

Limitations:

No scaling when cast at higher levels (no upcasting)

One discrete object per casting

Cannot reshape or thicken the object

Glassteel cannot be used as a material component for further castings

Dispel magic does not reverse this spell once completed. Wish can undo the effect.