Co₂Na₄O₁₆Si₆

Architectural potential

A cobalt-coloured soda-lime-silicate glass fired at 1150 °C to 500 MPa, 2.82 g/cm³, sitting in the same structural-translucent register as its amber-brown sibling but carrying the opposite end of the visible spectrum. Natural placements are pressed-glass piers in bath-house screens where cobalt blue historically belongs, translucent stair treads in gallery foyers where the descent-through-blue is a deliberate atmospheric choice, light-well panels pressed into floor slabs in atria, and transparent partition modules in crematoria and chapels where the colour carries ritual weight. The float-glass comparison holds on carbon to within 2 per cent with a sixty-two-per-cent cut in firing energy, so the environmental argument is process-based rather than chemistry-based. Where this entry departs from the amber-iron version is cost and colour: cobalt oxide is an order of magnitude more expensive than iron oxide per kilogram of batch, which brings the workshop batch to €19.91 against €1.74 for the amber. Specification therefore sits with the signature element — the pressed pier in the main entry, the floor insert below the oculus — rather than with repeated bulk modules. Annealing and edge conditioning belong to the tectonic as with any soda-lime glass, and walking-surface placements need a post-annealing polish.

Material character

Deep saturated cobalt blue through the 49 mm depth, read as a clean sea-blue on the face and deepening to a near-indigo when sighted along the 85 mm long edge where the slab thickens into full body. Pressed-glass surface with a shallow gravity crown and the quiet anneal striae that any soda-lime piece carries close to its long axis. Cold-cut edges lap-white, with the characteristic glassy chip that cobalt-coloured glass shows along a fresh cut. At 2.82 g/cm³ and 85×85×49 mm the slab sits one-handed but insists on it, slightly denser and firmer in hand than the iron-amber sibling of the same family. A distinct colour and weight step from the iron-amber variant — same glass bones, quite different room.