Five parts in ten thousand. Cobalt's colouring power is extreme — 0.05 wt% is enough to give bulk glass a saturated blue. Compare copper, which needs percent-level additions, or iron, whose pale green comes along free with untreated sand. That ratio is the whole economic story: a small, light, valuable quantity of blue ore could colour an enormous amount of glass, so it was worth trading over very long distances very early.

They had the colour without the element. Cobalt was not isolated and named until Georg Brandt did it in the 1730s, but cobalt-bearing minerals had been colouring glass and glaze for roughly three thousand years before that. Ancient glassmakers were not using cobalt oxide from a jar; they were using ores and roasted mineral preparations whose blue behaviour was known empirically. Be careful with any source that has Egyptians or Romans deliberately adding "cobalt oxide" — they added a blue-making mineral, which is a different claim.

Impurities as a fingerprint. Because different cobalt ore bodies carry different companion elements — nickel, zinc, arsenic, iron, manganese in varying ratios — analysing those trace elements alongside the cobalt lets researchers group ancient blue glass, glaze and enamel by source and trace trade routes. The impurity pattern is more informative than the cobalt itself.

What the blue actually is. In a soda-lime melt cobalt sits as Co²⁺ ions surrounded by four oxygen neighbours. That particular arrangement absorbs strongly in the yellow-to-red part of the spectrum and lets blue through. Change the surrounding chemistry — a different flux, a different coordination — and the same element gives a different colour, which is why cobalt in some glazes reads violet or pinkish rather than blue.

Not a hazardous blueprint, but not a careless one. The blue is a colourant here, in trace quantity, locked into a silicate network in the finished glass. What genuinely deserves respect is the process: 1300 °C molten material, and respirable dust at the batching stage. Handle both properly and this is a normal hot craft, the same category as glassblowing or kiln firing.

Coloring

Blueprints (3)

Tin Oxide Opaque White Glass

Silver Stain for Glass

Cobalt Blue Glass