Transparent glass, but so much cobalt added that light won’t get through. Glass colour is achieved by adding metals or metal salts that won’t burn out in the hot glass, but disperse to selectively absorb light, and appear as colour. This rich blue glass bottle would have contained sensitive matter, prone to ultraviolet damage. UV light is a kind of electromagnetic radiation from the sun - a powerful agent which can transform medicinal materials, robbing them of their potency. Apothecary bottles were colour-coded, blue glass denoted poison. Modern medicine bottles are amber, as are most beer bottles, as cobalt is increasingly expensive as the world uses it up for battery production.