How Glass Became Clear

[A] Early glass was not transparent, and for most of its history nobody expected it to be. The sand used to make it carried traces of iron, which tinted the result green or brown, and since the object was usually a bead or a coloured vessel this hardly mattered. [B] Colour was the point.

[C] The change came when glassmakers learned to counteract the iron rather than remove it. Adding manganese produced a faint purple that cancelled the green, in the way that two opposing tints on a colour wheel cancel each other, and the eye reads the result as colourless. [D] Venetian workshops refined the technique in the fifteenth century and guarded it fiercely. What followed was not simply better windows. Clear glass made the lens possible, and the lens made the telescope and the microscope possible, which is to say that two of the sciences of the following century depended on a solution to a problem that had begun as a cosmetic defect. The workshops that solved it were not trying to make instruments; they were trying to make a better drinking glass.