The glass

Glass is made of sand, limestone, soda ash, alumina, colourants and decolourants. The raw materials that compose glass are vitrifiers, fluxes and stabilisers. Silica, the essential raw material, comes in the form of sand and grey stone, and is found in riverbeds and quarries. After extracting the stone and sand and grinding the quartz, it is washed to remove clay and organic substances; the material is then placed in refractory pots to be melted.

The vitrifiable mixture reaches a liquid state at around 1,300°C and, as it melts, the insoluble substances rise to the surface and are removed. After refining, the mass is left to rest and settle until it drops to 800°C, ready to be cut.

In several Stone Age civilisations obsidian was used as a tool, knife or arrowhead, for its resistance and edges as sharp as a scalpel. The Aztecs had a type of sword with obsidian blades mounted on a wooden body, called a macuahuitl.

The glass industry began in Portugal in Marinha Grande, in the 18th century.