How is glass made?

The medieval glass makers discovered that adding different metal oxides to the sand and other glass ingredients produced different colours of glass. For example, adding copper produced red glass, cobalt made a beautiful blue glass, manganese made pink or purple and iron oxides could be used for greens and yellows. However, it soon became clear that red (ruby) glass made this way was too dark for windows as not enough light could shine through to light up the glass and the building.

The glass makers were extremely clever and decided to experiment with different ways of adding colour to the glass. To make a more transparent (lets more light pass through it) red, it was discovered that molten (liquid) white glass could be coated or ‘flashed’ by dipping it into molten red glass before it was made into flat glass for a window.

In the centuries that followed, new techniques allowed glass artists an even wider range of colours. In the fourteenth century it was discovered that a chemical called silver nitrate could be painted on coloured glass and the glass heated to create different hues (colours or shades). This technique known as yellow or silver staining allowed different shades to be shown on the same piece of glass. In the sixteenth century enamel paints allowed many more colours to be used on the same piece of glass. Many of these methods are still in use today.