After a great afternoon at Bath Aqua Glass this weekend, I thought I would share some interesting glass facts that we learned there, particularly the additives that are used to create different colours.
The glass used for windows, glasses and ornaments is made by heating silica (silicon dioxide – which also makes up quartz and sand), along with other chemicals such as sodium carbonate (to lower the melting temperature) and various oxides (including calcium, magnesium and aluminium, to make the glass more durable), to around 1200 degrees C in a furnace.
This glass can have a slightly greenish tint to it due to iron (II) oxide impurities (to see why some metal compounds can have different colours, watch my Naked Science Scrapbook on copper compounds). This colour can be removed by a combination of adding an oxidising agent, to convert the impurities to iron (III) oxide, which has a yellow-green tint, then masking this by adding small amounts of purplish and blue colour (from Nickel and Cobalt – see below).
When colouring glass, a combination of additives, oxidising agents, the temperature at which they’re heated and how the final piece is cooled affects the final colour, but here are some of the most common additives and the colours they give:
- Chromium – The chromium (III) ion gives emerald green when added to base glass with arsenic and tin oxide.
- Cobalt – Compounds of cobalt (II) ions give a deep blue colour when the base glass also contains potash (a mixture of potassium compounds), or pink when used with a borosilicate glass base (which is the type of very heat resistant glass used for Pyrex cooking dishes).
- Copper – copper (II) oxide gives the turquoise blue colour of Egyptian glass, one of the first types of coloured glass to be made.
- Gold – unlike the other colour-giving additives on this list, gold is added in its metallic form, rather than as a salt. The tiny particles scatter the light entering the glass, giving a ruby colour in high concentrations when added to lead glass with Tin and cranberry in lower concentrations.
- Iron – Iron (II) oxide gives the green colour to beer bottles, and when mixed with Chromium, gives the deeper green glass used for wine bottles.
- Manganese – anyone who remembers potassium permanganate from school chemistry lessons will remember its vivid pinkish purple colour. The same ion provides a purple colouration when added to glass, and was used as far back as Ancient Egypt.