Another manner in which colour can be introduced is by formation of colloidal particles. This simply means particles of a substance that are suspended throughout the glass. These particles are often formed as the result of treatment with heat, forming what are known as ‘striking colours’. The colloidal particles scatter light of particular frequencies as it passes through the glass, causing colouration. Examples of these colloidal particle colourants include gold, which imparts a ruby red colour, and selenium, which offer shades from pink to intense red.
The final main way in which colour can be introduced is merely via the addition of already coloured particles to the glass. Examples of this type of colouration include milk glass and smoked glass; milk glass is achieved by adding tin oxide.
Coloured glass finds obvious usage for decorative purposes – for instance, stained glass windows take advantage of the colouring effect of metal ions. The colour of glass can be more than just aesthetic, however. For example, beer bottles are coloured green or brown to cut out specific wavelengths of UV light which can otherwise contribute to beer spoilage and a ‘skunky’ flavour. It’s also used in some chemical bottles for a similar purpose.
There’s a lot more to the colouration of glass than is covered in this brief overview. If you’re interested in learning about it in a little more detail, check out the links below.