The colour of glass

3D-printing, along with other additive manufacturing (AM) and rapid prototyping (RP) techniques, involves building up structures in a layer-by-layer fashion based upon a computer design file. Such techniques are well-suited to the production of one-off, complex structures that would often be difficult to produce using traditional manufacturing methods. There has been rapid growth and interest in this field during recent years, and a range of techniques are now available which make use of many common materials such as plastic, metal, wood and ceramic. However, relatively little has been done to develop AM using glass. Since glass was first made, thousands of years ago in Mesopotamia, it has been appreciated because of its vibrant colours. To allow a successful design and print of any glass object, these colours have to be captured and classified in such a way that they can be incorporated in the CAD design of the object and lead to the desired result in the print. The colours of architectural glass are often classified by RAL charts [1] or by BS4800:2100 colour codes [2]. Both colour classification systems have been developed for paints and coatings, but are a good first approximation. What they cannot capture is, for example, that some glasses display different colours in reflection and transmission and/or the colour change occurring in glass when it is reheated. In 3D printed glass, gas inclusions are another source of colour changes. Scattering at the air/glass interface leads to the addition of white to the underlying glass colour. Using CIE chromaticity coordinates [3], glass samples are characterized before and after processing. We used two different measurement methods to determine colour coordinates as a function of sample thickness and frit size, to check how robust the results were as a function of the measurement method.