This is the first of a two-part post on glass, and in particular on the way in which light interacts with it. In this first instalment I’ll attempt to cover some glass basics: what glass is, its transparency, the way light behaves as it passes through and how one can introduce colours. Hopefully, this prepares the way for a closer look at stained glass in the second chapter and at a specific, Victorian, example of the sort of issues faced by conservators of Canterbury Cathedral’s stained glass windows.
Rather than spend a lot of time reiterating what I, and others, have written or spoken on in the past regarding what glass is, I’ll offer a brief description and then a couple of links to previous posts and videos. You can choose how wide-ranging you want to go, or how deep you’d like to dig – and by the same token, how long you want to spend on the topic. Perhaps the easiest place to start is via the assumption that most of us are familiar with what a crystal looks like. Even if you don’t have large diamonds or sapphires kicking around the place, you’ll have perhaps seen a crystal of quartz, or even grown salt or other crystals whilst at school. The one thing they have in common is the regularity of their respective shapes: all salt crystals are cubic, natural (i.e. uncut or polished) diamonds are, well, diamond-shaped and so on. The shape they display to us arises directly from the equally regular arrangement of their constituent atoms. Thus, the atoms in a quartz crystal – atoms of silicon and twice as many of oxygen – are also arranged regularly as though on an ever-repeating lattice. In this case however the atoms are arranged in a pyramid-like fashion (shown in the four-part figure below), which gives quartz crystals their characteristic shape. This provides for us a bridge into understanding glass, since the prototypical glass, silica, on which all our windows, bottles etc. are based, has an identical chemical composition to quartz: two oxygen atoms for each silicon atom. Just as in quartz, the atoms are in a pyramidal arrangement with their nearest neighbours – but the key difference is that silica typically solidifies too fast for the atoms to arrange themselves perfectly in 3D: the angles (and distances) vary just a little from one group of atoms to another. This addition of a small degree of disorder is enough to rob the material of any semblance of the regular facets observed with a crystal.
|From left to right: a crystal of quartz, showing the regular facets associated with all quartz crystals which arise from the regular arrangement of atoms shown in the second figure (after Prof. A.C Wright). If the key angles vary by a small amount – less than 10 degrees – from one group of atoms to the next then one has the sort of disordered atomic arrangement depicted in the computer-generated model shown in the penultimate figure (after Prof. A. Cormack); it is this disordered structure that is associated with glass, as in the virtual MineCraft® building I wrote about here and which is depicted on the right.|
Should you wish to read beyond this basic description I have written about glass, in its several guises, in several former posts, but this one is perhaps the most relevant; move on to this post if you would like learn something of the ‘human factor’ within scientific research into such materials. On the other hand, if you’d prefer to sit back and watch a video presentation on the subject, then look no further than the recording of a public lecture I delivered a few years ago in one of my local museums. The video is approximately 58 minutes long, although the introductory material is confined to the first eight minutes or so.
|Please see the text above for an explanation of these figures.|
The final stage of this first post in the pair brings us to the subject of coloured glass. The reason that window glass is reasonably transparent is explained very well at the atomic level in the video I recommended earlier (here): in essence, there are few mechanisms within the glass able to reduce the amount of light passing through. We can change and control that situation, and do so by design. What is needed is the introduction of small concentrations of one or more metals, each of which will offer at least one route by which light of a particular colour will be absorbed. Thus, adding a metal which absorbs light at the red end of the visible spectrum (i.e. from the ‘rainbow colours’) ensures that the light transmitted through the glass has no red within it. We have, in effect, coloured the glass. For example, to give a blue-coloured glass one could use cobalt, copper or ferrous iron; nickel, chromium or ferric iron would yield a yellow-looking glass. Moreover, one can play with the addition of more than one type of metal. For example, a glass containing both ferric and ferrous forms of iron would appear green since that mid-section of the visible light spectrum would be the only part not absorbed by one or the other forms of iron. In passing, I had the privilege of taking part in a project run by the Turner Contemporary Gallery a few years ago in which the topic of colour was explored by a local group of young people. This included a visit to the Glass Studio at Canterbury Cathedral to examine the artistic use of such coloured glasses; the video record of the project is here and my short voice-over on the scientific background to the colours of glass starts at about two minutes in.
Peter Layton’s studios; this piece is from his Mirage series.