Friday, 26 May 2017

Paintings in Light

Welcome to the second instalment of my two-part post on the interaction of light with glass. The first part (in which I attempted to cover some foundations regarding what glass is, its transparency, the way light behaves as it passes through and how one can introduce colours) may be found here. As promised in that initial post, the intention is to move on to stained glass windows, and to highlight the interdisciplinary work of conservators who are committed to passing on these inherited treasures to future generations. Before diving into the subject itself I must acknowledge several people. First, my friend Martyn Barr who generously allowed me to recycle the title of his excellent book and use it as my own (see here, second and third paragraph, or here for further details). However, absolutely central to this piece is Léonie Seliger and her wonderful team at the Glass Studios of Canterbury Cathedral; I never tire of visiting them and of being able continually to learn new things from them. I am also very grateful to Jane Walker, the Cathedral’s Head of Communications, for her permission to use the images I captured on my 'phone during a recent visit.
Before I focus on the glass of Canterbury Cathedral I’ll share with you a few images from elsewhere, ecclesiastical and otherwise. On the left is a window I photographed in Folkestone (Kent, UK; All Souls church) after delivering a talk on glass there: the window was donated by the artist, Gabriel Loire, in the year of my birth; it is made from ‘chunks’ of coloured glass rather than cut sheets. Middle top shows a small part of the Roots of Knowledge windows by Tom Holdman, with the Big Bang depicted on the left and prehistoric humans to the right. Below that is shown a stained glass garden sculpture by Joe Szabo, spotted during a visit to the Royal Horticultural Society’s Wisley site. On the right are two examples of Louis Comfort Tiffany’s work; the top one I was fortunate to see during a visit to Chicago but not, sadly, the collection of lampshades shown below .

I doubt there are many people who are unaware of stained glass, even if they’ve seen only images; there are windows and other works of art based on the use of coloured and painted glass in buildings right around the world. In Europe, the techniques employed to create them date back more than a millennium, and novel examples continue to emerge. In order to maintain focus and to avoid turning this into an overly-long post, I will not attempt to describe how a window is made; that job has been done many times over (e.g. in Martyn Barr’s highly accessible book, see above, and in videos like this one, and this). The essential stages begin with the artist’s design, then cutting and shaping appropriately coloured glass to that design before painting on the fine detail – which may be fused into the glass surface using a furnace or occasionally ‘cold-painted’ onto the glass. The individual pieces are slotted into place using lengths of ‘H-shaped’ lead which are soldered together at each junction. For a large window comprising multiple sub-sections of the overall design each part is then tied to a supporting frame, usually of iron, using copper wire which has been soldered to the lead. There are variants on this formula, as in the windows created from relatively thick ‘chunks’ of coloured glass broken from a large block to create a more abstract effect, but I’ll confine my coverage of those to one of the images above.
These images will hopefully illustrate the way in which glass pieces are assembled and then sub-sections of a window are fixed to the frame. Copper wire is first attached to the leading, as in the mock-up shown top right, before being twisted around the frame to support the assembly in its final resting place. The ties shown in use on the right help to support Canterbury Cathedral’s Great South Window, recently re-installed after the surrounding masonry was replaced/renovated and the glass disassembled for conservation work. The image on the left shows the scaffolding I climbed through – with permission and a hard-hat, naturally – in order to get the in situ image.

When light passes through them, the windows ‘come to life’. However, the way in which they do so is affected by more than just the nature of the incident sunlight. For instance, in glass sheets made by traditional methods rather than by the commercially dominant float glass process – blowing a tube shape, cutting off the ends, slicing along its length and allowing the cylinder to fold outwards – there will be variations in thickness apart from anything else. Add to that the fact that older glasses, medieval for example, will probably include cullet (waste or recycled glass) of varying provenance, and differences in colour/shading from one part of the sheet to another will almost certainly be apparent. The fragment shown here illustrates this effect. There is a great deal of fascinating
archaeological science undertaken on such specimens, and the origins of particular glasses may now be revealed in some detail by studying the material at a microscopic level. (For those wanting to dig a little deeper, into the red-coloured glasses of antiquity for example, I suggest a close look at the accounts published by Ian Freestone, who is also very much involved in the project I initially outline here (second half), and which I’ll update below.)

One of the more profound effects of a stained glass window on the light passing through it, beyond selecting out a particular colour that is, is associated with the phenomenon of light scattering. Whether we realise it or not, we have all seen the effects of light scattering: blue daytime skies giving way to red sunsets, the whiteness of clouds and of milk etc.; all of this is due to the way in which particles (dust, water droplets, suspended fat droplets etc.) scatter beams of light. So it is too with stained glass windows. If through the effects of corrosion or by the artist’s will the surface regions of a piece of glass become porous, or perhaps picks up a ‘powdery’ layer through chemical attack or the accretion of particulates, something similar happens. Viewing such a window from the inside, that is to say with the window back-lit, gives the impression that the window ‘glows’ – the light coming through it is being scattered in all directions, irrespective of the colour of the glass. This is beautifully illustrated in the images below, associated with a major exhibition mounted by Canterbury Cathedral’s Glass Studio in the USA (see here and here). Some of the oldest surviving medieval stained glass windows that were being removed as part of the Cathedral’s rolling programme of building conservation work travelled to the USA for a season, and as a part of the exhibition the Glass Studio team made a modern replica of one of those windows …
Look first at the image on the left: which window comprises old, ‘rough-surfaced’ glass and which is the modern replica? Both are identically back-lit. Notice that the window on the left looks relatively ‘dull’ yet casts a bright pattern on the floor, whereas the window to the right of the picture appears much brighter but casts only a shadow on the floor. This illustrates the effect of light scattering. The modern window is on the left: the light that passes through it simply travels on until it reaches a surface, in this case the floor. The original window on the right of the picture takes the light that has passed through the coloured glass and, at or near the surface, scatters it widely – so we enjoy the coloured glow from whichever direction we view it, but very little of that light is left to carry on through to the floor. The photo on the right shows the head of the Glass Studio, Léonie, and a senior member of the team, Laura, standing in the transmitted light of a large modern window: just think of the patterns of brightly coloured light that would have bathed Canterbury Cathedral when its medieval windows were young.
Now we move into the realms of conservation. One might naïvely suggest any surface layers ought to be cleaned off in order to return the glass to its original state, but nothing is that straightforward. Remember that some surfaces may have had detail added via the application of a paint, which may have been fused into the surface or simply be applied ‘cold’. Moreover, many of the older glass pieces may be fragile and there is a risk of irreparable damage – especially if the surface layer turns out to be deeper than anticipated. Then comes the need to know what the surface layer is made of since whatever is used to remove it must not also damage the native glass below; this itself can be a complex issue to resolve. However, the question becomes far more complex when the glass artists themselves apply a surface coating since current thinking is that an intentionally applied layer must be left in situ – irrespective of whether we might feel it was ill-advised, or whether it has changed over time. After all, many world-famous paintings change over time because their pigments or other media were not stable – this can be a serious problem with some of the J.M.W. Turner’s work for example because he was keen to experiment with novel paints – and we would be outraged if they were ‘tampered with’. In terms of stained glass windows this particular issue is widespread. For instance, it was not uncommon for Victorian (i.e. 19th century) stained glass artists to try to make their windows look older than they were: perhaps by sprinkling iron fillings onto the surface and then fusing them into the glass in a furnace. Ironically, this has in some cases left us with medieval windows that appear to be younger than Victorian ones. Adding a colour-wash to the surface was also practiced, perhaps to reduce the brightness of a particular section in order to keep it more in line with the window as a whole or artificially to generate the light scattering effect discussed above.

It is exactly this sort of issue currently facing the Glass Studio at Canterbury Cathedral: Victorian windows that are being removed as part of their wider conservation/renovation programme and which, to use the technical term, have a series of ‘blobs’ or patches in particular locations on the glass. The problem was outlined in a post I uploaded last year: here, second half. However, the good news with which I will end this update is that a strong international team of experts is now pooling its efforts in order to resolve the problem. Thus, added to the considerable experience and expertise of the staff of the Glass Studio is an archaeologist from University College London, Ian Freestone, who specialises in applying scientific methods to the study of old glass, and a conservation scientist from Lisbon, Márcia Vilarigues with a wealth of relevant knowledge. I met Ian a few years ago, and have been reading his papers for much longer, and had the pleasure of meeting Márcia for the first time at the conference on glass I wrote about in the post mentioned just above. We finally managed to get us all together a few weeks ago and spent the best part of a day touring the site and poring over examples of the problem at hand. Minute samples of the troubling ‘blobs’ have now gone back to Lisbon for analysis and I have high hopes that we’ll soon know what it is we’re dealing with – and that this will give Léonie and her team the additional scientific insights they need in order to undertake genuinely appropriate conservation work on the windows. The day itself provided a wonderful opportunity to learn from each other in a spirit of partnership – although I rather suspect that I had the most to learn, by far – and I doubt I could convey its excitement adequately in the words of a blog post. In lieu of the better prose required I’ll end by sharing some of the images I captured from the day …
Phase 1: the journey up using the construction workers’ cage lift gave us some extraordinary views of the Bell Harry tower, some heavy-duty masonry, amusing gargoyles and down towards the Cathedral Gate and the city beyond.
Phase 2: the working platform sits atop a huge scaffolding assembly which straddles the nave a long way below; some sense of the height is possible using the left hand image, taken through a hole in the safety netting at the end of the platform and towards the quire and the altar. Even with the nave far below us, the space up there was still enormous. However, the key thing was being able to see some of the affected windows which are still in their original masonry settings.
Phase 3: poring over one of the windows now in the Glass Studio in order to get a better view of the ‘blobs’, which are all-too-evident in the left hand images (these show the same area of the window but viewed from either side – i.e. external and internal surfaces). Tiny amounts of surface material were then carefully removed for detailed scientific analysis.
Phase 4 & etc.: the results, conclusions and conservation decisions are yet to emerge; as in all areas of research, perhaps especially in the area of Heritage Science, patience is a virtue: watch this space …

Further reading
Although I spent a large fraction of my career as a scientist studying glass – there are innumerable entries on the subject within posts on my blog, e.g. here – I have come relatively late to stained glass and its conservation. However, for what it’s worth, these are the books that now sit on my shelves:

Paintings in Light by Martyn Barr, ISBN 978-0-9563429-4-2
Stained Glass of Canterbury Cathedral by M.A. Michael, ISBN 1-85759-365-0
Stained Glass in Canterbury Cathedral by S. Brown, ISBN 0-906211-31-X
Notes on the Painted Glass of Canterbury Cathedral by F.W. Farrar, a digitised version of the1897 original from (I bought it online from a retailer specialising in out-of-print titles, here.)
Conservation of Glass by R. Newton and S. Davison, ISBN 0-7506-2448-5
The Conservation of Glass and Ceramics ed. by N.H. Tennent, ISBN 1-873936-18-4

Naturally, there is much also available online – both as text and as videos; you might like to take a look at the material uploaded from Canterbury Cathedral for example (e.g. here)

On the history of glass more generally, I find I have the following:
A Short history of Glass by C. Zerwick, ISBN 0-87290-121-1
Glass: a short history by D. Whitehouse, ISBN 978-0-7141-5086-4
5000 Years of Glass ed. H. Tait, ISBN 978-0-7141-5095-6
The Glass Bathyscaphe by A. Macfarlane and G. Martin, ISBN 1-86197-394-2

Monday, 8 May 2017

Colour my Glass

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.

Having established the basics, and keeping in sight the target of understanding the way in which light is altered as it passes through glass – and coloured glass in particular – one ought first to tackle the matter of glass ‘transparency’. We tend to think of the windows in domestic and commercial buildings, windscreens, display screens etc. when we think about glass in the everyday. We can see through glass: it’s ‘transparent’ (see here for an excellent insight into why this might be). Indeed, the secret of the success of world-wide fibre optic communications resides in the exceptional transparency of the silica glass at its core, first demonstrated in the early 1970s by Donald Keck and co-workers. However, ‘ordinary’ glass isn’t perfectly transparent and might not be very transparent at all under certain circumstances. It all boils down to what sort of glass it is (its chemical composition, whether it includes bubbles, impurities & etc.) and what sort of ‘light’ we’re talking about. I have tried to illustrate this in the images below. The two images on the left show three types of glass: a common (soda-lime) glass typically used in windows, bottles etc. which sits inside a tube of Pyrex glass (a borosilicate) and which, in its turn, sits within the outer tube of pure silica. Viewed side on (left) the composite glass rod seems reasonably transparent, but when viewed end-on (middle image) so that we’re trying to look through a far greater thickness of glass it is obvious that the transparency varies a lot between glasses. Turning now to the diagram on the right, this illustrates the degree to which transparency, or the ability of the glass to transmit light, varies depending on what sort of light is involved. This simplistic diagram provides a representation of the situation with a car windscreen for example: of course we need a high level of light transmission for the visible part of the spectrum – the rainbow colours – but we don’t want a lot of infra-red or ultra-violet getting through as it’s preferable neither to overheat nor to get sunburnt; however, it is important that microwaves are able to pass through as our passengers may wish to use their mobile phones. The situation is very similar for window glass, and a great deal of research and development has gone into the formulation of glasses tailored to achieve these ends.
Please see the text above for an explanation of these figures.
Having now introduced some of the caveats and subtleties behind apparently simple comments such as “glass is transparent” we ought also to mention the important ways in which even transparent glass affects light as it passes through. Key phenomena are refraction and dispersion, which allow us to fabricate lenses and use prisms as well as to explain why a swimming pool looks to be less deep than it really is and where a rainbow comes from. Refraction is the phenomenon by which light is ‘bent’ as it passes from one transparent medium to another, and dispersion tells us that the magnitude of such processes depends on the wavelength – the colour – of the light. I’ll not weigh this post down with a lot of detail since it would be a bit of a diversion from the principal thread. However, if you’d like to know more then please take a look at two of my earlier posts: one on the origin of rainbows (here) and another which illustrates theories of colour through the use of a prism (here).

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.
One can map the development of the chemistry of metals by looking at the coloured glass used by artists of the time. Within a very few years of their discovery, often less than a decade, a new metal would find itself being used within the glass industry. Some metals imbued not only a particular colour, but more exotic effects. Neodymium, for instance, will colour a glass blue in daylight – but this becomes more red in colour if the glass is illuminated with UV light (a ‘dark light’). Even more dramatic is the effect of UV on the green glass created by adding uranium – yes, uranium was used also – since it fluoresces and emits a very bright yellow-green light.

In the next post we’ll focus on one aspect of the artistic and architectural use of coloured/stained glass, and on the conservation issues associated with old stained glass windows. In the meantime, I’ll leave you with this image of one of the many delightful pieces to come out of Peter Layton’s studios; this piece is from his Mirage series.