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 bibliolife.com (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



1 comment:

  1. Hi Bob, yes that was an exciting day in the Cathedral. Nothing can beat the experience of getting up close to these ancient materials with a group of people who know their stuff. It enthralled me the first time it happened to me some 35 years ago, and continues to do so. Best, Ian

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