Repairing Fractures III

After the success with the polyester in the fabric tests we decided to see how it performed on fractures in stained glass.

white polyester on yellow glass

white polyester on clear glass sz14

The images above show that white polyester contrasts against the coloured glass and is visually distracting. Textile conservators suggested this could be improved with the use of coloured polyester. Because this is only available in a limited range of colours they proposed dyeing the polyester, as they do when the colours they require are not available.

Dyes were tested to create a range of coloured polyester that complements the colours of the glass, to reduce the visual impact of the treatment as much as possible. The white polyester was placed in a vat of dye and heated to around boiling point in order for it to absorb the colour.

Due to the large quantities of boiling water involved and the fumes released by the dyes there were health and safety implications. Because of this I used fume extraction and wore protective equipment: heat and water proof gloves, water resistant apron and visor.

Dyeing the polyester
Dyeing the polyester

It was not always possible to get an exact colour match to the glass, as some dyes absorbed into the fabric more than others. Reds were particularly hard to achieve as the red dye appeared pink when dried. I decided to aim for a colour that was lighter but closer in hue to the colour of the glass. I deliberately did not want the colour to be too strong in case it affected the tone of the glass.

As you can see below, the colour of the dye mixture didn’t always look too promising, but it managed to create some successful results!

Stirring the dye to ensure even colour distribution
Stirring the dye to ensure even colour distribution
Coloured polyester produced with the dyes
Coloured polyester produced with the dyes

Several pieces of glass of different colours containing fractures were selected for repair using the dyed polyester, of which a couple are shown below. The chosen colour of polyester was cut to shape and applied with wheat starch paste to the back of the fractures.

Example 1:

example 1 bigger writing

The repair provides good structural support and the polyester is remarkably invisible when viewed in transmitted light. When seen from the back of the panel it is still clearly discernable but blends in far more sympathetically than the un-dyed polyester. The colour of the polyester is sufficiently light that it does not alter the colour of the glass, yet it is enough to disguise the chink of light that had previously been visible through the fracture.

Example 2:

fabric repair example 2

Satisfied that this technique provides maximum support to the fracture with minimal intervention to the panel, and is durable yet easily reversed, we have now trialled this technique on several panels which we will continue to monitor regularly to assess its behaviour over the long term.


Repairing Fractures II

We have been investigating alternative methods of consolidating glass fractures. The techniques we have tested are inspired by methods used in other conservation disciplines.

Japanese paper applied with wheat starch is a standard treatment used by paper and paintings conservators and it has also been used to repair very fragile and corroded archaeological glasses as well as other objects.

We tried this technique to temporarily secure fractured glass in situ in an early 16th century stained glass panel from Fawsley Hall. This was initially intended to be a temporary repair, but two years on the repair is unchanged and still very effective in supporting the broken glass. We are monitoring the panel to assess the durability of the repair over time, although we do plan to replace the paper at some later stage.

Japanese paper

Japanese paper repairs to 45.317 Shield of Skenard from Fawsley Hall on display in the Burrell Collection Restaurant

The Japanese paper is barely noticeable in transmitted light, but in reflected light it is clearly visible. This makes it acceptable as it does not interfere with the imagery of the panel from the front, but it can be easily identified as a repair when viewed from the back.

Wheat starch as an adhesive is easily reversed using moisture, which is great if the stained glass is in a protected and accessible display, but obviously not suitable for stained glass displayed in a damp building. The other draw back of wheat starch is that the paste is organic and a potential food source for micro organisms, so again it will be less useful in a damp environment and in locations where the glass will not be looked at for years.


Based on our success with paper we decided to test materials that are stronger and more durable than paper but similarly transparent and without too much distracting texture.

We considered a variety of fabrics with advice from textile conservator colleagues, and decided to compare light weight nylon, silk, polyester and glass fibre fabric alongside Japanese tissue. We applied the samples with Klucel G (wheat starch paste) and Paraloid B72 (a thermoplastic resin) to investigate the pros and cons of each adhesive.

Fabric tests on glass
fabric and paper tested on glass

Our first test results are unambiguous: in terms of ease of application and quality of the results the samples applied with wheat starch paste look much better than those applied with Paraloid B72. We plan to carry out more tests with Paraloids in a variety of solutions to see if we can achieve a satisfactory result. Using a resin would make the method more suitable to use in a wider variety of display conditions.

As for the fabrics: we found that the honeycomb pattern of the light weight nylon net was visually distracting against a piece of transparent glass.

Nylon mesh
Nylon mesh








Silk crepeline has a fine, hardly noticeable texture and is semi transparent, but is awkward to apply and tends to distort. It is a natural fibre and while it is initially quite strong, it is not very durable and deteriorates in daylight.









The woven glass fibre fabric also has a tendency to distort during the application process, and it frays easily at the edges. We were initially quite positive about using this material – it seems logical to use a glass based fabric on glass – but we actually found that it was more visually intrusive than many of the other fabrics.

Glass fibre
Glass fibre










Based on our sample tests we found that a polyester fabric is structurally stable and provides good support, it has good durability and it does not have a distracting texture.










We therefore decided to test it on some fractured stained glass. In the next blog we will discuss the results.

















































Looking at Dirt, Corrosion and Paint

We have started conserving the Boppard stained glass. Our first task is surface cleaning which is not straightforward because there are so many different layers of dirt, original paint and retouching. It is not that easy to tell what is there by intention (original or restoration) and what has simply accumulated on the surface of the glass over time.
The glass is covered with a layer of loose dirt which can easily be removed using a smoke sponge. With a gentle dabbing motion the cells in the sponge pick up and trap dirt from the surface of the glass. This can be a very effective method of cleaning, especially if one is trying to clean loose particle dirt.

Removing loose particle dirt with smoke sponges
Removing loose particle dirt with smoke sponges

The Boppard glass also has a layer of black sooty dirt. In order to remove this, we tested a variety of solvents and found that saliva is the most effective. We don’t much fancy cleaning 34 m² of glass using spit, so we are also using water (de-ionised) with a conservation grade detergent (Synperonic A7) and this works almost as well.

Removing the black surface dirt using a cotton wool bud and saliva
Removing the black surface dirt using a cotton wool bud and saliva

Apart from surface dirt there are other layers: In the 10 commandments window many of the glasses have a matt brownish- grey coating. We have considered if this could be a painted layer that was applied in order to tone down the colour and shine of the glass but we don’t think it is. There are no brush marks to be seen and it coats the glass surfaces very evenly without any emphasis on the drawing. It cannot be fired paint because in some areas it has been completely removed.

Detail from the 10 commandments window
Detail from the 10 commandments window

The layer is relatively hard and almost like a crust. It can be removed with mechanical means (scalpel) but not with solvents. Seen under the microscope it sits on top of the trace lines. Where the layer is absent the glass looks remarkably transparent and unblemished.
We think that this layer accumulated on the glass when dust, moisture and time worked together and formed a crust on the surface. In some places this crust has been removed by accident (pulling off sticky tapes, selective cleaning).

Detail - Microscopic image of the left eye of the angel
Detail – Microscopic image of the left eye of the angel

The microscopic image shows how vulnerable all these layers are and how little else is there. If you removed any of it, you would be left with very little definition apart from the trace lines. This leads us to conclude that there was a problem with the original paint perhaps it was under-fired and not very stable. However it is astonishing (and slightly baffling) that the glass is generally so clear and un-corroded where the paint has flaked away. One explanation may be that the windows were removed from the church before the onset of serious industrial pollution.
On the back of the same window we also find a similar pattern of painted areas and accumulation of dirt and weathering. The glass was originally back-painted in order to emphasise the detailed drawing on the front. Again in most areas the original paint lines appear ‘faded’. Along some of these lines as well as around the edges of each glass and along the top of the panel (underneath the tie bar) moisture accumulated and in combination with dust started to form a crust on the glass surface. In the middle of each glass piece this crust is not present and the glass is in good condition. However one can see some iridescence which indicates that a chemical change has occurred.

Detail from Detail of 45.489.1.c
Detail from Detail of 45.489.1.c

Most of these layers on the glass, both front and back will not be removed during conservation treatment. They are inorganic and very hard and although the glass underneath appears to have little corrosion it can easily be scratched and the gel layer can be damaged. The areas where the weathering layer and paint have been removed will be carefully retouched and we hope that by a combination of cleaning and retouching the overall impact of the imagery will be much improved.

– Marie.

Project Update: June 2013

The Boppard stained glass panels have now all been photographed and every square centimetre of the 35 leaded and painted panels have been scrutinised from the front and back.

We have assessed the structure of each panel, looked at the condition of the glass, paints and lead and even the frames. We have recorded the details of any restorations and tried to work out when they were added.

Most of the panels are structurally very vulnerable. The lead used in the restoration of 1871 was very thin with not much body (less material = less cost) and as a result it is not very rigid or strong. It also does not provide any space for putty to be inserted so there is a lot of lateral movement in the panels especially when they are removed from their frames.

Lead repairs
Lead repairs

The original design incorporated few leads and large glass pieces and subsequently there has been damage to the glass and repair leads have been added, as in the image above.

Unstable wash.
Unstable wash.

In other places broken or missing glass has been replaced by a restoration. The face above was probably inserted as part of the restoration carried out in 1871. But later someone must have thought that it was too pale and added a brown wash to tone the glass down. The problem with the wash is that it has not been fired, so it is very unstable and can easily be removed – as has already happened!

Wash on reverse
Wash on reverse

In other areas where restoration glass was inserted, an unfired “wash” was applied from the back to help disguise the restoration. What is puzzling though is why this wash was applied to the lead as well. These sorts of findings raise the question whether the panels underwent significant restoration while they were in the Hearst and or Goelet Collections. It will be interesting to trace down some of the firms that carried out the work.

Surface cleaning test.
Surface cleaning test.

Some preliminary cleaning tests have been carried out as all of the panels are very dirty. The cotton swab in the picture above was moistened with water and synperonic A7 (a mild detergent) and even after several applications the swabs still turned black very quickly.

We have looked at paints under magnification….

Trace paint on blue glass
Trace paint on blue glass


Cold (unfired) paint applied glass and spilling onto lead
Cold (unfired) paint applied glass and spilling onto lead

… and studied glass corrosion.


On the whole the panels have been left in their frames for now. The only exceptions have been made where it is clear that a panel will have to come out of its frame anyway for treatment.

The frames are interesting in themselves. In the case of the windows William Burrell bought from the Hearst Collection (Tree of Jesse Window and the Two Standing Saints) the frames seem to have been added by Wilfred Drake in 1938, whereas the frames for the window bought from Goelet (Ten Commandments Window) the frames probably predate the sale of the window in 1939. We are definitively going to keep them for now and if possible we will try to reuse them.

Over the next 2 months the hundreds of photographs will be organised and filed and research questions will be summarised. At the moment it seems like there are more questions than answers!


De-Installation of the Virgin and Child and 9th Commandment Window

BBC cameras at the ready


BBC cameras at the ready

Practice makes perfect – using a cardboard mock-up

 Preparing Top Window  BBC Interview

Preparing the top panel for removal

Marie being interviewed before the de-installation

 Top Panel Down 1  Top Panel Down 2

The top panel comes down

First Panel safely down and recorded on camera – no pressure then!

 Middle Panel Down 1  Middle Panel Down 2

Now the big panel comes down

One small step for man, one giant leap for a Boppard panel

 Middle Panel Trolley  Middle Panel Protect

Phew – The panel has landed!

Megan protects the panel – nothing escapes the camera!

 Bottom Panel Down 1  Bottom Panel Down 2

Kenny and Andrew at work on the final panel

And here it comes…

 Bottom Panel Transport  De-Brief

Preparing the final panel for transport

Final debrief

 BBC Monitor  All Down

Job well done – and all recorded on camera

Only one Boppard window left on display (on the right)

Old Lead Repairs

Birth of the Virgin
Birth of the Virgin

The two stained glass panels that make up this image depict the birth of the Virgin. It is an intimate scene, showing the infant Mary at the centre standing on her mother’s bed – she is clearly a well developed child so soon after her birth! Saint Anne (also Anna or Hannah), her mother, is sitting up in the splendid curtained bed which is covered by a vibrant red blanket. An elderly woman reaches out to take the child; this may be a servant or perhaps the grandmother. In the foreground is another woman washing white cloths. She is referred to in the catalogue as a midwife¹.

Every time I look at this window I am distracted by the many repair leads; there are about 83 of them and they have been added over the years to secure and disguise glass that was broken. The area worst affected is in the beautiful red blanket and the face of the midwife, where the repair leads are so dense that they disguise the details of the paintwork and the intricacy of the design.

Birth of the Virgin - Detail
Birth of the Virgin – Detail

The damage in this area of the window was most likely caused by the impact of a thrown stone (or a similar object) as the breaks in the glass radiate from a central point. The only way that restorers in the past were able to repair such damage was to insert lead cames to bring the broken glass pieces back together. They used H-section leads with 1-2 mm cores and to accommodate the lead without making the whole area bigger the restorer had to take a little bit away from each glass piece (the equivalent of the added core).

Today we have sophisticated adhesives that can be used to bond broken glass pieces. There are many different types and some are formulated to match the properties of glass very closely. They can be tinted with pigments to match the colour of the glass and a skilled conservator can achieve an almost invisible repair. So in order to restore the area above, a conservator will carefully remove all the repair leads (which will be kept as samples and for analysis). A tinted resin is then injected into the spaces left by the lead. Once it is cured, the glass piece appears intact again.

It is a very time consuming process and it can be complicated – especially when dealing with such a complex break as in this window – but the outcome can be very successful and it can help to restore the image.

Before embarking on the mission to improve the visual impact of the panel and to make it easier to read, there are some issues that should be considered.

  • What are we aiming to preserve? Are we mainly concerned about the image and the painted details or are we also interested in using the object to tell stories about our history? For instance, the complex lead repair is possibly the result of an attack on religious imagery during the reformation and witness to why so much of our stained glass heritage was destroyed and lost during that period. Is it worth preserving that aspect to better illustrate our history?
  • As an object and piece of our heritage this window is as authentic as it will ever be and all marks on it including the repair leads and even the dirt are witness of its history. No amount of restoration will ever bring it closer to that state of authenticity. In this context it should also be noted that the window has been restored many times in the past and we cannot hope to ever return it fully to its “former glory”.

There are also some questions about practicalities and resources that need to be considered before we decide on the most appropriate conservation treatment:

  • The lead repairs that are currently holding the glass together have probably been there for  142 years and they will last for at least that much time again provided the stained glass window is well cared for, kept in a stable environment and handled and moved as little as possible.
  • A repair using an epoxy resin is said to last for as much as 80+ years but as this kind of product has only been in use since the 1970’s no one can actually confirm that. We have examples of epoxy repairs that were carried out 35 years ago and some of them are definitively failing. So we will also have to consider the cost implications of having to renew all these repairs in the future.



¹ Stained and Painted Glass Burrell Collection, 1965

The Corporation of the City of Glasgow written by William Wells

First observations

The stained and leaded panels from the “Life of Christ and the Virgin” window have now been assessed in the studio. We have taken images from the front and back in transmitted and in reflected light which records the maximum amount of information about the condition of each panel.

45.485.1.e Front – transmitted light
Front – reflected light
Back – reflected light

One of the most immediately obvious things about the panels from this window is that all the lead has been tinned.  This means that molten tin has been applied and is covering the surface of the lead cames.


Why would this have been done?

Tin is harder than lead, so usually tin is applied to lead that is too soft and flexible – and therefore not giving enough rigidity to the panel.

Could it have been applied in 1881 when the panels were restored in Berlin in the Royal Institute of Glass Painting?

I think this is very unlikely. It looks like the panels were completely re-leaded during this restoration and I would be surprised if the restorers would have tinned a newly leaded panel. It is more likely that the new lead was very pure – and therefore very soft. It was also probably very insubstantial with thin cores and flat flanges. So I assume that sometime perhaps 40 or 50 years later, they were tinned to make them stronger and more rigid.

It is possible that the tinning of the lead was carried out while the panels were in the United States in the collection of the newspaper magnate William Randolph Hearst. Tinning copper foil to make stained glass windows and other decorative artefacts such as Tiffany Lamps was an invention made in the USA in the late 19th century. Stained glass restorers there would have been familiar with tinning as a technique to be used to strengthen a panel.

In the coming months I will explore this issue as I progress with further assessments. The most obvious investigation will be to find an area of accessible lead where I can take a sample to analyse and find out if my theories are substantiated by what I find.