A Digital Restoration of the Birth of the Virgin Panel

window before conservation

The Digitally Restored Window

Our approach to the conservation treatment of the stained glass panels from Boppard is one of minimum intervention. There are many reasons for this:

• As conservators we adhere to international guidelines provided by the Corpus Vitrearum Medii Aevi, the Institute of Conservation (Icon), E.C.C.O. professional guidelines and ICOMOS Venice Charter.
• Because we are dealing with three monumental windows with a total of 34 individual panels we have to apply the same level of treatment to every one of them: some may be in a condition where restoration would be appropriate, others may not be.
• We have to consider time and cost.

Very early on in our blog we discussed the panels depicting the Birth of the Virgin and the issues surrounding restoration and authenticity. So we decided that our best option was to not interfere with the actual stained glass panel (other than cleaning it) and to try out a virtual restoration instead. This would allow us to recreate an image of how the scene might have looked without being limited by professional guidelines and time factors and most importantly we would not be interfering with the authenticity of the artwork.
With a virtual restoration using digital technology you can explore different levels of intervention and present different restoration options depending on and directed by the condition that the original glass is in.
Our approach with this panel was to virtually restore the glass to a level that could – in theory – be achieved in reality. So we removed all repair leads, re-bonded broken glass, retouched lost paint, removed some of the unsightly restorations where we could identify with a degree of certainty what would have been there before, but left restorations that have replaced original glass with no trace of what was there before.

The most obvious area of virtual repair was the extensive damage to the magnificent red bed cover. John took some detailed photos of the area and digitally enhanced them so that the almost invisible floral pattern came to light. The outlines of these were traced onto transparent paper which was scanned back onto the computer and used as a guide layer to recreate the pattern.
The virtual restoration allows us to recreate the detailed patterns in the fabric of the bed spread as it might have originally been seen. This is immensely exciting as it gives us a sense of how much detail the Boppard panels must have once contained!

Bed Sheet Pre-Restoration

The damaged area “virtually” restored

Trace of the Bed sheet

Trace of the bed sheet folds

The image on the left is the trace created of the fabric pattern painted on the back of the glass. The image on the right shows the folds of the cloth painted on the front of the glass. (Mary’s foot was included to allow accurate positioning as an overlay!)
One of the most amazing outcomes of all of this work trying to recreate an image of what the original stained glass scene would have looked like is that we have to conclude that the bed spread was cut from one piece of glass!

Detail of the Bed Sheet

The possible shape of the bed sheet in the original window

This is a very daring shape to cut, requires great skill and would have been predestined to break at the narrow point in the centre. We almost cannot believe that they would have done this, using the most expensive red glass, decorated front and back, but there is no evidence of an original lead line to separate the two sections.

Another key area we looked at is the head of baby Mary and the section of blue curtain just above her head.

Mary’s Head Before Restoration

virtual restoration of Mary’s Head

This area must have been severely damaged – both the head as well as the light blue insertion above Mary’s head are restorations from 1871.
There is nothing original left of the head and if we replaced it, we would be guessing wildly, so in some ways the 1871 restoration is as authentic as it will ever be. So we decided to give Mary a virtual face clean and leave it at that! (This, by the way, is something we cannot do on the original because the paint is not fired and will come off very easily…)
The restoration insertion in the blue patterned curtain above her head is different: so much original glass with curtain pattern remains in the surrounding area and it allows us to make a pretty accurate guess about how this would have originally appeared. We therefore decided to carry out a more sympathetic repair in our virtual restoration.
John reconstructed the area by copying and pasting sections of the original curtain area and blending it into the background along with some digital painting.

So much for a virtual restoration.

This is what the real stained glass panel looks like after careful cleaning and stabilisation.

The Cleaned Birth of Mary Window

Facts about glass: Assembling a stained-glass panel

Once all the pieces of glass have been cut to shape, painted according to the design, and fired, they are finally ready to be assembled using lead.

Megan assembling a panel

The method used to assemble a stained-glass panel nowadays has changed remarkably little from mediaeval times. According to Theophilus, a 12^th century monk, in his treatise “The Divers Arts”; a flat smooth wooden board large enough to take the window panel would have been whitewashed (using powdered chalk and water) onto which the design for the window would be drawn.

When paper first started to be produced it was very expensive and so it was much cheaper, if it was a one-off, to lay out the design for the panel on an assembly table, build the panel on top of it, then, just whitewash the table ready for the next panel. This also made it easier if there were repeated design elements in the panels, such as borders or canopies, for example, as these could be left and only the sections that were different needed to be whitewashed and redrawn. By the 14^th century, in Italy at least, there is documentary evidence that paper was being used as standard, as Cennino Cennini writes in his manuscript Il Libro dell’ Arte:

“you will take as many sheets of paper glued together as you need for your windows; and you will draw your figure first with charcoal, then fix it with ink, with your figure completely shaded, exactly as you draw on panel. Then your glass master takes this drawing, and spreads it out on a large flat bench or table; and proceeds to cut his glasses”

Profile of a lead came

The lead strips used to connect the separate pieces of glass in the panel are called “cames”. The same process of using H-shaped rods of lead has been used since Roman times, and the Latin name for the rods of lead was calamus, so this is probably where the term “cames” is derived from.

In mediaeval times lead was cast in a mould made either from wood or iron. Theophilus describes the process of making an iron mould, which consisted of two parts, hinged at one end, which, when closed created a hollow ‘H’ shaped channel down the centre. Molten lead would be poured into one end then removed once cool. If you didn’t have suitable iron available you could create a mould using wood. Inevitably, this casting process resulted in large variations in the shape of the cames, which were then cut by hand to the desired size, and so are quite uneven and irregular. It was around the 16th century that the use of a lead mill for shaping the leads into a more regular profile was introduced.

The lead used in mediaeval times, although up to 99% pure, still retained a small number of impurities such as silver, tin, copper and antimony, which considerably increased the tensile strength and hardness of the lead at ambient temperatures, making the lead network stiffer and less elastic, and also more resistant to corrosion. During the nineteenth century it became possible to produce lead without all of the other naturally occurring metals which resulted in softer and less durable cames. As a result of this, it quickly became necessary to replace these softer lead cames on a regular basis, which – ironically – lead to the myth that cames would have to be replaced every 100 years or so and to the routine re-leading of medieval windows. This is actually very rarely true, especially with medieval lead which is remarkably strong, and more modern lead which deliberately contains some additional trace metals that give a stained glass panel strength as well as flexibility.  The nature of the lead used, the standard of craftsmanship in the construction of the window, and the weathering the window has been exposed to are all key factors which determine the amount of maintenance a window will need. It might need to be re-leaded just a few years after the original panel was made, or it might last for centuries.

A panel during assembly – note the design attached to the board below the glass, and the nails holding the pieces in place.

Assembly of a window usually starts at one corner of the panel and gradually works outwards and upwards. Lead strips are cut to length using a lead knife and carefully shaped around each glass section. Horse shoe nails are tapped into the assembly table and added or removed as required to hold the sections together. This process today is almost exactly as Theophilus described back in the 12^th century!

A panel during restoration being re-leaded

Once all of the pieces of glass have been brought together using lead, the joints where the leads meet then need to be soldered on both sides to secure all the pieces in position. Theophilus explains that the solder (a mixture of lead and tin) was cast in a similar way to the lead cames, using wooden or iron moulds.  Each joint would be lightly scraped and wax would be used as a flux during the soldering The soldering iron was long and thin with one end round and shaped to a point. This would be heated with care and much experience because soldering at the right temperature greatly affected the stability of the final window. Not enough heat and the solder will not run and join the cames together securely, too hot and the underlying lead will melt and distort or even burn, potentially damaging the glass beneath.

Marie soldering using an electric iron

The final step nowadays, is to cement the panels. This process, in which a putty-like mix of a calcium carbonate and boiled linseed oil is worked between the leaves of the lead cames and the glass pieces, adds greatly to the stability of the panel as well as making it more wind and waterproof. There is however, no mention of this process in any of the treatises before the 15th century which suggests that no cementing took place at this time.

Facts about glass – Cutting and shaping glass

Once a design for a window has been finalised the coloured glass is then chosen and cut to shape ready to be painted. When the Boppard windows were made glass cutting techniques were different to those we use today. Some medieval treatises survive which shed light on these early techniques.

The earliest surviving reference to the medieval process of glass cutting is found in the 12^th century treatise “On Divers Arts”, written by a Benedictine monk known as Theophilus Presbyter. He very clearly explains in detail the techniques involved in painting, glassmaking and metalwork. Theophilus is believed to have been shown how to make stained glass windows by practicing craftsmen.  He tells us to:

“heat on the fire an iron cutting tool, which should be thin everywhere except at the end, where it should be thicker. When the thicker part is red hot, apply it to the glass you want to cut, and soon there will appear the beginning of a crack. If the glass is hard, [and does not crack at once], wet it with saliva on your finger in the place where you had applied the tool. It will immediately split and, as soon as it has, draw the tool along the line you want to cut and the split will follow.”

People have tried to recreate the medieval glass cutting  techniques described by Theophilus – below are some examples of this using slightly different shaped tools:

Using a red hot iron to cut glass. Image courtesy of Katie Harrison

Cutting glass with a hot iron

“When all the pieces have been cut like this, take a grozing iron, a span long and bent back at each end, and trim and fit together all the pieces with it, each in its proper place”. The grozing iron removes chips of glass from the edges to shape the glass, leaving a very distinctive chamfered edge.

Using a grozing iron

You can see, in the photos below of the Boppard glass during restoration, the textured edges of the shaped glass pieces where they have been nibbled with a grozing iron.

Grozed edges on Boppard glass pieces

Grozed edges on Boppard glass pieces

Another treatise, dating from the late 14^th century, also describes the use of a hot iron to cut glass. In contrast to Theophilus, this manuscript was written by an experienced glazier, known as Antonio da Pisa. An example of one of his windows, which was commissioned in 1395, survives in the nave of Florence Cathedral.

It is interesting to note that in addition to the use of a red hot iron to cut glass, he describes two other techniques; the application of a thread soaked in sulphur, wrapped around the glass and set alight (probably only used for very thick glasses) and the use of a hard stone, such as diamond or flint. This is the earliest known reference to diamond cutting of glass, which developed into the modern diamond wheel cutters we use today.

It is not known which was the preferred method at this time, it may well have varied from studio to studio. It is most likely that the change in techniques was gradual, and either the hot iron or a hard stone, such as a diamond, was used based on personal preference, before grozing the glass where necessary. The setting alight of threads dipped in sulphur would have been quite hard to control and therefore imprecise and so is unlikely to have been common practice.

Once all of the pieces of glass had been cut, they were then ready to be painted, fired and leaded together.