Preparing for pigment consolidation part 1

MS 201.4  © The Fitzwilliam Museum

A prevalent reason fragments were cut from manuscripts historically was so that illuminations could act as individual examples of medieval painting. The fragments were removed from their manuscript context and valued as art objects rather than as part of a text. While each fragment was highly valued for its imagery, removing it from the original context within a book can be very problematic for its physical survival. The artist painted on parchment which was meant to be stored within the controlled conditions of a book. Mounting systems for fragments do not recreate this environment and if not created carefully may cause damage. The pigments and their attachment to the parchment are threatened by this change in environment. Arguably, this compromises the integrity and aesthetics of the illuminations that made them appealing to cut out in the first place.




What goes wrong with the pigment?

Example of cracking, flaking and loss in
 MS 201.4 © The Fitzwilliam Museum

Pigment or paint deterioration can be caused by a variety of factors and it can be difficult to identify the exact cause in an illumination. Cracking can be caused by movement of the substrate; from the expansion and contraction of the parchment as it reacts to environmental factors; cockling; or other kinds of mechanical stress. It can also be caused by binders or the drying time of the paint.

Powdering can be caused if the artist used too little binder, or if the binder was too weak. Very coarsely ground pigments may be too heavy for the binder and so subject to powdering; the pigment may be attached until the parchment is dessicated and shrinks in low relative humidity, causing an already weak binder to fail.

Pigments themselves can also be the cause of the damage. Copper-based pigments can corrode, weakening or destroying the substrate they are painted on. Silver gilding tends to oxidize and darken over time. When some pigments are painted immediately next to one another they may react and cause damage. For example, the sulfide in orpiment will react with lead-based pigments such as lead white to form lead sulfide, visible as a blackening in white areas. Chemical deterioration is facilitated by moisture, and when cut from the protective environment of a manuscript, damage to a fragment may be accelerated as it is more vulnerable to changes in  relative humidity.

Cracking and flaking of media can be seen in most of the fragments being conserved in this project. Many fragments are in need of pigment consolidation to stabilise them because of this. While these problems are visible to the naked eye, until microscopic examination and documentation is conducted it is not possible to know the severity of the damage. Some areas of cracking and flaking may not be active damage and therefore will not require consolidation. For example, if a pigment appears to be powdering but does not seem loose when checked under magnification it may indicate that some of the pigment did not mix with the binder properly. While some pigment was lost, the rest of the pigment would be firmly attached and not at risk to future damage.

Practice

I am in the process of conducting microscopic examination and documentation of pigment damage before beginning consolidation of pigments. With my limited experience handling pigments and understanding of pigment damage, I am also practising pigment consolidation before treating pigments on any of the fragments.

For this exercise I created several samples imitating the flaking, cracking and powdering seen in the historic manuscript fragments. These act as test pieces on which I can practice to gain experience using consolidation techniques and adhesives. I did not attempt to imitate the same circumstances that lead to the damage of the pigments, but instead concentrated on imitating the current condition of the pigments.

Cracking and flaking:

 To imitate cracking and flaking, I used a heavily applied  iron oxide using Laponite as a binder. Laponite is a synthetic layered silicate sometimes used as a cleaning poultice in object conservation. Like many silicate compounds it will shrink and crack if allowed to dry in an uncontrolled way.

Iron oxide mixed with Laponite gel

I prepared the Laponite with water and after it dissolved and solidified as a gel I added the iron oxide. After a few small tests, I found that a thick application cracked and flaked as it dried.

Parchment tensioned as the pigment dried.

Powdering:

To imitate powdering I used carbon black mixed with water and did not add any binder.

Carbon black mixed with water

The pigment appeared to be well-adhered to the parchment until it was tested with a small brush, whereupon it detached. For this reason, on historic objects it is important to rely not only on what can be observed, but physical examination too.

Carbon black detached easily onto my finger.

Preparation:

I painted these pigments on the hair side of one parchment offcut and onto the flesh side of another offcut. On the flesh side I painted two squares of each pigment.

On the flesh side I wanted to better simulate the combinations and overlaps of different pigments as they would appear in the actual media. I drew a copy of a hare riding a hound from the Macclesfield Psalter and painted it carefully so that consolidating the two pigments would be more similar to those in the fragments.

Pigment on flesh side of parchment, using imagery from Macclesfield Psalter.

Original detail of Macclesfield Psalter MS 1-2005 © The Fitzwilliam Museum

Pigment consolidation has been slightly delayed while a broken component of our microscope is replaced. Future posts will discuss the results of consolidating the practice sheets and the consolidation of the fragments.

The next post will be about removing fragments from old mounts, and 'Pigment Consolidation part 2' will be posted in January. To be kept updated about new posts, please email me here with the subject 'blog subscription' and I will add you to my mailing list.