Image of CBL W 099 f. 42 mounted with Japanese paper corners.

A medieval masterpiece under magnification

1 March, 2019

It has been a busy year for the conservation team, with a steady stream of beautiful objects coming through the lab. One of the highlights for the team over the past year has been preparing a number of unique and precious objects for Gift of a Lifetime: Treasures from Chester Beatty’s Collection, an exhibition to celebrate the 50th anniversary of Beatty’s incredible gift to the nation. This included sixteen miniatures from a stunning illuminated prayer book known as the Chester Beatty Rosarium (CBL W 099). The illuminations are by Simon Bening, one of the most celebrated manuscript painters in 1530’s Flanders, so unsurprisingly the conservation team were completely blown away!

After an initial conservation assessment, it was decided that the miniatures should be removed from their glass housing so that they could be digitised and prepared for display. With the glass removed, we checked the pigments carefully under magnification and observed some severe but localised flaking, particularly in the painted purple borders.

Working under magnification, the media was carefully consolidated using a 2% aqueous solution of Isinglass, an adhesive derived from the swim bladder of the sturgeon fish. First, Isopropanol was applied to the edge of the flaking areas using a very fine brush, directly followed by a second brush which applied the adhesive. The alcohol acts as a wetting agent, reducing the surface tension of the adhesive and drawing it underneath the flaking pigment layer by capillary action. On drying, the adhesive secures the fragile pigment layer to the parchment folio below.

CBL W 099 f.42, Detail of a figure under magnification.

CBL W 099 f.26, Alice Derham consolidating flaking pigment at the microscope.

The double-sided Rosarium paintings are surrounded by a border of thickly applied purple pigment, a clear statement of the wealth of the person who commissioned the manuscript. Even after careful consolidation of the purple pigment, the paint layer was still fragile. The margins have also been extensively trimmed, with some folios trimmed right up to the edge of the illumination. When selecting an appropriate mounting system for these folios, we needed to ensure that there was no risk of the purple pigment detaching.

With parchment folios, our preferred mounting system usually involves attaching Japanese paper tabs to float the object within a window mount, allowing the parchment to react to environmental changes in the most controlled manner, but in this case, we wanted to avoid adhering anything to the delicate purple borders.

Using an adhesive-free system usually means that the object is over-mounted to cover the mounting system. However, as the margins on some of the Rosarium folios were extremely thin and the curator wanted to display as much of the remaining purple margins as possible, we needed to devise a discreet method which could support the weight of the folio whilst overlapping the object as necessary. Access to the opposite side of the folio was also important, making this a particularly challenging project.

Image of CBL W 099 f.22, before conservation.
W 099 f.22, before conservation.

After some discussion, we decided upon a mounting system that uses non-adhesive folded paper corners to secure the delicate folios. The paper corners were made in-situ, using a narrow strip of Japanese Usumino paper (4cm x 2cm) which is folded to create the corner. First, the lower edge of the Usumino strip was feathered to ensure a soft edge was in direct contact with the painted surface of the object, then the upper corners were folded down towards the middle of the feathered-edge to create a triangle. Next, the two flaps were folded back on themselves to create a narrow ledge in which the folio can rest. This very simple technique can be modified by adjusting the size of the ledge to suit the needs of each individual folio.

Diagram of folded Japanese paper corners.
Folded Japanese paper corners.

Each paper corner was carefully positioned on the mount board, before the central part and the two bottom corners were adhered to the mount with wheat starch paste. Using this system, the folios were mounted in pairs with a standardised window size to ensure the edges of the folio were protected.

These extremely fragile object are now safely housed, and can be displayed and enjoyed by the museum’s visitors in their full splendour. This adhesive-free mounting system also allows easy access to the back of each folio by gently pulling on the paper tabs to release them from the mount.

The Chester Beatty Rosarium’s exquisite paintings will be on display in Gift of a Lifetime: Treasures from Chester Beatty’s Collection until 28th April 2019.

Alice Derham & Julia Poirier

Image of CBL W 099 f.18, mounted in non-adhesive Japanese paper folded corners, after conservation.
W 099 f.18, mounted in non-adhesive Japanese paper folded corners.

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