Conserving glass objects is tricky business. The material itself presents many technical problems. These problems are compounded when dealing with a flat pane of glass, and especially a piece of painted glass. The multi-media aspect of reverse painted glass objects makes them an interesting challenge for any conservator.

A brief background on reverse painted glasses:

Reverse painted glass (sometimes called verre églomisé [1]) is an art form that entails applying paint to the back of a flat sheet of glass. The image is viewed from the opposite side the paint was applied on. This art form was very popular during the Middle Ages for sacral painting, especially in the Byzantine Empire. Later, its popularity spread to Italy, where it became common during the Renaissance. Since then, it continued to gain popularity, especially in Central Europe during the 18th century. During this time, reverse painted glass was favored by the Church and by European nobility. Many objects were created in this fashion; including religious paintings, landscapes, clock faces, etc. Until the invention of acrylics, these pieces were created using oil-based paints.

Conserving flat glass objects:

To briefly sum up the process, a padded template is made to arrange the glass shards in alignment for bonding. This template supports the object during bonding.

 After the template is made, the shards can be cleaned using a solution of 50% deionized water and 50% Industrial Methylated Spirit. The break-edges are typically cleaned with Acetone to rid them of any grease that may be present.

The object is then aligned and taped into place for bonding. To bond, a liquid epoxy resin is used. The epoxies conservators utilize for this process are clear and very closely match the Refractive Index [2] of the glass. To apply the epoxy, small amounts of the liquid are dotted along the break-line with a cocktail stick. The epoxy then travels into the break-lines by capillary action and cures there. After the epoxy is cured, excess can be removed. This completes the repair of the glass.

Issues in Conserving Reverse Painted Glass Objects:

Conserving reverse painted glass requires a bit more delicacy and planning than repairing plain flat glasses. The complications can be attributed to many factors such as: deterioration of any framing on the piece, how the frame may have affected the glass, selection of appropriate materials and methods of cleaning and bonding to mitigate risk to the painted surface, and how to repair any areas of loss in the paint (a process called retouching).

A case study:

In the case of the reverse painted glass object shown in the beginning of this article, proper planning was crucial to maintaining the integrity of the piece. Assessment of the object revealed that the likely cause of the damage (broken glass) was the way the object was secured into its frame. The glass had been secured into the frame with small nails. These were driven into the frame, however, many of them were also in contact with and putting pressure on the glass. Over time, the stress put on the glass from the nails caused the glass to crack and split where the two materials made contact. The nails had also rusted over time, which contributed the degradation of the object as a whole. Removing the glass from the frame was therefore critical in stabilizing the object.

The assessment also revealed that this piece contained photographs (the geishas in the foreground) and mother of pearl decoration. This mix of mediums made the treatment plan even more complex.

 To begin, the backing and nails had to first be removed. This was done carefully avoiding any unnecessary contact with the glass. 

The images above show the deteriorated nails in the backing and the removal process.

Once the glass was free of the backing, the extent of the damage was observable. The piece had clearly been previously repaired.  Old repairs included a reattached corner of the glass that had become detached and reaffixed pieces of mother-of-pearl decoration. The previous bond left the object with a large amount of excess adhesive, and the dislodged pieces of mother-of-pearl were secured with tape (which had yellowed and mostly detached from the painted surface). There was also an old cotton ball stuck to the paint. The removal of these old repairs had to be undertaken carefully.

Circled areas show the tapes, cotton ball, and old adhesive.

The cotton ball was able to be mechanically removed by carefully lifting it from the surface with tweezers and cutting any attached fibers with a scalpel. The tapes had become so embrittled with age that they were able to simply be gently pulled free from the surface. This did not affect the paint at all but did leave behind some adhesive. The bonding adhesive, however, had become completely attached to the paint on the surface of the glass. There was no way to remove it without also removing the paint in this area. This presented a problem: should the yellowing adhesive (which is no longer holding the glass in place) be removed so new adhesive can be applied cleanly? Or, should it be left to preserve as much of the original paint layer as possible? Ultimately, it was decided that the adhesive would be left, and the paint layer preserved.

 Next, the stability of the paint had to be assessed. This was particularly important to establish if solvents could be used to prepare the glass edges for bonding. To assess the paint, small solvent swab tests were carried out on a discrete area. Neither Industrial Methylated Spirit nor Acetone had any effect on the paint. This indicated that it was produced with an oil-based medium.

With the stability of the paint established, cleaning could commence. To clean the object, the front (the side without paint) was cleaned using cotton swabs with the 50% deionized water and 50% Industrial Methylated Spirit solution. This effectively removed the dirt from the surface, however, there was also rust attached to the edges of the glass where the nails had come into contact with it. This was removed with acetone and a scalpel (where mechanical action was required).

An area of the rusted glass edge with a chip from the nail.

The object has now passed onto the bonding stage. This has presented new challenges in terms of adhesive selection, which must be done carefully to preserve the pictures. After the object has been bonded, it can pass onto retouching. Unlike other objects, the retouching will not be applied directly to the surface, but instead to a clear Polyethylene sheet (Melinex) that will back the glass. Remounting the painted glass into the frame will follow for completion of the treatment. These phases of treatment will be discussed in Part 2 of this article.

[1] Morris, A. and Erhardt, M. (2012). Mary Magdalene: Iconographic Studies From the Middle Ages to the Baroque (Studies in Religion and the Arts). Brill.

[2] The Refractive Index is the value given to the bending of a ray of light when passing through a given medium. (Encyclopedia Britannica. (2018). refractive index | Definition & Equation. [online] Available at: https://www.britannica.com/science/refractive-index [Accessed 4 Jun. 2018].)