Reverse painting on glass became popular in Federal America and was practiced mainly by foreign-born artists in Boston, Salem, Philadelphia, and Baltimore. For example, "Walker and Chandless, Painters, in General, from Dublin and London," advertised "Painting on Glass and Transparent Painting" in the Maryland Gazette in 1790. Such paintings, with subjects ranging from flowers and geometric designs to allegory, mythology, and scenic views, were usually incorporated into mirrors, clocks, and other furniture. Ten reverse-painted glass panels, varied in size and shape, can be seen on a neo-classical sideboard in the collection of the Department of American Decorative Arts (Figure 1) that had been commissioned by General David Van Ness (1743–1818) for his Maizefield estate in Dutchess County, New York. Major elements of the designs used on these panels, including urns, cupids, foliage, lyres, and masks, were derived from plates in Thomas Sheraton's The Cabinet-Maker and Upholsterer's Drawing-Book, first published in London in 1791. In an accompanying text Sheraton wrote: "These may be painted, inlaid, or gilt in gold behind glass, and the glass being then beded [sic] in the pilaster, it is secure, and has a good effect."

At the end of the eighteenth century, domestic glass manufactories in several major commercial centers were competing with imported crown and cylinder glass from England, and as such the origin of the panes used on the Van Ness sideboard cannot be assumed. Samples from two panels were analyzed using energy-dispersive X-ray spectrometry (EDS) and both were found to be potash-lime glass with a ratio of potassium to calcium of approximately two to one, containing only relatively small amounts of sodium, magnesium, and aluminum. These results may point to a domestic source, as glass with very similar composition is known to have been made by at least one eighteenth-century American producer, the New Bremen Glassmanufactory in Maryland.

Well-executed reverse-glass paintings do not reveal the complexity of their manufacture. Since the designs are applied to the back of glass panes they must be built up in reverse—starting with the foreground and working "backwards"—which makes corrections virtually impossible. The technique used for the glass panels on the Van Ness sideboard is called metal-foil engraving, although in technical and art historical literature it is often referred to as verre églomisé. Gold leaf was applied to the back of the glass with a size such as clarified egg white, gelatin, or gum, and then engraved with a stylus of metal, wood, or bone. The design was completed by applying a colored background with paint (Figure 2).

From an advertisement in a 1795 issue of the Pennsylvania Packet, it can be established that "White Lead, Yellow Ochre, Venetian Red, Spanish Brown, Lampblack, Verdigrease, Prussia Blue, &c. dry and ground in oil; Linseed oil raw and prepared, Spirits Turpentine, [and] Painters Brushes" were among the materials available to reverse-glass painters. The paint layers on the six smallest glass panels contain a mixture of Prussian blue and lead white, while the green oval fields within the decoration of the two larger rectangular panels were painted with verdigris (Figure 2). A transparent layer of verdigris was found directly behind the engraved gold leaf on all rectangular panels. Using Fourier transform infrared spectroscopy (FTIR) and EDS, linseed oil was identified as the paint medium used on the Van Ness sideboard, excepting the large oval panels. The engraved urns on these two panels are painted with a transparent Prussian blue in a pine resin binder, while the red background contains vermilion, lead white, and dragon's blood, a dark red palm-resin traditionally used in glazes. Based on the nature of the pigments identified, it can be recognized that the glass panels on the sideboard were originally far more intensely colored, but they have altered—with Prussian blue fading to blue-gray, and verdigris and dragon's blood turning brown and orange-brown respectively—due to the combined effects of exposure to light and the presence of potassium hydroxide, a product of glass corrosion (see Glass disease). Something of this original brilliance can be seen in the saturation of paint layers where protected by the gilding (Figure 3).

Delamination is a problem innate to the reverse-glass painting technique, since paint layers do not bond very well to smooth vitreous surfaces and begin to separate as the binding media deteriorate. At first, this process creates voids between the glass and the paint, and the difference between the refractive indices of glass and air cause the colors of detached paint layers to appear lighter. On the glass panels from the sideboard the adhesion of the paint was additionally adversely affected by the presence of potassium hydroxide. On several, this combination of detrimental influences has caused nearly ninety percent of the paint to delaminate, and on the proper right oval panel, for example, the damage progressed to the point where more than half of the red paint had fallen away from the glass surface, leaving hundreds of loose flakes that were recovered when it was removed from the sideboard (Figure 4). The oval panels were found to have a whitish opaque film on their reverse, both on glass surfaces exposed by paint loss, and on intact paint layers and detached fragments. FTIR analysis served to identify this substance as beeswax, which probably had been used as a consolidant during an earlier treatment. In the presence of potassium hydroxide the wax esters have saponified, causing further deterioration of the paint layers.

The first step in the treatment of the glass panels was the removal of the disfiguring opaque residue with deionized water and mineral spirits, although this method could be used only on exposed glass surfaces and where the paint layer is sufficiently stable. Since the size used to adhere the gold leaf proved to be water-soluble, only mineral spirits could be used in the gilded areas. For the consolidation of the reverse-glass paintings both artificial resins and waxes were considered. Dilute resin solutions are very effective because their low viscosity allows for deep penetration into spaces between the paint and the glass through capillary action, but air bubbles often form in these cavities when the solvent evaporates. Although more difficult to apply, waxes are preferable for the stabilization of reverse-glass paintings since they do not develop air bubbles, while offering the benefit of thermoplasticity, which makes it possible to reform the consolidant when necessary. Wax was the only option for treating the Van Ness sideboard panels because the beeswax residue makes it impossible for resins to adhere properly. TeCero Wax 30445, a microcrystalline wax chosen for its good adhesive properties, was applied with hot spatula to the reverse of the glass panels. The risk of using heat on the corroded glass was tolerable because the melting point of this wax is relatively low, and the use of a small tip on the spatula ensured localized and brief exposure. No effort was made to complete lost sections of the designs, but visual integration of losses was achieved by placing toned, acid-free paper behind the glass.

Given that the reverse-painted panels are affected by glass disease, their stability depends largely on climate control. Unfortunately, the ideal relative humidity specified for such glass is well below the levels required for wooden furniture, further complicating the preservation of the Van Ness sideboard.

Glass Disease
Glass is an amorphous matrix of negatively charged silicate ions and metal cations. The main refractory ingredient is silica (SiO₂), to which alkaline substances such as potash (K₂CO₃) or soda ash (Na₂CO₃) are added as fluxes, together with lime (CaO) or magnesium oxide (MgO) as stabilizers.

The incidence of glass disease is directly related to the composition of the affected glass, and the deterioration that occurs can be thought of as a two-phase process. When high flux-low lime glass is exposed to a humid environment, the first step, known as alkali depletion, occurs as alkali ions contributed by the fluxing materials migrate to the surface of the glass matrix, where they are replaced with hydrogen ions present in water vapor. The resulting alkali-deficient, hydrogen-rich layer—the "gel layer" or so-called hydrogen glass—has a lower reflectance. The potassium and sodium hydroxides formed in this process react with carbon dioxide and sulfur dioxide from the air, and the resultant hygroscopic salts form a greasy, highly corrosive alkali-rich film on top of the depleted glass. In extreme cases, droplets form on the surface of the glass, a phenomenon known as "weeping". When glass affected by glass disease is placed in an environment with a lower relative humidity, the sodium and potassium carbonates form a white precipitate on the surface.

The second part of this process occurs due to the difference in size between the hydrogen and alkali ions. Replacement of the latter with smaller hydrogen ions causes surfaces to contract, leading to fracturing of the glass and exfoliation of the upper layers. Ion exchange will continue at the newly exposed surfaces of these breaks and losses, causing the damage to progressively worsen, eventually resulting in the disintegration of the glass.

BACK TO TOP

Simone Bretz has been active as a conservator of reverse paintings on glass in Munich since 1985, working with museums and private collections in Europe and the United States, including the Metropolitan Museum, where she has treated works of art on four occasions. After completing her training as a conservator of easel paintings, she acquired technical skills in the field of glass conservation and specialized in the treatment of reverse paintings on glass. Simone recently contributed her expertise to a research project on Swiss reverse-glass painting of the seventeenth century, and she has published widely on reverse-glass painting techniques.
info@bretz-hinterglas.com