Lost in a Haze
Investigating Surface Changes in Red Colored Glass
Figure 1. Preexisting hazing on red glass (right) and worsened hazing after application of paint stripper (left).
Materials testing can sometimes have unexpected results. JBC worked on a large, multi-colored Venetian glass art mosaic installed on the exterior of an office building in the 1960s. A thick polyurethane coating had been applied to the artwork, possibly to prevent water infiltration. During testing to remove the failed coating, it was discovered that chemical paint strippers etched some pieces of red colored glass (Figure 1). Hazing did not occur on all red glass pieces in the testing area, but only a specific color of bright red glass. The remaining colors, including other shades of red, were unaffected (Figure 2). But why?
Venetian glass is made of the base ingredients silica, lime, and flux. Metal oxide pigments, together with the firing conditions, create the colors. This means each color of glass has different chemical and physical characteristics.
Figure 2. Hazing in bright red colored glass after paint removal testing.
Under the microscope, the hazing appeared either as a dense concentration of small pits in the surface (Figure 3) or as rings (Figure 4) possibly resulting from inconsistencies in how the metal oxides and other ingredients were dispersed throughout the glass.
Figure 3. Concentration of small pits in surface of hazed red glass.
Using x-ray fluorescence (XRF) to identify the elemental composition of the multiple glass colors, the bright red glass was determined to contain only half the silicon (and therefore silicon dioxide) found in the other glass colors, decreasing its stability. The red glass was also deficient in calcium, zinc, and potassium, which make up other stabilizing oxides in glass.
Figure 4. Preferential etching in red glass.
Additional testing with x-ray photoelectron spectroscopy (XPS) found that areas of the bright red glass exposed to chemical paint strippers were missing elements present in the other colors, indicating that chemicals were being leeched out of the glass. While this process occurred to some degree in all of the glass, it was especially pronounced in the bright red glass because of its lack of stabilizing chemicals.
The analysis ultimately allowed for the development of a safe and effective method of protecting the bright red glass during coating removal and a maintenance plan to prevent further deterioration of the glass in the future. JBC also successfully restored the original surface appearance of the hazed red glass.