history of flat glass

The industrial revolution led to mass-marketing of many materials, a good number of which found their way into 19th-century cultural heritage. One such material was flat glass, the demand for which was driven by the window glass and mirrored glass markets. Flat glass was usually of an alkali silicate composition. With the invention of photography, flat glass, also known as sheet glass or plate glass, was quickly adopted by photographers for both picture glazing and image substrates. Continuing developments in the flat glass industry influenced the quality of glass that eventually found its way into 19th-century photographs, which now make up an important body of cultural heritage collections worldwide. The history of flat glass development and its effect on photographic advancements in the 19th century go hand-in-hand with developments in photography and contribute to the amount of instabiity found in glass used in this new art form.

Historical test samples

A small group of ambrotypes and tintypes, i.e., objects containing cover or image glass, were purchased from the open market and analyzed to get a sense of typical photographic glass compositions. Seven samples of glass from the objects were ground into powder for fully quantitative XRF analysis. Results show that among these random samples, six are high lime soda glass (also known as soda lime glass) with:

~13-15% CaO (lime)
~10-14% Na2O (soda)
~0-500 ppm MnO (manganese oxide)
~0-250 ppm As2O3 (arsenic oxide)

Although these results do not disqualify any of the cover glasses as being modern replacements (as often occurs), no boron or other elements that are typical of modern glass were detected.  

The remaining sample, an uncased ambrotype (pictured above as FG-5I), is a higher lime potash glass, with a composition of:

19% K2O (from potash)
8% CaO (lime)
1.5% Na2O (soda)
467 ppm Sb2O3 (antimony oxide)
155 ppm MnO (manganese oxide)

UV examination of historical photographic glass is often difficult to determine and can be influenced by the light source and viewing set up, but is best assessed on the glass edges. UVA examination technique is only appropriate for cover glasses in encased photographs, since image glass normally has a dip-coating and/or emulsion, which itself will fluoresce. Nevertheless, UV examination of photographic glass reveals some possible patterns related to compositional types. Fairly pure soda lime glass usually has only weak fluorescence in UVA (365 nm). However, the presence of MnO, a decolorizer, is quite common and causes distinct fluorescence in UVA of both soda lime and potash glass, producing a range of yellow colors---from orangish to greenish. This variation in color depends on the concentration of Mn, and more particularly on the presence of other trace elements which can act as fluorescence co-activators or quenchers. For example, the presence of As2O3 seems to cause a more lemony yellow fluorescence, while Fe2O3 may quench and mute fluorescence. In UVC illumination (254 nm), coatings normally do not fluoresce, and soda lime glass usually has a weak fluorescence, depending on the elements present. UVC illumination of potash glass that contains antimony oxide is interesting and yields a strong milky light blue in historical glass, as viewed as part of this study. The latter visible fluorescence color can, however, be confused with clear light blue fluorescence caused by PbO, which is normally only present in very trace amounts in photographic glass.

Two encased daguerreotypes from a private collection were examined without the opening of the encasement using non-invasive XRF, light microscopy, and OCT. XRF shows that the cover glass in one of the two objects is a potash glass composition containing an antimony oxide fining agent, while the other is soda lime glass.

Both objects display significant deterioration on the underside of their cover glasses, including a high concentration of liquid droplets and precipitated salts, exemplifying that the micro-environments of encasements can cause deterioration of any type of cover glass. OCT, a medical imaging technique that has become standard in the field of ophthalmology, was used in situ to non-invasively view and document deterioration on the underside of the cover glass by imaging through it (without opening the encasement). This allowed liquid droplets on the underside of the cover glass to be examined without danger of drying out, and is the first time OCT has been used for this purpose, as far as we know. OCT images show that the liquid droplets extend about 100 µm below the glass surface. Images taken of the potash cover glass also allowed distinction and measurement of dry particulates.

historical society surveys

Several institutions were visited to survey a randomly sampled portion of the photographic collections, including: The Virgina Museum of History and Culture, The Historical Society of Washington DC, The Historical Society of Pennsylvania, and The Special Collections Research Center at The George Washington University. Three of these collections contain early photographic materials that previously had poor storage conditions and have not been treated by a conservator, as is typical of such under-resourced institutions.

Approximately 145 objects, including daguerreotypes, ambrotypes, tin types, glass plate negatives and glass lantern slides were examined with the non-invasive, portable instrumentation shown in this image. No sealed enclosures were opened during the surveys. XRF analysis in general showed a range of glass types similar to that represented in the model glass study. Unfortunately, signs of deterioration among the objects examined were far more common than predicted from composition alone, particularly in encased or enclosed objects. For example, about 90% of one historical collection’s encased photographs display liquid droplets and/or crystalline particles on the underside of the cover glass. Most surprisingly, these deteriorated cover glasses include both unstable glass formulations, where stabilizers are insufficient in quantity compared to alkalis, and what are normally considered to be stable formulations of soda lime glass. Glass deterioration symptoms that we observed are mostly on the underside of the cover glasses in the form of tiny precipitated-looking particles spread evenly around the surface, liquid droplets, and/or liquid droplets with a solid center. Contrary to deterioration symptoms predicted by model studies, no cracking in glass was observed, but some cracked emulsions of glass plate negatives was found, signaling possible glass deterioration underneath the emulsion. Biological growth on some cover glass was also observed, suggesting exposure to humidity. For more discussion of glass deterioration symptoms, see visual vocabulary.

Historical collection surveys determined that roughly 25-30% of the overall collections examined may show condition issues, either as liquid droplets or crystalline deposits, which is far greater than expected. Materials at the greatest risk for deterioration include encased photographs as well as glass lantern slides, which are usually sandwiched between plates of glass. Tests performed on liquid droplets on a soda lime glass lantern slide from about 1858, in which the tape seal had detached, showed that the droplets have a pH of about 9.0-9.5. This high alkalinity confirms their identification as symptoms of severe glass deterioration. These results also underscore the potential for any type of 19th-century glass to undergo deterioration in adverse environments.

Decision Tree

The results highlighted above have been used to construct a decision tree for assessing risk in collections that contain 19th century photographic glass. The tree guides museum professionals through simple tools, followed by XRF analysis, if available, for better confidence in assessment of preservation priority.