JAIC 1993, Volume 32, Number 3, Article 2 (pp. 231 to 240)
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Journal of the American Institute for Conservation
JAIC 1993, Volume 32, Number 3, Article 2 (pp. 231 to 240)

THE HARVARD GLASS FLOWERS: MATERIALS AND TECHNIQUES

RIKA SMITH McNALLY, & NANCY BUSCHINI



2 GLASS ANALYSIS: ELECTRON BEAM MICROPROBE

To identify the type of glass used to make the Glass Flowers, the Botanical Museum provided 30 fragments collected from damaged models. Of these, 14 were chosen for analysis using electron beam microprobe. In selecting the fragments to be analyzed, an effort was made to obtain an even distribution of fabrication dates within the 50-year working period in order to detect any change or evolution in materials. In addition, since the majority of available fragments were leaf parts, these fragments were selected when possible to provide a foundation for the comparison of glass type and colorants over time. The presence and amounts of the oxides of 13 elements were sought in each sample (table 1).

TABLE 1. THE WARE COLLECTION OF BLASCHKA GLASS MODELS OF PLANTS ELECTRON BEAM MICROPROBE ANALYSIS OF GLASS COMPOSITION (WEIGHT PERCENTAGES)


2.1 GLASS, ENAMEL, AND COLORANTS

The analyses indicate that the glass used by the Blaschkas was a soda glass containing a smaller amount of potassium as the secondary alkali. It appears that the Blaschkas used a similar type of glass over the 50-year working period, as the elemental composition of the analyzed samples is fairly consistent. After Leopold's death in 1895, Rudolph began experimenting with enameling glass rather than cold-painting it to achieve color, and in flowers from this time there appears to be some variation in the glass composition. The glass Rudolph used after his father's death had a higher percentage of sodium and a lesser amount of potassium than the earlier glass, but it remained binary alkali glass containing sodium as the primary alkali.

The microprobe analysis indicates that a much wider range in the elemental composition of the enamel was used to color the surface of the clear glass substrate in the years 1895–1936, probably reflecting Rudolph Blaschka's keen interest in enamel and his experimentation with it. After Rudolph first began using colored glass and enamels in 1895, he wrote to Mary Lee Ware of the difficulties he was encountering with the new technique for coloring the botanical models: “The working with the colored glass is specially more exposed to risk in springing and some other unpleasant properties … so it is requisite to work in awfully hot temperatures, on purpose to prevent this damage” (Blaschka 1896). From the letters in the archives, it appears that the biggest problem was springing and shattering of the glass as it was enameled or after enameling as it was annealed and cooled. Annealing is the process of placing a completed glass object back into a section of the glass furnace or oven to allow it to heat uniformly and then cool gradually, releasing strains that have built up in the glass during the forming process and preventing sudden fracture. When Leopold and Rudolph worked together, the technique involved only lampworking (see sec. 5) without the additional complication of enameling over the clear glass. Rudolph also had problems with color in the enamel, including color changes upon reheating. Letters show that he purchased a new hearth and a muffle stove for enameling and annealing as he experimented with the new techniques.

Enamel is made of finely ground glass mixed with a medium such as oil or gum, applied to the surface of a clear glass substrate and then heated to melt the powdered glass and fix it to the substrate, in the process burning off the medium. Although the term “enameling” is often used to describe the application of glass to metal, it is also used to refer to glass on glass and was referred to in this way by Rudolph Blaschka. Seven samples of enamel from the Glass Flowers were analyzed using the microprobe, and all contained lead oxide, ranging widely from 8% to 61%, with corresponding decreases in alkali and silica and an overall decrease in lime. The lead glass has a lower melting point than the alkaline glass, allowing Rudolph to apply and then heat the enamel without melting the clear glass substrate. The wide range of composition may be the result of Rudolph's experimentation with melting temperatures and/or the effect of lead on color. Rudolph appears to have searched continually for better heating and melting properties and better color, for as late as 1906 he wrote of the difficulty he was encountering getting the right color for the model of autumn maple leaves.

The microprobe analysis indicates that metal oxide colorants were used in the colored glass and enamels from the period 1895 to 1936 (in the earlier period, 1886–95, only clear glass was used). The deep green glass used for some of the leaves contained an average of 1.4% chromium oxide and 0.3% copper oxide by weight. Other leaves with a brown-green color contained small amounts of the oxides of nickel, manganese, and iron in addition to copper and chromium. Tin and lead oxide were used to produce the white-colored enamel on the petals of Asclepias syriaca (model 770, 1923). The brown-green enamel on Ceratophyllum demersum (model 738, 1913) contains small amounts of the oxides of copper, nickel, chromium, manganese, and iron as colorants (table 2).

TABLE 2 GLASS AND ENAMEL COLORANTS (WEIGHT PERCENT)


Copyright 1993 American Institute for Conservation of Historic and Artistic Works