To delve into a deeper understanding of uranium glass, such as its elemental makeup, micro-XRF is the perfect technique for measurement. Providing visualisation at a high-spatial resolution up to 4µm, combined with its output data, the micro-XRF conducts in-depth non-destructive elemental analysis.
As antiques go, uranium glass is one of the most unique and visibly striking ornaments that one can display in their home. Its history is tied to the grandeur that was 19th century Habsburg Austria. The creation of uranium glass is credited to 19th century glassmakers Franz Xavier Riedel, and his nephew Josef. Both men were fifth and sixth generations of the Riedel glass manufacturer, which is said to be the oldest family owned and operated global crystal glass brand worldwide. Although the mineral uraninite was used in Bohemia as a colouring agent for glass since the middle-ages, it was the Riedel’s who popularised uranium glass with their innovate designs. To create their pieces, uranium was engineered in an oxide diurnate form and added to a glass mix to produce the glass. This type of uranium glass became popular and highly sought-after in the late 1800s.
Figure 1: Uranium glass under fluorescent lighting in a dark room.
Given its popularity, glass manufacturers in Britain, France, and the United States (to name a few), adopted the technique to produce their own versions of uranium glass. Although eventually the production of uranium glass in the U.S. ceased in the middle years of World War II. This was because of the government’s confiscation of uranium supplies for the Manhattan Project (1942 – 1958). Despite lower production levels, uranium glass is still circulating today and can be found in select vintage and antique stores globally. To indicate the authenticity of uranium glass, a UV light can be used. The fluorescent light when pointed at the glass will trigger any uranium present to fluoresce. This will cause the glass to appear as though it is ‘glowing’ (fig. 1). Another method is to utilise a Geiger counter to measure for ionizing radiation (fig. 2).
A sample of uranium glass was scanned with parameters at 50µm resolution and 30ms/pixel using the Bruker M4 TORNADO at Portable Spectral Services. The glass, shaped in a leaf design, was purchased from a local antique store that held a collection of uranium glass.
Figure 2: Thermo Scientific RadEye B20-ER Survey Meter showing a reading of 2.4µSv/h (approx. 150 times the natural background reading of 0.14µSv/hr).
The precise amounts of materials used to create uranium glass have been seen to vary between manufacturers. A recipe that was used up until the 1940s describes the ingredients for uranium glass as:
Commonly: 850 pounds of sand, 330 pounds of soda, 100 pounds of feldspar, 42 pounds of lime, 50 pounds of nitrate, 36 pounds of lead, 10 pounds of arsenic, 43 ounces of uranium, and 13 ounces of copper oxide.
In figure 3, the elemental maps make it clear that abundant silicon, calcium, and arsenic are present. The latter of these being a key ingredient as a refining agent to reduce excess O2. This was key for creating a clean and smooth finish without bubbles on the surface of the glass. The high calcium present can be explained due to the use of both lime and calcium uranates in the manufacturing process (fig. 3). As evidenced in the 1940s recipe, high silicon is expected, as quartz (or sand) and feldspars are key minerals in the base glass mix.
Variation in element concentration is visible in the micro-XRF elemental scans due to differences in the topography and thickness of the glass. As seen in the scans, there are apparent highs of elemental concentration along the midrib and veins of the leaf design.
Figure 3: Individual element maps.
Along with elemental analysis, micro-XRF provides the ability to undertake a chemical analysis of the sample, with quantifiable results. In table 1, an oxide calibration was used during quantification to identify the percent of key elements expected from the 1940s uranium glass recipe. This was done to confirm that there was no Pb within the sample, even though it was to be expected from the recipe. The quantification shows very trace amounts of Pb is present within the sample.
Table 1. Detected elements from Uranium Glass Recipe.
As shown in figure 2, the Geiger counter picks up a reading at 150 times the natural background radiation. For context, that is half the dose rate one receives whilst having a dental X-Ray. However, as seen in figure 3, there seems to be a high proportion of uranium throughout the sample. This means that the colouring agent used when applying uranium to the glass mix was blended appropriately.
Table 1 noticeably displays uranium oxide within the sample to be at 0.191%, this is not uncommon for uranium glass, but sits below the average. On average the amount of uranium within a uranium glass sample is 2%, with the maximum amount ever analysed to be at 25%. PSS’ elemental analysis displays this also, with considerable peaks apparent for Si, Ca, and As (fig. 4); whilst uranium sits under 6cps/eV (counts per second per electron-volt).
Figure 4: Map Spectrum from uranium glass scan.
Things get more interesting by pulling apart the measured spectra for elements of lower abundance and contrasting their high spectral peaks with their mean spectra (fig. 5).
Figure 5: Contrasted elemental maps.
With these elemental maps, imperfections made during production are apparent. As seen in figure 5, there are different spots of contaminates of Al, Fe, and Ti – Cr and Ni were also identified as trace elements. It was confirmed that these elements detected were within the glass and not external – such as dust accumulation or smears – by corresponding the highs of impurity spectra with comparative lows in Si. Though Si is present throughout the whole sample, areas of lower Si could allow for the presence of a contaminant inclusion during the mixing process, and with a further look at figure 3, Si ‘holes’ are apparent in the displayed element map.
It is known that copper oxide (CuO) is often present in glass in minute amounts for colouration purposes. K and Na present also corresponds with the 1940s uranium glass recipe. The amount of observed sulphur in this sample also provides a unique insight into the quality of uranium glass – or any glass for that matter. Like arsenic, sulphur-bearing compounds are involved in the final part of the fusion process, with sulphates acting as an oxidizing agent, and sulphides acting as a reducing agent. The amount of sulphates present in the glass melt will influence the sulphur reactions and redox state. This determines the efficiency of bubble removal. Glass producers may add more sulphur to a batch than what is required, hoping to ensure good quality – in these cases excess sulphur is exhausted as a flue gas.
Another point of interest is the trace amount of Au detected. This has been confirmed by analysing the object spectra for the points of interest (fig. 6). The presence of trace Au is complexing at the outset. It could be explained by the rare association of gold with uraninite, such as at Witwatersrand in South Africa, and in Northern Finland within the Peräpohja Schist Belt. However, the gold detected within this glass sample is far more likely to be merely the product of dust or a smear on the glass surface rather than an inclusion, with a strong Si peak still identified within the gold object spectra.
Figure 6: Au objects, and object spectra (1 and 2).
Though glass is often expected to be a homogenous composition of materials, using micro-XRF technology the imperfections made during the foundry process can be detected. Figure 7 visually displays this with a gamma-reduced element overlay of Al-Si-S (a), and S-Ca-U (b). Topographic features have been exaggerated for easier visualisation of the alumina impurity and highs in sulphur.
Figure 7: Element distribution map overlay of Al, Si and S (a), and S-Ca-U (b).
Uranium glass has a history dating back to 19th century Europe. Its growth in popularity saw a rise in production across the globe. The micro-XRF was able to scan a uranium glass sample to investigate its elemental composition. Clear links were found between historic recipes that were used to produce the glass and the major elements found in the micro-XRF scans. Additionally, imperfections made during production were also able to be detected.
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