Absorption spectroscopy is the technique that measures the absorption of radiation as a function of wavelength, due to its
interaction with the material. During a research project funded by our home university, we were able to investigate the
possibilities of this technique to study ancient glasses. One of our main conclusions is that UV-VIS-NIR absorption
spectroscopy is especially suited to characterize colored artifacts in terms of composition and furnace conditions.
Moreover, for naturally colored window glasses, we have shown that this technique allows us to classify fragments based
on differences in iron impurity levels. It is a semi-quantitative analysis tool that can be applied for a first-line analysis of
(large) glass collections. Thanks to the commercial available portable instruments, these measurements can be performed
at relative high speed and this in-situ if necessary.
To illustrate the possibilities of this technique, we describe in this paper two case-studies. In a first test-case we analyze
63 naturally colored window glasses and demonstrate how groups with different iron concentrations can be identified by
calculating the absorption edge position from the measured optical spectrum. In a second case-study 8 modern naturally
colored and 31 intentionally colored Roman glass fragments are the point of focus. For these samples we first estimate
which samples are potentially fabricated under the same furnace conditions. This is done based on the calculated color
values. Finally we identify the type of applied colorants.
Raman spectroscopy is presented as a suitable and fast non-destructive technique to obtain qualitative information about
glass samples of various origins (ancient and modern/industrial glass).
A first application is the broad corpus of archaeological window glass that still needs to be investigated. For many sites,
archaeologists have to deal with large collections of excavated glass samples and a selection of the most appropriate
samples for chemical analysis is necessary. A fast classification can be made based on Raman spectra: different kind of
glasses (Alkali-glass, High Lime-Low Alkali glass (HLLA)) have their own typical Raman signature. Even for glasses
giving strong fluorescence, a classification is possible after a simple treatment of the Raman data.
Raman spectroscopy has also been utilized to identify iron containing glasses. The effect of the iron content in glass
samples is reflected on the topology of the Raman spectra: a strong link between the ratio of the Q2/Q3 vibration units of
the silica tetrahedral structure is seen. Even (semi-) quantitative results can be determined from calibration lines if
matrix effects are taken into account (similar glasses). In amber colored glasses, an extra peak ~415cm-1 in the Raman
spectra indicates the presence of a Fe-S chromophore. Finally, in the fluorescent signals of some yellow and red glasses
two peaks of Zn-Se-Cd-S nanocrystals have been identified.
In this paper we studied the transmittance spectra of a collection of several glass samples taken from a 16th century
stained window of the Church of Our Lady in Bruges, Belgium. We recorded the optical spectra for all the samples in the
region between 350 and 1600 nm. The goal of our research was to reveal information about the composition of the glass
artifacts in a fast and non-destructive way.
Analysis of the optical spectra allowed us in the first place to identify the type of colorants that were used. It was
possible to recognize metal ions, such as Fe2+, Fe3+, Co2+, Mn3+, Cr3+ and Cu2+. Also colors made of metal nanoparticles,
such as silver and copper colloids were successfully identified. The recognition of the coloring agents is of particular
interest in dating the glass pieces. This is because some colorants were only used in certain periods. Green glass colored
by chromium was produced only after the mid 19th century onwards. Our study showed that 3 of the 10 pieces were
colored by this element and they originate as such from a later period. A second conclusion refers to the applied fluxing
agent. By analyzing the spectral position of the first cobalt absorption band, we were able to classify the glass pieces as
potash based (used in medieval times) or soda-based (used in modern times) and therefore to classify them as original or
as restoration material. From the 10 blue colored samples, 7 of them were recognized as original material.
Finally, for the naturally colored parts the analysis of the spectra allowed us to group them based on cobalt impurities.
In this publication optical spectroscopy is considered to be a supplementary technique to study ancient colored glass. It
results from a systematic study of the UV-VIS-NIR transmission spectra of intentionally colored glass fragments from
various archaeological and historical sites and dated from the Roman period to the 21th century AD. The main goal
consists of defining optical sensing parameters for this type of material. The considered colorants are iron, cobalt,
manganese, copper and chromium.
It is proved that many cases exist where optical spectroscopy can be seen as a straightforward, non-destructive, low-cost
and in-situ applicable technique in identifying authentic material or to obtain information about the origin of the material.
Possible sensing parameters are defined as the absence/presence of absorption bands characteristic for a specific coloring
metal oxide and the spectral position of these bands. These parameters could reveal information about the applied
furnace conditions and/or to the composition of the glass matrix. It is shown that the cobalt absorption band situated
around 535 nm for soda rich glasses (Roman and industrial times) is shifted towards 526 nm for potash rich glasses
(medieval and post-medieval times).
Window glass fragments from four Belgian sites were studied and for a set of eighty-five samples the UV-VIS-NIR
transmission spectra were analyzed. This collection contains historical and archaeological finds originating from
religious buildings namely the Basilica of Our Lady of Hanswijk in Mechelen (17th-20thc) and the Church of Our Lady
in Bruges (16th-20thc) as well as from secular buildings as a private house/Antwerp (18th-1948) and the castle of
Middelburg-in-Flanders (1448-17thc). All sites contain material on the hinge point between the medieval and the
industrial tradition. The variation in composition of the analyzed samples can be explained by the use of different
glassmaking recipes, more specifically the use of different raw materials. The composition of window glass differs
essentially in the type of flux, using a potash rich fluxing agent until the post-medieval times and industrial soda from the
19th century onwards. A second difference concerns the iron impurities in the glass. For all fragments a clear
compositional classification could be made based on the iron concentration. These conclusions were based on
archaeological research and drawn after submitting samples to expensive, complex, time-consuming and destructive
chemical analyzing methods.
Our study indicates that similar conclusions could be made applying the proposed optical based methodology for plain
window glass. As a whole, the obtained results make it possible to cluster the fragments for a particular site based on
three different sensing parameters: the UV absorption edge, the color and the presence of characteristic absorption bands.
This information helps in identifying trends to date window glass collections and indicating the use of different raw
materials, production technologies and/or provenance.
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