Aging of PLS-glass: study of the alteration phenomena
L. de Ferri
1,a, D. Bersani
2,b, Ph. Colomban
3,c, P.P. Lottici
2,d,
G. Simon
3,e, G. Vezzalini
1,f1 Dipartimento di Scienze della Terra, Università di Modena e Reggio Emilia, Largo S.
Eufemia 19, 41100 Modena, Italy
2 Dipartimento di Fisica-Istituto Nazionale di Fisica della Materia, Università di Parma,
Viale G.P. Usberti 7/A (Parco Area delle Scienze) 43124 – Parma, Italy
3 Laboratoire de Dynamique, Interaction et Réactivité (LADIR), UPMC, CNRS, 2 rue Henry
Dunant 94320 Thiais, France
a[email protected], b[email protected], c [email protected] d[email protected], e [email protected] f[email protected]
Keywords: PLS- glass, Raman , FT-IR, glass alteration Abstract
The wood ash-based glass (PLS, potash-lime-silica), employed for medieval cathedral windows in Northern Europe since 1000 A.D., is particularly sensitive to the alteration phenomena.
Three PLS medieval-like glass were produced with different K2O content and
characterized by FT-IR and Raman spectroscopy. Aging experiments were carried out by sulfuric acid and water, checking, in this last case, the pH over time. The weight loss and the alteration layer thickness were measured and the interacting solutions were analyzed through AAS. Raman maps were recorded on the altered glass cross sections. Changes in the glass network structure were observed and the presence of neo-formation products was determined.
Introduction
The most characteristic chemical feature of glass used in medieval cathedral windows of northern Europe is the high content of K2O and CaO [1] due to the use of beech and
fern ashes as flux instead of the previously used soda-based materials, such as the Egyptian Natron. This PLS (Potash-Lime-Silica) glass has lower alteration resistance than soda-lime glass [1] and can be attacked by atmospheric pollutants conveyed by water in liquid or vapor form. A thin water film acts as a trigger for the corrosion process: a former leaching step gives a de-alkalinized layer, enriched in silica and H+ [1]. The further step
can be the dissolution of silica network through an hydrolysis reaction due to the pH increase [2]. After the evaporation of water, alteration products can crystallize on the surface.
In this work an aging study of reproduced PLS glasses, verifying the dependence of the alteration resistance by the alkali content, is presented.
Three glass samples (V1,V2 and V3,) were produced with 12.29, 16.74 and 23.49 K2O
wt %, respectively. Their vibrational properties were investigated by FT-IR ATR (Attenuated Total Reflectance) spectroscopy (FT-IR Vertex70, Bruker) and Raman spectroscopy (488 nm line of an Argon laser), using a Jobin-Yvon HR 800 LabRam
Spectrometer. The Raman background subtraction was performed with linear segments,
then the Gaussian de-convolution of the spectra into different Qn’s vibrational contributions
was made through the Origin® peak-fitting software (method developed at the LADIR
laboratory [3]).
Aging in acid conditions was obtained by dipping small glass pieces (thickness 1-1.5 mm) in concentrated boiling sulfuric acid from one hour to two weeks (for the most stable V1 glass). Water-attack tests were performed using autoclave filled with bi-distilled water: glass samples were left in the autoclaves for two weeks (V1 and V2) and one week (V3) at 300°C and at 80 bars, monitoring the water pH variations [2].
The chemical analysis on the interacting solutions was carried out by using a Fast Sequential 220 Varian Atomic Absorption spectrometer equipped with a SIPS 10 sample introduction system, air/acetylene flame; solutions were diluted 40 times in weight and adequate quantities of CsCl were added for the detection of K.
Results and Discussion
The IR band located at 800 cm-1 in pure silica glass, and usually red shifted for
alkali-glass, in the here synthesized samples is found to move from 774 to 756 cm-1 by
increasing the amount of flux which weakens the Si-O bond. The two components in the broad band at 850-1100 cm-1 - the antisymmetric stretching motions Si-O [4] and the
stretching Si-O-K+ [5] - become more defined by increasing the potassium quantity.
Moreover, the second component is found to move from 874 in V1 to 893 cm-1 in V3, in
agreement with the literature data [5].
Changes in the glass structure connected to the modifiers quantity were also investigated through Raman spectroscopy. The five Qn's components in the stretching
band (750-1250 cm-1), which describe the short range structure of silicate glass [9], are
clearly distinguishable in the reproduced samples: Q1 and Q2 configurations increase with
the K2Ocontent, at the expense of Q3 and Q4 (Fig.1) [6].
Fig.1 – (left): Raman frequencies of the Qn's components. A% is their
relative area with respect to the stretching band total area.
– (right): Weight of the Qn's components as a function of the K 2O
The three samples differ for the CaO/total alkali content (1.47, 1.07 and 0.74 for V1, V2 and V3, respectively); as a consequence a progressive increase in the Q2 units frequency
(mostly originated by alkaline earth ions) was observed as Raman band tends to move toward lower frequencies by increasing the atomic weight of cations [7].
The “polymerization index” (Ip), as defined by Colomban et al (2003) [8], gives
information on the degree of connection of the glassy structure. In literature Ip was calculated for samples differing both in composition and production technology, while the glasses produced for this study differ only in the chemistry. We determined Ip values very
similar (0.3-0.4) in the three samples, hence this parameter seems influenced more by working temperature than by the composition [9].
All the samples were weighted before and after the aging treatments. Data recorded after acid attack for the three glasses indicate that the maximum weight loss is 0.6, 3.6 and 30.8 wt% for V1, V2 and V3, respectively. The weight loss is very small for V1 glass, which appears transparent even after two weeks. A higher weight loss was detected after the water aging: 52.5, 58.4 and 67.0 wt% for V1, V2 and V3, respectively.
The thickness of the depleted layer depends on glass composition [10] and differs in the here reproduced samples: V3 shows low durability and the alteration extends from ~ 121 μm to the entire volume, as a function of the exposure time when aged in sulfuric acid. In V1 glass, on the other hand, the alteration process, even after a two weeks aging, seems limited to rare small areas, ~ 70 μm depth. Also in the water-aged glasses the thickness of the layer grows at the increase of the flux amount: 357, 587 and 523 nm for V1, V2 and V3, respectively.
A strong increase in the water pH value occurs during the alteration tests: it changes from 5.1-5.5 to values higher than 10 for all the samples (Fig. 2).
The release of potassium was measured by AAS: V1 glass is very stable, while a great loss is found for V3, even after only one hour exposure. During acid treatment the K2O loss
for this glass varies from 5 % (one hour) to 23 % (six hours).
Fig. 2 – Time dependence of the water pH increase during the aging experiments.
Raman linear maps were performed on the altered samples sections to check possible changes in the glass network structure and to identify new phases crystallized after the aging process. In agreement with literature for altered stained glass windows [11], we identified in the acid-attacked samples a mixture of gypsum and bassanite, while in the
case of water-attacked glasses, gyrolite, a Ca-phyllosilicate, was identified (Fig. 3).
Fig. 3 – Raman spectra taken at 488 nm on a V3 glass cross section at a 10 μm step, showing the features of the Ca-sulphates at ~1010 cm-1 near
the glass surface.
In all glasses the Ip values found in the altered layers (0.5 – 0.6) are higher than the ones obtained in the unaltered samples. However, this increase is only apparent and may be due to the release of the less bound components Q0 and Q1 with the alkaline and
alkaline-earth metals during the alteration process [9]. Summary
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References
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