Naturwissenschaften und Technologie in der Kunst; Akademie der bildenden Künste Wien; Schillerplatz 3; 1010 Wien
Research > Projects > Archive Print

Qualitative and Quantitative Evaluation of the Weathering Attack of
Air Pollutants to Potash-Lime-Silica Glass

In the late 1980s a large scale field exposure program has been started by the Working Group on Effects on Materials Including Historic and Cultural Monuments of the Economic Commission for Europe within the United Nations (UN/ECE). The aim of this project is to perform qualitative and quantitative evaluations of the effects of various air pollutants as well as climatic parameters on the atmospheric corrosion (weathering) on important materials like zinc, aluminium, copper, steel, bronze, stone, painting materials and glass. More than 15 countries, among them France, Germany, Great Britain, Italy and Austria are taking part in this International Cooperative Programme. The Institute of Science and Technology in Art (Academy of Fine Arts / Vienna) is responsible for the preparation and evaluation of glass having a composition similar to medieval stained glasses. In 2001 a succeeding project (“MULTI-ASSESS”, Contract No. EVK-4-CT-2001-00044) was initiated by the European Union. Within MULTI-ASSESS exposures were performed mainly in urban and industrial atmospheres as well as under artificial conditions in the laboratory. Its aim is the assessment of threshold levels for each of the materials mentioned above.

Structure of Glass

Glasses produced in the Middle Ages show a relative low amount of SiO2 (network former) and high concentrations of network modifiers such as Na, K and Ca in comparison to modern glass. A further characteristic of medieval glass is the preferred use of potassium instead of sodium compounds as a flux. The reason for that might be a lack of sodium-containing raw materials (soda) or the difficult procedure of their production: the evaporation from lakes produces sodium-rich salts of varying composition, but 7000 parts of water had to be evaporated to yield one part of salt. However, medieval glassmakers began to use K-rich plant ash as a source of alkali producing glass, which is highly susceptible for aqueous and atmospheric corrosion as the alarming condition of many medieval glass windows in churches and cathedrals, show.

[SiO4] – tetrahedron in crystalline (a) and glassy form (b) Typical network-structure of a soda-glass in 2 dimensions

The Weathering Process

Although the process of glass weathering is a very complicated matter due to many influencing factors such as the composition and surface structure of the glass, the nature and concentration of the air pollutants etc., its principle can be explained in a few steps:

1. Formation of a water film & ion exchange

The first step of the weathering process consists of the formation of a water film on the surface of the glass. Even on vertically positioned glass this film can reach a thickness of up to 50 µm without forming drops. Due to an ion exchange reaction between H+/H3O+ from this moisture film and network modifier cations like K+, Na+ and Ca2+ a so-called leached layer is formed with enrichments of hydrogen bearing species and depletions of alkaline and alkaline earth ions. This leached layer shows a thickness of a few nm to hundreds of µm depending on the type of glass, the corrosion medium and several other physical and chemical parameters such as the temperature. Therefore, the pH in the water film increases enabling the destruction of the glass network (Equ. 2).

2. Absorption of acidifying gases

The next step in the weathering process is the absorption of air pollutant gases like O3, NO2 and SO2 in the water film. As a consequence, the pH decreases favouring the ion exchange reaction in Equ. 1.

3. Formation of weathering products

After the evaporation of the water film, crystalline weathering products such as CaSO4·2H2O (gypsum), K2SO4 (arcanite) and K2SO4·CaSO4·H2O (syngenite) remain on the surface of the glass specimens.

The Exposure and Investigationof the Glass Samples

The exposure within the UN/ECE and MULTI-ASSESS projects was carried out in two modes: sheltered in a box and unsheltered enabling the investigation of the effects of a direct contact of the glass samples with the precipitation as well.

Sheltered exposure Unsheltered Exposure

After exposure periods from 6 to 72 months, the weathered glass specimens were investigated in the scanning electron microscope in combination with energy dispersive X-ray microanalysis (SEM/EDX).

Results

Identification of Weathering Products

The objective of this study was a qualitative investigation of the weathering products formed on the glass samples. It turned out that mainly sulphates such as syngenite (K2SO4·CaSO4·H2O), gypsum (CaSO4·2H2O) and arcanite (K2SO4) were formed on the sample surfaces during natural weathering, but also Si-containing weathering products and organic crystals could be detected.

Secondary electron (SE) image of the surface of a glass sample exposed under unsheltered conditions (area approximately 80 x 60 µm2, magnification: 1000x) showing the cracked glass surface but no weathering products.

Secondary electron image of a glass sample exposed under sheltered conditions for 4 years at test site 26 (Aspvreten/Sweden), showing weathering products mainly consisting of CaSO4, K2SO4 and organic material
(magnification: 600 x)

Statistical Evaluation

As the monthly environmental and climatic data (such as the concentration of acidifying gases SO2, NO2 and O3, the temperature T, the relative humidity RH and precipitation data) were measured at all test sites, a multiple linear regression (MLR) model could be set up. The objective of this statistical investigation was to find correlations between these environmental parameters serving as independent or explanatory variables (xi) in the regression model and the degree of weathering (dependent or explained variable y), which was measured in terms of surface coverage with weathering products or by measuring the leaching depths of the network modifier elements. The quality of these linear dose-response functions can be assessed by the coefficient of determination (R2) and the significance of the entire model and of each regression coefficient.

Secondary electron image (magnification: 12000´) showing the cross section of a weathered glass sample. The leached layer (middle of the image) shows a thickness of about 1µm. The horizontal line exhibits the position of measurement in terms of a linescan.

The best fit of the measured data can be achieved by the regression curves in the equation 3. d(K) stands for the leaching depth of potassium (K) and can be seen as a measure for the environmental attack on the glass surface due to weathering. The equation contains the relative humidity (RH), the temperature and the concentrations of NO2 and SO2 as significant parameters.

Laboratory Exposure

The laboratory exposure was performed in co-operation with SVUOM Ltd. – Research, development and testing in the field of corrosion protection (www.svuom.cz). More than 60 glass-specimens were exposed in climate chamber for periods between 48 and 500 hours to different levels of SO2 and RH, but the main objective of these investigations was to investigate the influence of particulate matter on the weathering process. The most important results were:

(a) Even after the shortest exposure period (48 hours) in atmospheres with a relative humidity between 50 and 100%, with and without SO2 as well as with and without dust clear weathering effects on the sample surfaces could be detected. Most of the weathering products formed contain sulphur (syngenite (K2SO4·CaSO4·H2O), gypsum (CaSO4·2H2O) and arcanite (K2SO4)).

(b) Relative humidities below about 70% seem to slow down the whole weathering process significantly, which can be observed in a decreased number of weathering products on the glass surface compared to the other samples. Probably this low amount of moisture is not sufficient for the formation of a water film on the glass surface, in which the acidifying gases can be dissolved and the ion exchange process can proceed. Hence reducing the water content in the ambient air (if feasible) is an effective method for protecting glass material from further degradation (Fig. 31).

SE-images of weathered glass surfaces after 100 hours of exposure (no SO2, no dust). The images (a) and (b) show the relatively unaffected surface of a specimen, which was exposed at only 50% RH, while the images (c) and (d) depict the much more corroded surface of a 70% RH-sample. The wire-like features in the latter are inhomogeneities in the sputtered C-layer and are hence no weathering phenomena.

(c) Dust particles turned out to influence the weathering of the glass. Firstly, deliquescent components in the dust seem to attract water from the atmosphere and enhance the formation of a water film on the glass surface, even at low levels of RH. Hence, the presence of particulate matter may “compensate” low levels of humidity.

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