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The saint in the window

Yet another myth of glass

Since July, an image on a small window (figure 1) has been mobilizing thousands of people in Ferraz de Vasconcelos, on the outskirts of São Paulo. It is a face that is reminiscent of the Virgin Mary, the mother of Jesus.  After countless television reports, similar effigies started to be noticed in homes from several cities in the interior of the state of São Paulo and in other states.

Scientists have issued their opinions in television and radio reports on the supposed miracle, some of them reasonable, but due to the limited time, these explanations were not properly noticed by the lay public and by the academic community.  To clear up the mystery, we shall demonstrate that these images are the result of natural phenomena (corrosion and iridescence), known for decades both by materials scientists and by the manufacturers of sheets of glass.

The glass in the window
The first glasses made by man date from roughly 4,000 B.C.  Since those days, the most common compositions, which includes glasses for bottles and for windows, contain mainly silicon (SiO2), which is the component that makes up glass’s structural network; its main source is sand.  The other components are sodium oxides (Na2O), potassium (K2O), calcium (CaO), magnesium (MgO), aluminum (Al2O3), minority elements and impurities.  It is odd to note that these are most abundant elements on this planet, and hence the extensive use of these glasses, known by the generic name of soda-lime silicon.

Table 1 shows the typical compositions of modern and medieval window glasses.  While the current compositions are practically uniform, the older compositions used to vary substantially (Muller et al. 1994).  Medieval glasses have a much higher level of such impurities as iron oxide (Fe2O3), manganese (MnO) and phosphorus (P2O5), and they are rich in potassium, while contemporary glass windows have a higher sodium content.  We shall show further on that these last two elements (alkalis) are responsible for corrosion and the formation of multicolored rounded images on the surface of the glass.

Aqueous corrosion
When glass comes into contact with an aqueous solution, chemical and structural changes take place on it surface (figure 2).  Moreover, in the course of the reaction, the formation and the accumulation of the products of corrosion set off changes in its chemical composition and increase the solution’s pH.  It is important to emphasize that glass does not resist high pHs, which leads to dissolving the silicon network that forms its structure.  Since the 50s, dozens of researchers (for example, Clark et al, 1979) have demonstrated experimentally that the reactions between water and glass can be divided into two stages:

First stage
The primary attack is a process that involves exchanges between ions of sodium (Na+) and potassium (K+) from the glass, and hydrogen ions from the solution; and the other constituents of the glass are not altered.  Reaction (a) in Figure 2 tends to predominate during the first stage.  During this stage, the rate of extraction of alkalis (Na, K) from the glass is slow, and it decreases approximately with the square root of the time.  An increase in the effective surface area of the glass in contact with the corrosive solution is to be observed during these reactions, due to the increase in rugosity.  This increase in the surface area is related to the leaching (selective dissolving) of the alkaline ions from the glass, leaving a silicon rich layer contained hydrated micropores.  The extent of the leached surface area depends on the original composition of the glass.  In materials containing lower levels of silicon, this increase in the surface area is more significant, so that leaching therefore increases with the level of alkalis.

Second stage
The second stage of the attack is a process where the breakdown of the main connections Si-O-Si) occurs, causing the structure of the glass to dissolve.  Reaction (b) in Figure 2 is the predominant mechanism in the second stage.

In glasses of the soda-lime silicon kind, the motive power for the process of diffusion of the Na+ ion from the glass to the aqueous solution in reaction (a) is the difference between its concentration in the glass and in the surface water.  However, to keep electrical neutrality, hydrogen (H+) ions from the water have to spread from the solution to the glass, and to occupy the spaces left by the sodium ions that have migrated to the solution.  As a consequence, this ion exchange leads to an increase of pH in the aqueous solution.

Intemperism
The degradation of the surface of the glass, due to the interaction with the humidity in the atmosphere, is called intemperism.  Intemperism is classified under two kinds, static and dynamic.  Dynamic intemperism can be caused by one of the processes whose description follows:
i) Condensing-leaching, where humidity is deposited on the surface of the glass, until a natural process of leaching takes place, washing away the products of the reaction.  There is therefore no accumulation of reaction products (Figure 3b).
ii) Adsorbing-condensing-evaporating, where a fine layer of “mist” is formed on the surface, but it evaporates before drops of water are formed.  In this case, there is an accumulation of reaction products (Figure 3d).

Static intemperism can be caused by raindrops getting trapped between two sheets of glass, as described in Figure 3c.

Type (i) dynamic intemperism is a kind of aqueous corrosion, as the surface of the glass is continuously restocked with the solution, at a specific rate.  During the period for which the drop remains on the surface, the de-alkalization (impoverishment in Na+ and K+) of the glass takes place, and, simultaneously, there is an increase in the pH of the water.  If the S/V ratio between the exposed surface area of the glass (S) and the volume of the solution (V) is high in the raindrop (> 1 cm-1), the increase in pH causes the glass surface in contact with the drops to dissolves completely.  The localized deterioration and the resulting rugosity on the surface of the glass lead to the solution to become trapped during leaching.  The degradation of the surface continues, where the solution has been deposited in the corroded pores.  This corrosion manifests itself by the formation of small craters (pitting) and peeling (spalling) of some regions of the surface [RJS1] .  This may lead to stains visible to the naked eye.

Type (ii) dynamic intemperism is characterized by the presence of reaction products on the surface of the glass.  This kind of intemperism, produced under cyclical alterations of temperatures or humidity, is the principal cause for concern amongst glass manufacturers.  To start with, the attack can be observed by a slight misting up of the surface of the glass, followed by the formation of a visible film that varies in thickness in accordance with the composition of the glass and the severity of the attack.  Various authors have studied these films extensively, using transmission electron microscopy and X-ray diffraction, and they have shown that they are rich in sodium.  Furthermore, films rich in Na react with the majority of acid gases, such as CO2 and SO2 (present in polluted environments), to form the respective salts: Na2CO3 and Na2SO4.  As CO2 is present in significant quantities in the atmosphere, Na2CO3 ought to be a product of reaction in the majority of glasses that suffer from this kind of exposure to the elements.  The presence of Na2CO3 was confirmed by X-ray diffraction analyses on the surface of commercial glasses that have been exposed to the elements.  If chloride or sulfur gases are present in the environment, the salts from these reactive species may also be formed on the surface of the glass.  The layer formed by type (ii) intemperism can act as a protective barrier to retard future corrosions and so to limit its own thickness.  This film, however, can peel spontaneously (scumming), or be removed by chemical solutions or abrasion.  Be that as it may, the remaining surface will show explicit signs of corrosion, such as rugosity.

The various kinds of aqueous corrosion or intemperism previously discussed are illustrated schematically in Figure 3, for the typical conditions of exposure.

Static aqueous corrosion occurs, for example, on the inside of glass containers holding a liquid solution (Figure 3a).  If the S/V ratio is small, the increase in the solution’s pH will be slow, and the first stage of Figure 2 (leaching of alkalis) will be the main mechanism for corrosion.

During dynamic aqueous corrosion (such as the corrosion caused by washing windows by jets of water), the surface is continuously restocked with corrosive solution (Figure 3b).  The same happens with windows exposed to rain, characterizing dynamic intemperism.  The pH of the raindrops is not therefore significantly altered, in spite of the S/V ratio being high.  Once again, the main mechanism for corrosion is described by reaction (a) in Figure 2.

Static intemperism, where the corrosive solution is not renewed, is probably the most severe kind of corrosion.  Drops of water may remain stuck in the cracks between sheets of glass piled up and stored in places subject to humidity (Figure 3c). If the sheets are piled up with a small distance between them, the result may be a high S/V ratio.  Consequently, the pH of the retained water increases rapidly, and this is accompanied by localized dissolving of the glass.

The most frequently found condition of intemperism is shown in Figure 3d.  The glass reacts with the humidity and with gases present in the atmosphere, resulting in the formation of salt precipitates on the surface of the glass.  The conditions needed for drops of water to form and wash away these precipitates are not present in this case.

The resistance of the glass to chemical attack depends on the medium to which the glass is exposed.  The results that are obtained from a particular kind of environment should not be extrapolated to other conditions of storage.

The window saint – the explanation
In the light of what has been set out above, our hypothesis is that the images in Ferraz de Vasconcelos and other places result from a chemical attack arising from one or more of the mechanisms described above.  Since Mr. Antônio (the owner of the house in Ferraz de Vasconcelos) apparently got the pane of glass from a shop selling building materials from demolitions, a probable cause of the corrosion could be the prolonged storage of this pane next to another, in accordance with the arrangement in Figure 3c. An alternative could be the storage at the factory itself or with a distributor causing the same effect.  This would also explain the rounded shape of the image, reminiscent of a saint.  A certain volume of water trapped between two sheets of glass during storage can generate that particular look.

To reduce the wet area to the minimum, the surface tension always produces rounded geometric forms.  Drops of water resting freely on sheets of glass have a perfectly circular section, though when pressed by another sheet and partly spread, the perfect circularity is broken, but the rounded shapes are maintained.  Tests carried out in the Vitreous Materials Laboratory of the Federal University of São Carlos (LaMaV/UFSCar) have demonstrated that an unlimited number of geometric figures (including some that are very similar to saints) can be obtained by squeezing a drop of water between two glass slides.

The image, then, already existed when the glass was installed, but it was noted recently.  The perception of these figures really is subtle.  For example, in the Materials Engineering Department at UFSCar, there have been, for years, several panes of glass with geometrically rounded images, similar to the one in Ferraz de Vasconcelos (but not so similar to a saint).  Very few, however, were the lecturers and students who had noticed them!

Iridescence
It still remains to explain the multicolored “aura” that surrounds the image and reinforces its similarity with a sacred apparition.  Science can also explain this phenomenon, known as iridescence.  Iridescence is defined as a rainbow effect, the color of which changes in accordance with the angle of observation or lighting.  Iridescence is common in old glass that has been subject to intemperism (Table 1 shows that they are rich in alkalis), and it is caused by optic interference – light reflected from several layers of products from corrosion, the thickness of which is in the order of magnitude of the wave length of visible light (400 – 700 nm).

Some artists know about this effect and enhance it.  They use acids to set off a controlled corrosion, or they apply vapors of SnCl2 or PbCl2 – heating it up later in a reducing atmosphere, poor in oxygen – to precipitate thin layers of metal that cause the desired rainbow effect in glass for decoration.

For the incredulous, we suggest scraping off a few micrometers (10-6 m) from the surface of a pane of glass (where the images are), with abrasive sandpaper, and then to polish it with diamond paste or cerium oxide, to recover the flatness and the shine.  Any specialized optician can offer this service.  The image will certainly disappear, demonstrating that it is a question of surface corrosion.

Conclusion
To sum up, glasses of the soda-lime silicon kind, when subject to the elements, suffer chemical attacks, which commonly produce rounded geometrical figures – reminiscent of sacred images – which become visible due to the resulting iridescence.  Little by little, then, science reveals the enigmas of nature, and, in this case, teaches us that “Our Lady of the Window Pane” is not an other world phenomenon.  It is just the fruit of chance; a fine example offered by nature itself – it is nature, rather, that is perfect, a veritable miracle!

My thanks to Ana C. M. Rodrigues, Oscar Peitl, Christian Ravagnani, Marcio L. F. Nascimento, Miguel O. Prado, Eduardo B. Ferreira, Ana C. G. Curro and Deonísio da Silva da UFSCar for their valuable suggestions.

References
W. Muller, M. Torge e K. Adam, “Ratio of CaO/K2O > 2 as an evidence of a special rheinish type of mediaeval stained glass”.  Glastech. Ber.- Glass Science & Technology, 67, n.2, 45-48 (1994).
D. E. Clark, C. G. Pantano e L.L. Hench – Corrosion of Glass, Magazines for Industry, Inc. New York (1979).

Edgar Dutra Zanotto is a Ph.D. in glass technology, the incumbent professor of materials engineering at the Federal University of São Carlos (UFSCar), and assistant coordinator of FAPESP’s Scientific Directorate.

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