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Treatise on Materials Science and Technology, Volume Glass IV covers the developments in glass science and technology. The book discusses the use of.
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Please enable JavaScript to access the full features of the site or access our non-JavaScript page. Table I: Overview of the experiments to generate features in leached layers that cannot be explained with the currently accepted theories describing glass corrosion. The increase in acidity was empirically measured with a pH meter Sentron Argus , which was calibrated before each measurement.

After the experiments the surface structure of the samples was observed with a binocular optical microscope with reflected and transmitted light Olympus BX The cross-sections were examined with an optical microscope. In the near-surface zone, the leached layer showed cracks perpendicular to the glass surface. On the surface a blue copper precipitation was present, probably due to a fast but local increase of the pH of the solution Fig.

At the initial pH it was indeed possible to completely dissolve all the metal salts used in the experiments. However, when corrosion proceeded, the pH of the solution increased, especially in cracks and close to the surface. This resulted in the precipitation of compounds of the type Cu Cl,OH 2. It can be concluded that even when shaking the solution, local pH-increases have to be taken into account. Although the precipitation was already visible after one week, a small amount of copper had migrated into the leached layer as can be seen in the X-ray spectra measured after 3 months of immersion.

Remarkable was also the migration of Cl - -ions Fig.

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The incorporation of Cl - -ions has already been mentioned in other publications e. To maintain the electrical neutrality in the glass after incorporation of anions, two possible mechanisms can be considered: 1 a cation migrates together with the anion into the glass; 2 ion exchange takes place with an anion present in the glass. The last mechanism is less plausible as there are normally no mobile anions present in the glass.

As a consequence of the formation of a leached layer, a decrease in mass was expected. Both in experiment 2 and 3, a brown manganese-rich deposition appeared on the glass surface after a few days of immersion. Analogous to the Cu-experiment, this was probably caused by a local increase of the pH.

After 3 months of immersion, the cross-sections of the model glass samples showed irregular leached layers.


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The higher concentration of MnCl 2 in experiment 3 had caused a more explicit pattern of Mn-rich cracks with clear vertical fissures, interconnected by horizontal fissures Fig. With light microscopy the manganese precipitations appeared as dark brown, irregular inclusions in the glass. The glass surface showed a cracked pattern. Remarkable was the formation of a heterogeneous lamellar structure in the leached layer. After 15 days however, no manganese had migrated into the structure, but some chlorine and calcium enrichments were visible.

The enrichments followed the lamellar structure Fig. However, the high amount of too many uncontrollable experimental parameters did not allow for a selection of a possible cause for the formation of the lamellar structure. Nevertheless, it can be supposed that external ions are not necessary for the formation of a lamellar structure. As reported before Aerts , the formation of a lamellar structure is also not dependent on cyclic changes in temperature and humidity. It is probably caused by a reorganisation of the glass structure itself.

The experiment demonstrates that lamellar structures can be generated under laboratory conditions.


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Prolonged exposure to the electron beam has led to additional cracks in the leached layer as can be seen in the X-ray images. A comparison of the sample mass before and after the experiment confirmed this assumption, as the mass had decreased by 1. SEM-images of the cross-section clearly showed an increased amount of lead in the already existing corrosion pit, following a specific structure. Around cracks, the Pb-amount was remarkably larger. In the bulk glass, no significant amount of Pb was measured Fig.

To investigate whether the Pb was already present in the corrosion pits before the artificial corrosion experiment, the spectrum of the pit after artificial corrosion was compared with a reference sample without artificial corrosion. This made clear that the Pb had indeed migrated into the leached layer of the corrosion pit during artificial alteration Fig. Therefore, only experiment 6 has been treated in the present article. The circumstances determine to a significant extent the thickness and value of the leached layer. First of all, the mobility of the external ions is important: only mobile ions can migrate into the glass.

As the mobility of the external ions can be related to the pH of the aqueous environment, this can be indicated as the reason why the migration of external ions was limited in the model glass experiments: the metal ions were quickly deposited on the glass surface and could not migrate into the leached layer anymore. Precipitation reactions thus reduce the mobility.

The exposure time is also a possible parameter: the migration of external ions into the glass is likely to be much slower than the leaching process. However, cracks in the leached layer certainly promote the migration. The primary corrosion processes are strongly coupled to each other and can take place in conditions where the glass is exposed to an aqueous environment.

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Besides, secondary corrosion processes such as the formation of precipitations, also have to be taken into account. Each process will be separately described in the following paragraphs. It should be noted that subtle differences in micro-environment, internal stress or the fraction of channels in contact with the surface can lead to completely different corrosion patterns, even when the glass originates from the same object and when they were buried close to each other. Inside such a channel, ion exchange transforms a channel filled with network modifiers i.

From a macroscopic point of view, this inter-diffusion process causes the formation of a leached layer, poor in alkali and alkaline earth ions. The interface between the leached layer and the bulk glass is sharp and therefore clearly distinguishable, as can be seen in Fig. As the corrosion progresses, the interface moves further into the glass. The rate of water diffusion is mainly determined by the size of the voids present in the glass network. When the voids are large compared to the size of the water molecule, a rapid diffusion is possible.

The smaller the openings in the structure, the slower the water diffusion will be. When the voids are too small for the penetration of water molecules, they can react with the network by breaking Si-O-Si-bonds hydrolysis and thus open the structure Bunker According to Sterpenich and Libourel water diffusion is not restricted to the leached layer, forming a gradual transition between hydrated and non-hydrated glass. In the experiments, it is already suggested that external ions probably do not play a role in the initial formation of a lamellar structure. Besides, it is known that diffusion processes coupled to precipitation reactions tend to create chaotic patterns.

Of late, Scott interrelated heterogeneous morphologies in the corrosion layer of metal artefacts with self-organising systems. He mentioned fractal geometries and layered or banded structures. An example of such patterns are Liesegang bands.