Glass disease, also referred to as sick glass or glass illness, is a degradation process of glass that can result in weeping, crizzling, spalling, cracking and fragmentation. Glass disease is caused by an inherent instability in the chemical composition of the original glass formula. Properties of a particular glass will vary with the type and proportions of silica, alkalis and alkaline earths in its composition. Once damage has occurred it is irreversible, but decay processes can be slowed by climate control to regulate surrounding temperature, humidity, and air flow.
Chemical composition and decay Glass disease is caused by an inherent fault in the chemical composition of the original glass formula. Glass contains three types of components: network formers establish basic structure, network stabilizers make glass strong and water resistant, and network modifiers, also known as flux, lower the melting point at which the glass can be formed. Common formulations of glass may include silica (SiO2) as a former, alkali oxides such as soda (Na2O) or potash (K2O) for flux, and alkaline earth oxides such as lime (CaO) for stabilizing. Structurally, silicate glasses are composed of a network of SiO4-tetrahedrons. In addition to the network former silicon which establishes its principal structure, glass contains network modifying agents such as the alkali ions Na+ and K+ and the alkali earth ions Ca2+ and Mg2+. Glass does not have a defined stoichiometry, rather the network is flexible. It can incorporate other ions, depending upon factors such as the main composition and firing conditions of the glass. This causes almost all glass to be chemically unstable to some extent, but maintain their physical and chemical properties over usable timescales due to a phenomenon known as metastability. Electron charge differences of ions within the structure form the basis of its bonding. Both viscosity and transition temperature are related to the availability of oxygen bonds in the glass's composition. Modifying agents tend to lower the melting point of the silica. Higher contents of SiO2 increase acidity of the glass. Higher contents of CaO, Na2O, and K2O increase basicity. The chemical stability of glass decreases when only Na2O and K2O are added as flux, because bonding becomes weaker. The chemical stability of glass can be increased by adding CaO, MgO, ZnO, and Al2O3. To be stable, glass composition must balance temperature lowering agents with stabilizing agents. Exposure of a glass surface to moisture, either in solution or from humidity in the atmosphere, causes chemical reactions to occur on and below the surface of the glass. The exchange of alkali metal ions (from within the glass) and hydrogen ions (from outside) can cause chemical and structural changes to the glass. When alkali metal cations in the near-surface layer are replaced by smaller hydrogen ions, structural differences between the affected surface layer and the unaffected lower layers of glass cause increasing tensile stress, which in turn can cause cracking. The likelihood of degradation due to glass disease depends on the amount and proportion of alkaline compounds mixed with silica, and on surrounding conditions. Inadequate calcium oxide causes the alkalis in the glass to remain water-soluble at a low level of humidity. Exposure to higher levels of relative humidity during storage or display causes alkali to hydrate and leach out of the glass. Repeated changes in humidity can be particularly damaging. Any glass object can deteriorate if it is exposed to unsuitable environmental conditions. Crystal, historic glass, or treasured family items should never be exposed to the high temperatures and water pressure of a dishwasher.
Stages of deterioration The processes involved in glass disease can reduce the transparency of the glass or even threaten the integrity of the structure. Glass disease causes a complex disintegration of the glass which can be identified through a variety of symptoms, including weeping, crizzling, spalling, cracking and fragmentation.
Example The following description of glass beads from an object in the collection of the British Museum, effectively illustrates the range of symptoms that can occur with glass disease:
"Two factors indicated that the deterioration was the result of the phenomenon commonly referred to as ‘glass disease’; first, damage was limited to beads of one particular colour (pale yellow) and second, visible signs of all the various stages of glass disease were present on these beads. This included the presence of small white crystals on the surface of most pale yellow beads and a fine network of uniform cracks or ‘crizzling’ crossing the surface of 55 of the 69 beads. This crizzling appeared to be more prevalent around the bead holes. A total of 32 beads had areas of spalling, or advanced crizzling, where cracks had extended further into the glass structure... Many had areas that had already spalled and the fragments lost. Vertical cracks extending through the glass were present on 37 beads and four beads had become detached due to complete fragmentation."
Stage One The initial stage of glass disease occurs when moisture causes alkali to be leached out of the glass. This becomes apparent when hygroscopic alkali deposits on the glass give it a cloudy or hazy appearance. This may occur within as little as five to 10 years of the glass's manufacturing. The glass may feel slippery or slimy and tiny droplets, or weeping, may be seen in high humidity (above 55%). The hydrated alkali can form fine crystals on the surface of the glass in low relative humidity (below 40%). At this stage, it may be possible to gently wash the glass and remove the surface alkali. This will help to stabilize the glass by reducing the surface pH, and by removing dust, soiling, and hygroscopic components that attract further moisture.
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