Laminated altered layers in historical glass: Density variations of silica nanoparticle random packings as explanation for the observed lamellae

• Describe the internal structure of degraded glass containing lamellae.
• Describe transformed glass as random packings of nanoparticles.
• Explain contrasts between lamellae by a difference in packing density.
• Explain several properties of transformed glass using the random packing model.

One of the most striking but unexplained phenomena in the natural degradation of glass is the transformation of an almost colorless, transparent and homogeneous glass into a colored, opaque and heterogeneous degradation layer. In many cases, the degradation layer consists of numerous lamellae with a thickness between 0.1 and 10 μm. However, both internal structure and formation process of laminated degradation layers remain unclear. In this paper, a model is proposed where we assume that transformed (degraded) glass consists of a random packing of nano-sized silica particles while the lamellae are the result of different packing densities. The model is able to connect the texture of numerous lamellae observed by several types of microscopic techniques with the structure at molecular level determined by means of chemical analysis. In addition, the model is able to explain numerous properties such as the parameters responsible for the contrast between lamellae. This contrast can be caused by differences in color, density, elemental composition, or surface roughness.