Staining of Archaeological Glass From Manganese‐rich Environments*

Abstract

It has been suggested that both internal and external sources of manganese may produce the black or brown staining that often occurs in buried archaeological glass. Modern potash glass of a manganese-free medieval composition was scratched, etched and immersed in an aqueous solution containing dissolved manganese. After several weeks, immersion samples were sectioned and examined by SEM-EDX. Manganese was shown to be present in fissures and cracks within in the glass, where alkali had been depleted. Manganese from an external source is capable of blackening buried medieval potash glass.

Browning Phenomenon of Medieval Stained Glass Windows

“…Experimental alterations have been conducted on model samples in order to evaluate the parameters responsible for the appearance of the browning zones. A Mn-free glass has thus been immersed in a Mn-rich solution leading to the formation of black areas 18 thus confirming the possibility of an external source of Mn in the browning process. Microbial effect has also been investigated.…”

THE SECTILIA PANELS OF FARAGOLA (ASCOLI SATRIANO, SOUTHERN ITALY): A MULTI‐ANALYTICAL STUDY OF THE GREEN, MARBLED (GREEN AND YELLOW), BLUE AND BLACKISH GLASS SLABS

“…Nevertheless, it is worth taking these reconstructions with caution; in fact, the technology of these slabs is not straightforward to decipher and the oxidation state of sulphur has not been determined. The external layer shows a well‐known weathering pattern of archaeological glasses due to the leaching of alkaline ions, which involve the ‘release’ of Fe 2+ and Mn 2+ ions, subsequently hydrated and oxidized (Pollard and Heron 1996; Freestone 2001; Watkinson et al. 2005).…”

Morphological and chemical characterization of weathering products on buried Sasanian glass from central Iraq

“…EDS analysis shows that no significant difference emerges among plug cores, plug borders and superficial patinas for the same sample, once data of composition are recast to 100; in fact, all of them mostly consist of silica, with about 2-5 wt% Al 2 O 3 , 1 wt% Na 2 O and K 2 O, and less than 1 wt% CaO and MgO. However, a difference generally emerges between the D100 value observed for lamellar areas (about 12) and for ''spongy" plug cores (about 25), and this accounts for the different luminosity observed in back scattered electron images. This is not the case for sample VA89, where D100 amounts to about 15 for all the observed decay morphologies.…”

“…Manganese compounds may form within the decay products due to the contribution of Mn(II) ions introduced by environmental solutions, as well as leached out of the glass [25]; oxidation of Mn(II) may then cause the precipitation of Mn(III) or Mn(IV) dark compounds within cracks, pits and lamellar layers. Since the occurrence of a homogeneous gray or black crust is always related to detectable levels of manganese in the unaltered glass, it seems reasonable to assume that the contribution of manganese of internal origin plays a major role in determining the dark color of the crusts in visual group 4.…”