Micrograph of the Month: Melted Silica Bubbles

Melted silica in ashy deposits at Neolithic Boncuklu (Turkey) above, and Kamiltepe (Azerbaijan) below.
Silica is one of the most common components of plant ash, and is often seen in the form of phytoliths. In the case studies I have worked on, phytoliths in ash can have >50% abundance in thin section, and there may be millions of them per gram of sediment. Less common are these features, also composed of silica but with a very different appearance. These are 'bubbles' of melted silica, and they occur when silica is present in conjunction with alkali salts. Heating under these conditions causes the silica to melt and form what has been termed a 'glassy slag' or vesicular glass (e.g. see Canti 2003). The word vesicular refers to the gas bubbles you can see within the larger silica mass (also bubble shaped!). I prefer to call it melted silica rather than use the word 'glass' as it can be confusing in an archaeological context. The two examples here are from Boncuklu (above) and Kamiltepe (below), both occupied well before the appearance of glass production. It is quite remarkable that we get these bubbles - pure silica melts around 1600°C, a much higher temperature than could be achieved in these outdoor bonfires we see in the Neolithic. However, the melting point is lowered in the presence of alkali salts, and small pockets within the fire can to a high enough temperature to cause the silica to melt.

Canti, M. 2003. Aspects of the chemical and microscopic characteristics of plant ashes found in archaeological soils CATENA 30: 339-361.