A layer of vesicular blue-green glass occurs in the 12th century–B.C. level of Tel Yin'am. The glass was formed by fusion of plant parts rich in opal phytoliths, and is 75% SiO2 with a high K2O/Na2O ratio. Surviving plant structures (now converted to tridymite pseudomorphs) indicate that the material fused was from a grain of some type, perhaps wheat beards. The glass layer overlies nests of probable rodent droppings, now converted mainly to aragonite by the heat of combustion. We believe the glass to be the product of accidental burning of a layer of straw or chaff from threshing, though there is still a possiblity of a deliberate attempt at glass manufacture.
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Folk et al. (1982) studied this question.