ABSTRACT Siliceous aggregates are a minor but diagnostically significant component of wood ash, consisting of allochthonous dust or soil particles embedded in a biogenic silica (opal) matrix. Their formation mechanism and precise origin within wood tissues, however, have remained unclear. Using Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy with energy‐dispersive X‐ray spectrometry (SEM/EDS), we show that in the Sycamore Fig ( Ficus sycomorus ), the outer bark epithelial cells silicify their cell walls. Overlying this layer is a consolidated crust composed of diverse mineral particles cemented within an impure biogenic silica matrix. This crust is compositionally and structurally indistinguishable from the siliceous aggregates previously identified in archeological wood ash. We, therefore, conclude that siliceous aggregates originate on the bark surface, and infer that epithelial cells bind adhering environmental dust into a silicified matrix. Because these aggregates are more resistant to dissolution than calcite pseudomorphs, they provide a durable indicator of wood ash in archeological contexts. This information clarifies the biogenic–allochthonous interplay underlying the formation of siliceous aggregates and strengthens their interpretive value in microarchaeological studies.
S. Weiner (Thu,) studied this question.