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Raman microprobe spectroscopy and scanning electron microscopy were used to study the initial nucleation and growth of YBCO in thick precursors by the BaF2 ex situ process. For quenched films of 2 mum thickness, the data indicate a low density of c-axis nuclei near the substrate, apparently due to a reduced oxygen concentration deep inside the precursor layer. Significant non c-axis growth was also observed; the majority of this material nucleates away from the substrate. Measurement of the conversion rate by in situ XRD for films in the range 0.2-2 mum suggest a weak thickness dependence.