The study of rare objects requires the use of mobile non-invasive methods such as a portable X-ray fluorescence instrument (pXRF), but this involves an analysis from the outer surface, while the depth analyzed depends on the element measured and, in addition, the material can be very heterogeneous at different scales. The concept of elemental composition, therefore, has no "absolute" meaning for painted enamel decorations. This work evaluates this concept by comparing the pXRF measurements made with different configuration procedures, allowing to evaluate the consequences on the variability of the XRF signals, and discusses the contents of certain chemical elements. For this, two shards from the Qianlong period are analyzed, a shard of blue and white (underglazed) porcelain and a fragment of an 'imperial' bowl with painted enamel decoration (huafalang). The variability of measurements is compared for visually appearing homogeneous or heterogeneous areas.