XRF Analysis of a 14K-Gold-Filled Disc.
In the case of gold-filled material, the XRF detector will see slightly below the surface, delivering a weighted average of signal from surface and interior (See Heterogeneous Materials). For example, 14K gold is 58.33% by weight, and this sample 14K-gold filled material looks like net 41.46% gold, which is happens to be close to the composition of 10K gold (41.67% gold). In order to make the distinction, a second non-destructive test is required.
For example, in this case a specific gravity test yielded a value of ~9 g/cm3, consistent a material having a higher proportion of base metal than 10K gold. Specific gravity tests are an option for samples that are solid metal with nothing attached and cannot trap air bubbles.