Ancient Chinese bronze vessels from the Shang to Zhou dynasties (c. 1500 - 221 BCE) were cast using piece molds. Several large vessel shapes, especially those with legs, have been found to be made in several steps with the vessel body cast onto previously-made legs or other appendages. In this situation the molten metal contacts and must attach to the solid pieces from the first casting. Similar interfaces are found at solid metal mold spacers or chaplets used in the casting process. Previous researchers showed that the casting can exhibit concentration gradients and evidence of slip in the regions around these interfaces. In this work, we studied the thermal expansion of the bronze at cast-on joints in ancient Chinese bronzes using thermomechanical analysis (TMA). The results are compared with those from reference lead-tin bronze alloys.