Ancient Chinese bronze vessels were cast in piece molds from the Shang period (c. 1500 BCE) until at least the middle of the Eastern Zhou period (6th to 5th century BCE). 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 joins to the solid pieces from the first casting. Similar interfaces are found at solid metal mold spaces or chaplets used in the casting process. 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. Modeling and microstructural analysis support TMA results that broad variations in composition still allow for successful castings.