Diffusion processes in typical metals are slow at room temperature but there are many applications for which very long-tern use in envisaged and stability needs to be assured over a timescale of 10,000 years, where even slow processes can be important. It is common to perform accelerated tests at higher temperatures and extrapolate the necessary information from the measurements so obtained. We have tested the validity of this type of extrapolation for room-temperature, grain boundary diffusion in the copper-silver system, by measuring low-temperature diffusion profiles in antique samples of Sheffield plate.