Welders are exposed to high levels of manganese (Mn) and iron (Fe). Since Mn is neurotoxic, being able to quantify brain Mn deposition by MRI is of high interest. This phantom calibration study is a first step towards understanding the combined effect of Mn and Fe on R1. Our results show that the presence of serum in the solution requires a Mn-Fe interaction term to improve the fit, likely due to the competing binding to the protein. The presence of Fe reduces the R1 effect of pure Mn, which would lead to an underestimation of brain Mn if Fe was neglected.