Disclosed herein are superconducting composites, and preliminary products therefor, having a core comprising a superconducting phase, a first casing surrounding the core, and having an inner area abutting the core and having a first magnesium concentration and an outer area having a second magnesium concentration greater than the first magnesium concentration, wherein the second magnesium concentration is, on average, between 5 and 40 atomic percent. Desirably, the superconducting phase comprises a MgB2 phase. This arrangement allows for methods for producing the composites that reduce or eliminate subjecting the superconducting phase to mechanical stresses.