A mode of treatment of iron castings that are allowed to remain in storage, or to stand out in the open, for a more or less extended period, e.g., 6 months, to effect a reduction in the residual stresses and consequently in the degree of distortion during subsequent machining. A very similar result can often be obtained by a comparatively short period, e.g., 30 min, of tumbling. Since stress relieving by heat treatment is a more certain process, and seasoning involves much delay and the use of considerable space for storage, stress relieving is more usually employed.