Problems with residual and additive elements and their control through specifications

Circumstances responsible for a specification approach to the control of residual elements are reviewed. For deleterious elements, response varies from insistence on 100% use of virgin materials, to setting maximum limits on a large number of residual elements, to specifying the relative quantities of two or more interactive species. For beneficial elements, minima must be set and testing techniques must assure that the desired element is present, not only in the proper amount but also in a state and location where its meliorating effects can be realized. Examples from both the alloy and ceramic fields are discussed.