Many older interpretations of wrought iron quality are suspect because its slag-containing nature renders it sensitive, in strength tests, to specimen size and shape. Reviews three aspects of its microstructure that are important: 1) form and distribution of carbon (eg., as pearlite); 2) partitioning of phosphorus between slag and ferrite, and its distribution within the ferrite; 3) amount, size, and distribution of slag inclusions. This third characteristic accounts for the variety of skills or failures seen. An important factor in the supersession of wrought iron by steel was the much easier quality control obtainable for steel, especially in large-scale operations.