About 900 slags and iron objects from the period 700 B.C. to 1850 A.D. have been examined and analyzed by classical optical microscopy and by energy dispersive analytical methods. Although ancient iron objects are extremely heterogeneous, a definite correlation between the metal phase and its slag inclusions is shown; for example, wustite-rich slags are located in ferrite, whereas glassy slags are located in pearlite. The ratios SiO₂/Al₂O₃, Al₂O₃/CaO, and so forth, of the slag inclusions are shown to be helpful in identifying the production site or provenance of an ancient iron object. These ratios are further shown not to be the same in irons produced by the direct process and in irons produced by fining or puddling. Emphasis is on iron production methods in Sweden, Norway, and Denmark, illuminating Scandinavian iron-making practices through 2500 years. The slag analysis method developed here may, however, be extended to yield information on other European sites.