In the approximately 2000 years that iron was produced in Denmark only the bloomery method was in use. The steps in the iron production has been followed analytically from bog iron ore, through reduction slags and planoconvex slags to slag inclusions in finished iron objects. For comparison a few analyses on blast furnace slags have been presented. For a better semiquantitative evaluation the parameter G has been introduced. G is defined as the sum of the oxides to calcium, magnesium, aluminium and potassium, divided by the sum of the oxides of iron, phosphorous, manganese and barium. G increases from low values in reduction slags to high values in slag inclusions in iron and steel. In addition, it is shown that slags from the Danish environment are rich in phosphorous, but poor in aluminium relative to similar slags from Scandinavia. Slags from ancient iron production have not received much attention, because they are difficult to understand. They are usually classified according to shape and size, and there are many chemical analyses, but they are rarely seen in a larger perspective. In Denmark iron production was, as long as it lasted, entirely based upon bog iron ore. All iron was produced in bloomer processes, and large quantities were made from about 200 B.C. to about 1600 A.D. No blast furnace and consequently no fining or puddling process have to the best of our knowledge ever been used in Denmark. This means that slags excavated in Denmark with certainty may be referred to some stage in the bloomer process. In the present paper it is proposed that any slags may be tied to a specific production step by virtue of its shape, size and chemical composition. It is further examined whether it is possible to characterize slags from different geological provinces on basis of their main and trace elements. If successful the analysis of slag inclusions in iron might then assist in identifying the production site of an artifact of unknown origin, a problem of some importance to the historian and the archaeologist.