During archaeological excavations in Iceland, samples of slag and iron were selected for advanced metallurgic analyses to increase the understanding and knowledge of iron production and metal work in Iceland during Viking Age and medieval time. Samples from four sites have been investigated so far. The Viking age and early medieval house Hofstaðir in Garðabæ outside Reykjavik, the medival turf house at Keldur in Ragnarvöllum south Iceland and early medieval harbour Kolkuós and churchyard in Keldudalur in Skagafjördur, north Iceland. Sometimes churches were used as a smithy after abandonment. The samples from Hofstaðir and medival turf house at Keldur are connected to a smithy process from which there are traces in the form of microscopic hammer scale. An interesting result, from both, Hofstaðir, Keldur and the early medieval churchyard in Keldudalur, was the presence of bone or calcium and phosphor in the samples. In the case of the medieval settlement at Keldur, bone was probably added in the production to make the iron harder for the progress in the iron making process. But in the bone material found in samples from the other sites, Hofstaðir and Keldudalur, it is clear that bone most probably was used as fuel in the process of iron production. In that time the forest had decreased and wood got rare in Iceland because of the expansion of the colonisation. In the northern part of the country there are resources in drift wood coming with sea currents, but this is not the case on southern Iceland to the same extent Therefore, there is a lack of wood for charcoal production. Tests have shown that equal parts of wood and bone as fuel makes almost the same effect in fire.