Analyses of iron in blooms
Metallurgical word for smelting is reduction. It is step by step reduction of iron oxides to metal iron in the carbon monoxide atmosphere (CO). At the same time slag is formed which is secondary product of different metalic and non metalic oxides. The third process is carburation of iron. All this happens below melting point of iron, in ancient smelting process it never became liquid. Carbon comes from fuel, charcoal and has big affinity to iron. Produced iron is therefore solution of iron with small quantity of carbon
Bloom is product of ancient smelting process; it is heterogeneous composition of iron and slag. It is created under the air inlets of the furnace at about 1200 °C. Small particles of reduced iron gather inside bath of liquid slag on the layer of charcoal and there become solid after cooling.
We recognized that ancient smelters were able to make slag with the lowest melting point which is necessary for the ancient process. The further question is how they made homogeneous bloom free of slag and also how they got different carbon content in iron.
The structure of iron under microscope shows quality of iron, indirectly also quantity. Chemical analysis gives us the exact quantity of carbon.
On the top picture there is bloom from smelting in 2001. It stands as it was formed in furnace. Most of its iron was inside liquid slag that protected iron from carbon penetration. Sides of bloom were out and in reductive atmosphere and were highly carbonized. Metallografic structures and chemical analysis of both parts of bloom prove that.
Mild iron was the majority of production in Iron Age. This quality of iron has ferrite and ferrite-pearlite structure. Only a small part of iron was made with higher carbon (pearlite structure). That was also the analysis of A.v. Morlot, the first researcher of the ancient Bohinj.
High carbon was probably reached at the last phase of smelting with tapping of slag and prolongation of heating. The slag found in smelting places in Bohinj proves that almost half of the found slag is light and porous without FeO. The other part is heavy smelting slag with high percentage of FeO and SiO2.
We have to mention also ability of ancient smelters to concentrate iron in bloom. When the slag is dropping out of the bloom it becomes porous and has to be hammered together, welded.
On photos there is the second bloom found at Dunaj in Bohinj and its side cut. The bloom shows high level of homogeny and has also high carbon content up to 1 %. There are no signs of hammering.
Both blooms from Dunaj in Bohinj show a method of refining process that can be described as »final shaping« ( Peter Crew).
We must say again that old smelters in Bohinj had incredible skills and controlled process of smelting as it was known in the ancient Alpine region called Norik. The Romans said : » Durior et ferro quod Noricus excoquid ignis«.
Ancient ironworkers got such a quality of iron at the final smithing as it is produced nowadays: low corbon steel and high carbon steel. Nowadays production of steel is two steps process; in blast furnace and in smelting shop at high level of energy.
Analyses of slag ..... read more
Experimental smelting ..... read more
cut of bloom, smelting 2001
ferrite structure of iron, incr. 100 x, carbon 0,13 %, bloom, smelting 2001
pearlite structure, incr.100 x, on sides of bloom, smelting 2001, carbon 0,8 %
ancient bloom, 3 kg, from Dunaj at Jereka with side cut for samples for metallurgical analysis
structure of bloom, Dunaj at Jereka, 3kg. Basic structure is pearlite with microhardness 261 HV 0,1 and ferrite in needle shape with hardness 191 HV 0,1. There are also graphite lamellas. The creation of graphite is possible by decay of cementite (Fe3C) or carbonized at heating in reductive atmosphere (Filip Marinšek).