Iron casting was performed in structures described in archaeological literature as slag-pit furnaces. This type of furnaces is known from the territories of Central and Eastern Europe and their spreading was connected with the Germanic peoples and eastern Celtic tribes. A furnace consisted of two main parts, the lower one called a pit and the upper called a shaft. A hollow was a simple hole dug out in the ground; it had 40-45 cm in diameter and not more than 50 cm in depth. Its main function was storing of slags coming from the reduction zone. Directly over the hollow a shaft was constructed which was a part of the furnace over the ground level. In the discussed region it was usually made of regular clay blocks – bricks strengthened with cut straw. Assuming that the driving force for the process was natural air blast, the height of this part of the furnace had to reach about 120 cm. A shaft was mainly used for feeding ore and charcoal to the furnace enabling their slow transfer to the reduction zone. Just over the ground, in the lower part of the shaft there were blast holes which provided air. In the Świętokrzystkie Mountains these were properly prepared ‘blast bricks’ with holes in the shape of a funnel.

Schematic cross-section of a slag-pit furnace of the Świętokrzystki type according to K. Bielenin: 1. Input: iron ore and charcoal. 2. Air blast holes. 3. Reduction zone. 4. Sponge-like slag-pit furnace iron bloom. 5. The surface of a free solidification in the upper part of a slag block filling the hollow dug out in loess. 6. Furnace shaft constructed of shallow blocks – bricks. 7. Ground surface.
In order to view an animated film on the reconstruction of the slag-pit furnace process – click on the picture.

The iron casting process carried out in this type of structures is called in metallurgy a direct reduction process. It differs from the presently used technologies of obtaining iron called an indirect reduction process.  It is worth highlighting that because of relatively low temperatures in slag-pit furnaces usually not exceeding 1250-1300°C iron reduction by means of liquefaction was impossible. Let’s remind that the theoretical point of iron melting is 1537° C. Currently in large furnaces such temperatures are achieved with no effort and iron acquires form of liquid pig-iron which is then processed in oxygen furnaces called convectors to obtain iron or steel of various stages of carbonization. Contemporary iron casting consists of two stages hence its name – indirect reduction.
This process was completely different when slag-pit furnaces were used. Reduction consisted in gradual deoxidization of ferrous oxides contained in ore until metallic Fe was obtained.
Reducer or an agent absorbing oxygen was ferrous oxide coming from the process of burning charcoal. Practically the process was conducted as follows. Through the mouth of a shaft layers of ore and charcoal were alternately fed. Ore transferred though zones of higher and higher temperatures underwent consecutive stages of reduction, that is the ferrous oxides it contained gradually disposed of oxygen finally to get rid of it at the level of blast holes and to turn to a metallic form. Sponge-like iron bloom created from microscopic pieces of reduced iron got stuck to the shaft walls over the blast holes. Metal, however, did not flow to the hollow as it was not yet liquefied. In a slag-pit furnace process only barren rock was melted which together with not fully reduced ferrous oxides created liquid slag.
It was calculated that in order to obtain a block of slags weighing about 100kg about 200kg of ore needed to be melted and 250-300kg of charcoal to be burned. The whole process would have to last almost 24 hrs. Semi-product obtained in such a process were an iron bloom contaminated mainly with slags and charcoal. Only after it was purified and its surface was melted was iron passed to smiths who used it for creating various useful objects.
The presented data are a slightly simplified version of a complex process which dependent on a variety of factors is very difficult to be reliably reconstructed. What we mean by that is the difficulty in establishing relation between ore used in the ancient process, the slags obtained in this process and expected effect of metallurgical activities – that is iron. On the basis of the slag-pit furnace slags it is impossible to define what type of ore was used by ancient metallurgists or to know what qualities the produced iron would have had. The latter we can find only in ready products which went through a number of processing stages. It is even more complicated if a mixture of ore types was used. The  process itself was not the same in every slag-pit furnace thus while conducting research on their remnants preserved in the form of an underground part filled with slags, we can realize the complexity of the whole process.  It is clearly visible in well-preserved slag-pit furnace sites which unfortunately are a scarcity. One of them has recently been discovered within the ‘Wykus’ range between Bodzentyn and Wąchock and still remains a subject of research. The mentioned problems impact experimental work on the reconstruction of the slag-pit furnace process so their long duration should not be surprising. Considering that the process was completely forgotten and only in the 50s of the 20th century its strenuous reconstruction started, one should not wonder why the current effects are still far from the results obtained by ancient metallurgists.