The resources of limonite and hematite existing in the west of Romania they were the object of a metallurgical activity at the beginning of the Middle Ages, with some extent in the 18th and 19th centuries. This led to the construction of ore smelting furnaces in the area and the obtaining of refined iron. One of these furnaces, the Zimbru furnace, is the object of our study. The analysis of the state of conservation of this unique technical monument shows the accelerated pace of degradation in recent years. In order to monument preserve analytical studies are needed regarding the materials from which the furnace is made (mortar, brick, stone) and the establishment of their state of preservation. For this purpose we used microscopy, spectrometry, analytical and petrographic techniques. The results led to the identification of materials and authentication to start the restoration process.