The optical microscope, scanning electron microscope, X-ray diffraction and petrographic analysis can be used to determine the types and distribution of minerals in slag. The minerals present in the slag are good indicators of the gas atmosphere in the furnace, the cooling rate of the slag and the homogeneity of the slag. The type of ore and flux used in the smelting process can be determined if there are elements of un-decomposed charge or even metal pills trapped in the slag.
Slag minerals are classified as silicates, oxides and sulfides. Bachmann classified the main silicates in slag according to the ratio between metal oxides and silica.
:::::Ratio MeO : SiO silicate examples
:::::::2 : 1 fayalite
:::::::2 : 1 monticellite
:::::::1.5 : 1 melilite
:::::::1 : 1 pyroxene
Fayalite (FeSiO) is the most common mineral found in ancient slag. By studying the shape of the fayalite, the cooling rates of the slag can be roughly estimated.
Fayalite reacts with oxygen to form magnetite:
:3FeSiO + O= 2FeO·FeO + 3SiO
Therefore, the gas atmosphere in the furnace can be calculated from the ratio of magnetite to fayalite in the slag.
The presence of metal sulfides suggests that a sulfidic ore has been used. Metal sulfides survive the oxidizing stage before smelting and therefore may also indicate a multi-stage smelting process.
When fayalite is replete with CaO, monticellite and pyroxene form. They are an indicator of a high calcium content in the ore.