Empirical value, in its simplest form used for expressing the weight ratio between CaO and SiO2 in metallurgic slags. In practice, the following degree of basicity B is mostly used for assessment of the basicity of slag (also referred to as as slag ratio, Eq. 1):

Eq. 1:

If the basicity value is greater than one, the slag is referred to as basic, with a basicity value smaller than one, it is referred to as acidic. The alumina content (Al2O3) is normally not taken into account.

Due to their weaker FeO and MnO bonds, basic slags can be reduced more easily than acidic slags with stronger bonds. Slags with a higher basicity degree, have a lower content of FeO and MnO with otherwise comparable conditions. The higher the basicity degree of a slag, the more intensive the carburization process. In this process, the slag itself only has an indirect effect since it is not capable of releasing any carbon. In basic slags, the silicon weight loss by oxidation is higher than in acidic slags, since the oxide in the silicon performs an acid reaction, i.e. it easily bonds with the basic components.

The degree of basicity in an acidic cupola furnaceslag is between 0.5 and 0.7, that of a basic slag is between 1.5 and 2, neutral slags have a basicity degree between 0.7 and 1.5.

According to F. Neumann, the composition of a typical cupola furnaceslag for a furnace with acidic lining is:

45 - 55 % SiO2
25 - 45 % CaO 
8 - 15 % Al2O3
1 - 3 % MgO
1 - 4 % MnO
1 - 3 % FeO

Additional references:
Neutral slag