A method of determining a chemical composition of a slag portion is provided. The method includes providing the slag portion having a surface, collecting light reflected from the surface using an optical system, obtaining a data set from the collected light, the data set at least defining a matrix containing values representative of an intensity of a part (LM,λ) of the collected light, each part being collected from one of a plurality of points at one of a plurality of wavelengths, the matrix being indexed at least by: a plurality of space coordinates of the plurality of points, and a plurality of spectral parameters representative of the plurality of wavelengths. The method further includes conditioning the matrix in order to obtain a reduced set of values and performing a mathematical algorithm using the reduced set of values in order to obtain the chemical composition. A corresponding installation is also provided.