In the tundish, purification of steel is given by the use of an oxide powder on the steel surface. This powder perform several important functions, such as thermal insulation, protection against liquid steel surface oxidation and capturing inclusions from the bath. This paper presents an experiment carried out with an acid powder based on thermal power plant ash to the casting of steel with carbon content from 0.30 to 0.33% and 1.00 - 1.15% manganese. Powder behavior during casting can be summarized as follows: rapid spreading of liquid steel surface, good thermal insulation, revealed by the gradient temperature from the slag. The average specific consumption recorded was very low, 0.4 kg/t liquid steel. It has been analyzed the chemical composition of the slag from the tundish during casting, to prelevate 6 consecutive samples taken during casting. The compositions of these slags were compared to the oxide powder composition initially given on the liquid steel. There was a good absorption of inclusions, as evidenced by the last composition of the analyzed slag sample. There was a strong increase of MnO content and changes in values of the other oxide compounds, such as SiO2. The paper highlights the experimental slag capacity, a hard acid slag based on thermal power plant ash, to capture the MnO type inclusions, up to 38%. This process can be interpreted as negative, meaning that the acid slag is absorbed from the steel alloying element, manganese.