The paper considers the issues of determining undissolved CaO and MgO particles in slags formed in a ladlefurnace unit. The assessment of slags by the presence and quantity of undissolved oxides CaO and MgO, depending on chemical composition, was carried out using a polymer model developed at UrFU and improved at IMeT UrB RAS. To determine the saturation of a multicomponent melt by CaO and MgO oxides, it is necessary to compare two parameters: thermodynamic activity of oxide in the melt, which depends on chemical composition, and saturation activity, which depends on temperature. The authors propose a method for estimating the content of undissolved particles in the slags formed at the steel ladle treatment at JSC VMZ. Most slags contain undissolved lime in an amount less than 10 %, which is sufficient for successful steel desulfurization. Theoretical calculations for determination of undissolved particles were confirmed in laboratory conditions during studies of industrial slags with a Stengelmeyer viscometer. Laboratory experiments showed the accuracy of the calculated method for determining the solid phase in the slags formed in ladlefurnace unit by comparing the viscosity changes with a decrease in the slags temperature. Solidification temperature of heterogeneous slag is 200 °C higher than that of homogeneous one. When temperature of heterogeneous slag decreased, enlarged agglomerates of solid oxides were formed, which fell under the measuring device, respectively, it showed an increased viscosity of the oxide system in the temperature range of 1570 – 590 °C. Laboratory experiments confirm the methodology for determining the solid phase in the slag.