The application and use of magnesium alloys in the transport sector is unbowed. For this, it is necessary dealing with the topic about behavior of alloys with melting equipment, which is the main focus of the described investigations, and it is closely connected and also interesting for the recycling [1][2][3][4][5] of magnesium alloys.Typically, magnesium alloys are molten in steel crucibles because the solubility of iron in pure liquid magnesium and in magnesium alloys with suitable manganese contents is small. [6][7][8] Manganese is added to control the iron content. But especially at high temperatures and with inappropriate manganese content, magnesium and the alloying elements can react with the steel crucible. Changes in the melt temperature lead to sludge formation. It is necessary to remove sludge and dross, which forms by the reaction of the melt surface with air, getting a clean melt. However, this process causes considerable metal loss. Additionally, improper iron contents and inclusions of intermetallic particles affect negatively the quality of castings. Furthermore an inadequate ingot composition reduces the lifetime of the crucibles. Studies investigating the reactions of magnesium alloys AZ91 and AS31 [9] with steel of the crucible confirm the formation of intermetallic layers between steel of the crucible and magnesium alloy melt. The intermetallic phases are assumed to be in equilibrium with the melt. Thus, using this information, the iron content of the melt is determined. The knowledge of equilibrium phases and their behavior makes statements possible about crucible wear and the formation of sludge. The alloying elements aluminum, manganese and silicon, which are responsible for the formation of intermetallic phases, play a crucial role.In literature, information on the precipitation of aluminum-manganese phases has been reported. The Mg-Al-Mn system was thoroughly investigated by Ohno et al. [10] They identified two equilibrium phases precipitating from the metal, which are Al 8 Mn 5 for 9 % aluminum and b-Mn and Al 8 Mn 5 for 3 % aluminum. Their results also make possible to determine the manganese content, which depends on the aluminum content and temperature. These phases are confirmed by Bakke [11] for 660 and 700°C. Additionally, the influence of 0.005 % Fe is specified. This low iron content shifts the manganese solubility to considerable smaller values and changes the equilibrium phases into Al 8 (Mn,Fe) 5 in alloys with high aluminum content and to a-AlMnFe in alloys with a 3 % aluminum content. The solubility of Mn in Mg-Al-Mn is described by Thorvaldsen et. al. [12] as well, valid for 5-11 % Al, < 1.5 % Zn and < 20 ppm Fe. The mutual solubility of iron and manganese in AZ91 and AM50 melts is stated by Hillis et al. [13] Figures 1 and 2. The knowledge of solubility and precipitating intermetallic phases supports the estimation of sludge amount, located in the melt after decrease of temperature. 0 0,01 0,02 0,03 0,04 0,05 0,06 0 0,1 0,2 0,3 0,4 0,5 %M n %Fe A8 680ºC A8 730ºC A8 770ºC ...