The possibility of using diatomite from the Ilyinsky deposit as unshaped thermal insulation for open risers of castings from aluminum alloys of different alloying systems, e.g., technical aluminum A5, AK7, AMg10, and VAL10, has been studied. It was found that an exchange reaction occurred with the formation of a silicon transition layer if the continuity of the oxide film on the surface of a diatomite particle was broken. Since silicon is wetted by aluminum, the diatomite particle sank in the melt, forming a blockage. The Pilling(Bedworth factor of the alloy components was proposed as a criterion for the continuity of the oxide film. A high level of the Pilling(Bedworth factor, even at a low content of the component (~1%), was shown to lead to the formation of blockages.