Abstract The phenomenon that a pyrometric cone does not bend over at its own soften-ing temperature is called "freezing". Such phenomenon has been studied for thecones of middle temperature range (SK7-16, or ПК123-146). It is shown that the freezing of a pyrometric cone is due to the formationof certain refractory crystals from molten phases in the cone when the furnacetemperature is either unsteady or kept for a rather long period just balow its sof-tening point. In the cones SK7-10, the refractory crystals formed are of chry-santhemum shape and found petrographically to be isometric cristobalite In thecones SK11-16, apart from crystals found in SK7-10, many needle-like crystalsare formed, X-ray examination shows that they are possibly mullite. If the sodium oxide content is high relative to total alkali in cone materialSK7-10, or potassium oxide is high in SK11-16, may cause the cones to freeze.Thus in order to prepare a series of non-freezing cones the potash felspar shouldbe used as raw material for SK7-10, and soda felspar for SK11-16. It wasattempted to reduce their freezing tendency by substitution CaO by MgO intro-duced as magnesite, but the result was very unsatisfactory.
