Special refractories for use at high temperature

The choice of the most suitable refractory for the laboratory melting of pan metals at temperatures from 1,700° up to 2,200° C. without detrimental con- tamination of the metal by the refractory depends upon a Dumber of factors such as the stability of the refractory at the higher temperatures, the m< available for forming and firing of crucibles, the type of melting furnace, chemical action of the particular metal in question, etc.Methods are described in detail for the preparation of crucibles and other shapes from the oxides of thorium, magnesium, zirconium, and beryllium.Thorium oxide, with a melting point above 3,000° C, is the most refractory of the four oxides named.The method used for fusing this oxide is described.Crucibles made of fused thorium oxide have been used in the determinations of the melting points of platinum and rhodium.Crucibles of the commercial fused magnesium oxide arc not satisfactory fox work with metals of the highest purity, but crucibles prepared from the so-called "C.P." or reagent grade of powdered magnesium oxide have been used at tem- peratures up to 2,000° C. without detrimental effect upon the metal melted.The commercial electrically fused or sintered zirconium oxide contains small amounts of silica.Crucibles of this material have been used at temperatures up to 2,000° C, but unless the melting is done in air, there is danger of contamina- tion of the metal by the silicon in the refractory.From a few experiments made with a small amount of the very pure oxide there are indications that, if available commercially, beryllium oxide might be a satisfactory refractory for use at high temperatures.