Boron nitride ceramics and molten metal container provided with members made of the same ceramics

A container structure means adapted to hold molten metal therein comprising;

• a container body comprising at least one wall and a bottom, which is adapted to receive and hold said molten metal, and which has at least one hole through at least one wall thereof;
• a refractory block replaceably fitted in and through said hole in said wall in a hole-closing condition;
• a passageway through said refractory block adapted to allow communication therethrough, to said molten metal;
• a protective end cap on the side of said refractory block directed toward said metal substantially covering and sealing said passageway from contact with said metal, which cap is closed at the end thereof directed toward said metal, and which cap has a hollow interior communicating with said passageway;
• and
• thermocouple means disposed in said passageway and in the hollow portion of said end cap;
• wherein said protective cap consists essentially of a ceramic material having a high bending strength, which consists essentially of:
• at least 50 percent by weight boron nitride;
• at least 1 percent, but less than 50 percent, by weight aluminum nitride;
• and 1 to 10 weight percent, sufficient to increase the bending strength of said ceramic to at least about 5 kg/mm.sup.2 of yttrium oxide, based on the weight of aluminum nitride;
• said protective cap having been made by sintering at normal pressures, a composition comprising at least 20 weight percent of at least one of amorphous boron nitride and boron.