since the dense and high purity pyrolytic - carbon - coated layer is formed with a thickness of 10 - 100 μm on the surface of the c / c composite , even when metallic impurities of the inside c / c composite are 5 - 100 ppm in ash content , similar effects to those of a known crucible made of the c / c composite whose inside was subjected to high purification can be produced to achieve full applicability to the structural elements of the cz apparatus . also , since the process time required or the c / c composite itself to be subjected to the high purification can be shortened , the effect of reducing the manufacturing costs considerably can be obtained .