PURPOSE: To achieve a higher analysis sensitivity by orienting an expanded graphite, thermally decomposed carbon or graphite in a layer on the surface contacting an assay sample of a crucible for atomic absorption analysis mainly composed of expanded graphite to prevent the permeation of the assay sample into pores of the graphite.

CONSTITUTION: A powder of natural graphite, kish graphite or the like is oxidized to form an intergraphite compound. Then, interlamellar compounds are removed by washing and undergoes a thermal treatment or the like in an oxidizing atmosphere to obtain an expanded graphite, which is used as intact or to which a binder such as tar pitch is added and kneaded as mixture. Then, it is crushed and undergoes an extrusion molding to form a cylindrical material in which the expanded graphite is oriented in a layer with no square pore on the surface thereof. Then, a horizontal cylinder crucible is cut out thereof. When the expanded graphite undergoes a compression molding, vertical pores are liable to be left on the surface at the right angle to the direction of compression and hence, a hydrocarbon gas, halogenated hydrocarbon gas or the like is passed through it in a kiln of an inert gas atmosphere to settle the thermally decomposed carbon or the thermally decomposed graphite in a layer on the surface thereof thereby eliminating vertical pores.