The mechanism of corrosive action of molten glass upon pyrophyllitic crucibles has been studied with a polarization microscope and by means of X-ray analysis. Specimens made of three kinds of materials of crucible i.e. clayey material, kaolinitic Roseki*, and diaspore, were fired at 1450°C for one hour. They were dipped into two kinds of molten glasses of the compositions, Na₂O⋅CaO⋅3SiO₂ and Na₂O⋅PbO⋅3SiO₂, at temperature 1200°C and 1400° for 0** and 3hrs. The corroded surface of the cooled specimens was microstructurelly tested. While the temperature range in which the reaction between glass and specimens took place, was determined by the thermal analyses on the mixture of the powdered specimen and the glass. The conclusions obtained from these experiments were as follows:
1) The kaolinitic Roseki, containing the small amounts of fine particles of diaspore, was observed to have changed on firing into the minute aggregates of mullite, high cristobalite and other matrix parts. When specimens with such aggregate texture were immersed in molten glasses, a soda component of the glasses permeated through fine cracks or pores into specimens and thus, the surface of the specimens expanded about 10μ, forming a glassy “exudation layer” of the reaction product. The longer the immersion time, the thicker became the exudation layer to 30μ or more by a process of dissolution of the surface parts into the layer, until the carnegieite crystals were formed with small amounts of nepheline. The formation of the crystal was very distinct where the exudation layers well developed. High cristobalite leached out from the body was found in the nearest zone to the body, the two kinds of crystals above mentioned in the middle, and locally, some corrundum was formed outside of the layer where the glass was rich in alumina. The exudation layer diminished gradually when the immersion temperature was raised up to 1400°C, and the glass invaded into the body resulting in the formation of a white corroded layer.
2) Where diaspore pseudomorphs of the body came into contact with the molten glass at 1200°C, minute corundum crystals occuring in such parts was gradually migrated into the glass and then arranged themselves in parallel with each other, perpendicularr to the surface. On raising the temperatuae up to 1400°C, the corundum layers reacted with glass, changed into that of the nepheline of thickness about 100-500μ. After the corundum layer had reacted with glass on heating at 1400°C for a long time, soda component of glass diffused to the body through the layer and vitrified the body at the surface. Then the layer disappeared, being mechanically cleaved or chemically dissolved leaving the white vitrified surface.
3) The thermal analysis on the mixture of powdered glass and specimens showed two different thermal effect in the range of 1050°-1200°C, one exothermic in the case of kaolinitic Roseki, the other endothermic in the case of diaspore pseudomorph. The former represented the crystal formation in the exudation layer, while the latter the reaction between the corundum layer and molten glass.
4) In clayey specimens, the exudation layer was not distinctly found, because the arrangement of mullite crystals forming on their surfaces was such that materials leached out could not retain on the surface but diffused easily. Thus the white corroded layer was appeared readily without the formation of exudation layer.
5) After completing the formation of white corroded layer, the corrosion proceeded on stationarily, the layer advancing into the body by dissolving materials on the front surface and by diffusing in the glass on the rear.
* Roseki is a kind of rock of hydrothermal origin, chiefly composed of pyrophyllite with small amounts of kaolin and diaspore. It is widely distributed in southeastern Japan and commonly

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Fusion cast type blocks which are usually used for the flux block of glass-melting tank furnaces are not always free from internal cavities which seriously affect the furnace compaign as well as the quality of the glass melted.
The measurement of the bulk density, the inspection of the appearance of the gate scare, and the examination by the supersonic waves were not satisfactory for the purpose avoiding the above mentioned risks.
The radiographical method for the inspection using the radioisotope Co⁶⁰ was quite successful. The experiments were carried out for Corhart Standard Blocks and mono-flux M Blocks of 6″ and 12″ thick, using 400 mc of Co⁶⁰. The results were compared with those obtained by X-rays.

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