The quality of casting depends on many factors, and moisture in the mould is reported to be one of the important factors by many investigators.
  Generally speaking, the defects such as scabs or pin holes are likely to generate when the mould contains too much moisture.
  But, most of the past studies have investigated only the relations between the casting defects and moistures of sand on mixing and neglected the change of moisture in mould surface layer after moulding.
  The author noted that the mould conditions just before the pouring differed from that of just after moulding in the previous reports.
  It is rather often that “the steel castings in green sand mould” is made in the mould treated by washing and natural or artificial skin drying in practice.
  It takes an unavoidable time in preparing the mould before pouring. So it is very important to investigate the behavior of moisture in the mould surface layer before pouring to get a sound steel casting in green or skin drying mould.
  In this paper, the behavior of moisture - especially, natural drying mechanisms and diffusion of moisture in the mould surface layer before pouring have been researched.
  The results obtained were as follows :
  (1) Generally, the natural drying mechanism of steel casting mould consists of the 1st and 2nd stages in the decreasing rate of drying according to the drying characteristics of the mould.
  (2) Assume the passing time after moulding and natural drying depth of mould X (hr) and Y (mm), in case of moisture content of moulding 4.2%, atmospheric temperature 30.5°C and humidity 58%,
              Y=X⁄(0.207+0.045X)
  (3) Assume the passing time after moulding and hardness of green mould surface X (mn) and Y, in atmospheric temperature 21∼23°C
        {at humidity 54% X=0.206e^0.064Y
        {at humidity 70% X=0.026e^0.097Y
  (4) Diffusion constant of moisture in the mould is approximately 39.4 X 10⁻⁴ cm²/h at 19°C.

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  The quality of the porosity layery of inner wall which influences upon the reduction of working time, as well as the pinholes which influence on good or bad of the commercial castings are important matters in the production of centrifngal steel of complicated forms.
  Authors has investigated on these subjects for various parts of the rock drills, and particulaly an inspection by X-ray was applied on the pinholes of the centrifugal castings of proper wall thickness.
  The results were obtained as follows,
  (1) The state of porosity layers of inner wall consist of non-metalic inclusions and gases included in the molten steel, and these are apt to concentrate in the thick part of the wall or in its vicinity.
  But the amount and the location of them are different according to the revolutions per minute (or times gravity, G).
  IT is important to decide the inner diameter economically in the centrifugal castings of complicated form.
  (2) The pinholes are apt to appear in the parts of thin wall thickness.
  The caus of is supposed to be due to more rapid rate of solidification than removing speed of H₂ gas to the inner wall.
  Wall thickness which will be able to control more or less pinholes is considered to be 18∼13mm.

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  Examinations were performed in preparation method of sample on the determination of hydrogen in cast iron.
  The following results were obtained :
  It was recognized that there was no influence on the hydrogen content of white and spheroidal graphite iron to prepare samples same as in steel. But water and moisture gave an influence to hydrogen content only in gray cast iron.

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  Skin dried mould, that is composed mainly of silica sand, bentonite and water, is commonly used for medium and large steel castings of about 0.5ton to 10ton weight.
  Skin dried mould is prepared by removing the surface moisture to 50mm depth heating the surface at about 200°C for 2 to 4 hours.
  In this paper, the relations between properties of bentonites and dry strength of sand mixtures were sought in order to determine the influence of bentonite, the main bonding material, on the strength of sand layer of skin dried mould surface.
  The results may be summarized as follows.
  (1) When skin dried moulds of steel castings are being assembled, such characteristics as high strength of mould coupled with preservation of mould edge as well as good adhesion of sand particles against rubbing or shaving are mostly desired in the shop.
  (2) When the bentonite of higher liquid limit value is used, higher dry strength mould can be obtained.
  (3) When the bentonite used is one with higher liquid limit value, better adhesion can be secured of the sand particles.
  (4) Therefore, it is in accordance with the practical demand to employ bentonite of higher liquid limit value in assembling the satisfying skin dried mould for steel castings.
  (5) Sand contained 3.2% or 4.2% moisture, and when moisture content was in this range, it may be concluded that the test results were not influenced by aggravation of sand mould due to crack formation on drying.

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