Shrinkages apprear as a Dark Paches or Streaks in Radiograph but am unable to visualize as to how the cross section would appear. Are there Photographs of the cross section that shows the Morphology of Shrinkages?
Mainly spherical voids, often have a high concentration of "quench" or even butterfly valve tramp materials.  Think soap bubbles.
Sterl is correct for describing "gas porosity", a common type of defect found in iron castings.Shrinkage porosity can also develop in areas (thin sections, tight corners) where the alloy solidifies between the feed gate of the casting and an unsolidified area of the casting.  The molten liquid cools and shrinks, and a void forms because more liquid metal can't reach the solidifying zone to to upstream blockage.  A section through the wall of such a porous area typically shows  a ragged, "torn" appearing gap between the cooled and solidified walls or "skin" of the casting.

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