They are known as " piping," " blowholes " and " segregation."
Abundant and larger the blowholes are, the smaller will the pipe be.
This enables it to take up enough silicon from the walls of the crucible to prevent the evolution of gas during solidification, and the consequent formation of blowholes or internal gas bubbles.
Hence in the act of solidifying it expels any excess of gas which it has dissolved while liquid, and this gas becomes entangled in the freezing mass, causing gas bubbles or blowholes, as at A and B in fig.
Because the volume of the pipe represents the excess of the contraction of the inner walls and the molten lake jointly over that of the outer walls, between the time when the lake begins to ebb and the time when even the axial metal is too firm to be drawn further open by this contraction, the space occupied by blowholes must, by compensating for part of this excess, lessen the size of the pipe, so that the more FIG.