Why is an ice slab of cuboidal shape holoow from the sides towards inside.?

Question

Some one asked me this question about refrigeration of water. Ice slabs of dimensions about 120X20X30 are made in ice plants and it is generally seen that they are empty towards inside from the two side surfaces.

Answer 1

Ice made in blocks at ice plants freezes against the sides first as that is where the cooling coils are.

As teh ice freezes the amount of disolved gas relative to the amount of liquid left increases and bubbles start to form.

This creates the pattern you see.