Describe the grain structure of a metal ingot that was produced by slow-cooling the metal in a stationary open mold.

In the scenario of a metal ingot cooling slowly, the microstructure tends to be coarse. The surface, exposed to higher temperatures for extended periods during cooling, features smaller grain sizes as they have less time to form. However, as we delve deeper into the ingot, the grains gradually extend, leading to equiaxed grain formation at the center.