There are various types of salt in Japan.
Among them, table salt, new cooking salt, cooking salt, and refined salt are smooth because sodium chloride is 99% or more and magnesium carbonate to prevent condensation is added.
It is good to use for finished cooked products, but magnesium carbonate is difficult to dissolve in water, so it becomes turbid when used for soup.
When you go to a restaurant, you may find roasted rice in a salt bottle. The reason for this is that the heated starch, which is a component of roasted rice, has the property of absorbing water, so it is devised to improve hygroscopicity and prevent it from hardening.
Sometimes raw rice is added, but the water absorption is not high. Although the meaning is thin, shaking the container has the effect of breaking the salt that is a little hardened by the raw rice colliding with the salt.
From here on, it’s subjective.
As a method of making salt smooth, there is a method called empty roasting.
It is said that by converting salt into roasted salt, magnesium chloride (nigari), which has high hygroscopicity, becomes magnesium oxide and loses hygroscopicity.