A hydrometer is a slim sealed glass tube with a weighted bulb at the bottom to measure the specific gravity of any liquid against the specific gravity of water. Any solid in the liquid will affect its flotation.
Principle operation of the hydrometer is based on [Archimedes’ principle] that a solid suspended in a fluid is buoyed by a force equal to the weight of the fluid displaced by the submerged part of the suspended solid. Thus, the lower the density of the substance, the farther the hydrometer sinks. Thus, it is based on the principle of flotation.
In ceramics slip casting, the hydrometer will gauge the amount of clay to water. Too much water slows down the clay wall thickness formation and less casts as the plaster mold gets over saturated.