Pressure and density are directly in compressible materials. As the isostatic pressure applied on a material increases, the material density increases as the atoms or molecules of the material are pushed more closely together.
Density is defined as the mass per unit volume. Pressure applied equally on all sides of a material, called isostatic pressure, causes a reduction in volume, consequently leading to an increase in density. Volume reduction only results from the application of isostatic pressure. Applying pressure uniaxially or biaxially causing the material to expand in directions that it is not constrained. The amount of volume reduction resulting from the application of a certain magnitude of isostatic pressure depends on material properties such as compressive bulk modulus and porosity.
Porous materials are more compressible than solid materials because pores are filled with gases, which are more compressible than solids. The bulk modulus is a material parameter that defines the amount of strain in a single dimension that the material undergoes when it is compressed. Materials with high bulk moduli require a large amount of pressure to undergo a small compressive strain, while materials with low moduli are compressed more readily under the application of pressure. Isostatic pressure is applied to metals and ceramics in hot isostatic pressing to compactify the material and eliminate unwanted porosity.