Density of inorganic pigments
Similar to hardness, the density of inorganic pigments is determined by their crystal structure. The denser the structural units of a pigment crystal are adjacent to each other, the greater the density. The density of different pigments can vary greatly. Lead black lead is one of the "heaviest" inorganic pigments. Its density is about 8600 kg/m3. The "lightest" is considered to be azure. Its density is about 1850 - 1920 kg/m3.
Pigment density can be found out by two methods: volumometric and pycnometric. The volumetric method is the following: when the pigment is introduced into a closed system, the volume of the gas is measured (it decreases). The pycnometric method consists in measuring the volume of liquid (white spirit, kerosene, etc.), which is displaced from the system by the introduced pigment.
Measuring the density of inorganic pigment is necessary in order to calculate the mass of the future paint coating. The bulk density is used to calculate how much pigment takes up space (necessary for calculating shipping containers and storage tanks). Bulk density shows the mass of a unit volume of pigment when shaken or free-flowing. There is another indicator, bulk volume. It shows the volume of a unit mass of pigment when shaken or free-flowing. Bulk volume and bulk density depend on the dispersion of particles and their shape. More often than not, the physical density of a pigment is greater than its bulk density. For example, the bulk density of lead crowns is 700 - 1000 kg/m3 and the physical density is 5100 - 6100 kg/m3.
Hardness of inorganic pigments
The hardness of pigments has an effect on their distribution in the dispersion medium (film-forming agent) and on wet and dry milling conditions. If the pigment has a higher hardness, the above-mentioned operations require more energy, complicating the technological process. In addition, some parts of the equipment wear out much faster, so they need to be replaced frequently. In some cases, the hardness of the pigment can have a significant impact on the physical and mechanical properties of the paint coating. For example, iron oxide pigments impart abrasive properties to a paint coating.
The hardness of inorganic pigments is determined by the packing density of the component units of the crystal. If they are close together, the hardness of the pigment is higher. In the series of sulfides HgS, CdS, ZnS the hardness increases as the size of the cation decreases and the density of the crystal packing increases accordingly. The hardness of different modifications of the same substance can vary greatly.
The hardness of inorganic pigments is evaluated on the Mohs scale (10 points). Diamond has ten points and talc has one. On the Mohs scale, the hardness of rutile titanium dioxide is 6.5.