Barytes is used in two ways—as a pigment and for making colouring matter.
(a) Pigment: Pigments serve mainly to impart or reinforce or extend certain properties of
a paint for suiting specific purposes. Barytes mainly serves to increase the weight of
oil paints so that they can settle on the painted surface better. In addition, whitecoloured
barytes serves to augment the hiding power of white paints. There are two
types of barytes-based white pigments: (i) very high grade natural barytes with high
specific gravity as near to that of pure barytes (4.5) as possible, and (ii) chemically
precipitated pure BaSO4 called blanc fixe (although its specific gravity is less i.e.,
3.36, it is still heavier than many other substances).
Natural barytes pigment is used in both white and coloured paints depending on
its purity—the bright snow-white barytes (high reflectance of blue wave light and
high ‘l’ value) for white paints and off-coloured barytes for coloured ones.
Chemically precipitated BaSO4, on the other hand, is generally very pure and is used
in high-quality white paints. The higher refractive index of barytes (1.64) than that of
linseed oil (1.48) serves well to enable the paint to hide spots. Chemical inertness of
both natural barytes and precipitated BaSO4 is an advantage.
Oil absorption should be optimum. Since oil is the medium, very high oil
absorption will mean that much of the oil will be absorbed by the pigment leaving
less for the main colouring matter, but slight absorption is desirable as otherwise the
paint will not act well. Size of particles is important as only very fine-sized ones with
large surface areas can keep the paint particles in suspension. Volatile matters is
undesirable because, when the paint dries, they may go off leaving holes on the
painted surface. Carbonates (e.g., BaCO3, CaCO3) are generally hard with a tendency
to have an abrasive effect on the painted surface besides contributing to volatile CO2
and hence are considered deleterious.
In 1972, the Bureau of Indian Standards (BIS) has recommended the
specifications of both natural barytes and precipitated BaSO4 as follows.
Barytes 25
i. Natural barytes: 95% (min) BaSO4, 2.24% (max) BaCO3, 0.5% (max) volatile matter,
6-12% oil absorption, 0.5% (max) water-soluble matter and particle size varying
from less than 40 micron (400 mesh) to 53 micron (300 mesh).
ii. Precipitated BaSO4: 97% (min) BaSO4, 0.45% (max) BaCO3, 0.5% (max) volatile
matter, 15-30% oil absorption, 0.5% (max) water-soluble matter and particle size less
than 40 micron (400 mesh).
The higher oil absorption specified for precipitated BaSO4 is because it is of
bright snow-white colour and is mainly used as a white colouring substance which
should be saturated with oil for durability of the paint coating.
Earlier, barytes used to be a popular material for adding weight to paints because
they used to be sold by weight. Nowadays, they are generally sold by volume, and so
adding barytes to paints for the sole purpose of increasing weight is no longer a
common practice unless, of course, other purposes are required to be served as
explained above. It however continues to be used as a weighting agent in some
primers.
(b) Lithopone: Lithopone is a chemically prepared white-coloured mixture of BaSO4 and
ZnS used as an indicator in X-ray photographs. For preparing it, snow-white coloured
barytes is first roasted with carbon (coal or coke) at 13000C to produce BaS, which is
then reacted with ZnSO4 solution to precipitate the mixture of BaSO4 and ZnS or
lithopone.
The particle size of barytes is important because too fine a size will result in dust
loss and too coarse a size will not mix well with the powdered coal/coke at the time
of roasting. Iron oxide is objectionable because it melts at about 1,1000 C, i.e., below
the roasting temperature and inhibits the roasting operation. alumina and silica in the
form of aluminium silicate (e.g., clay) melts below 1,3000C, the roasting temperature
of barytes and inhibits the roasting operation. Besides, iron oxide, alumina and silica
react with barytes to yield water-insoluble compounds and remain mixed with the
BaS reducing its reactivity with ZnSO4, and getting precipitated along with the
lithopone. Further, compounds of iron as well as of manganese, if present, reduces
the efficiency of reduction of BaSO4 because some carbon is used up by them
reducing its availability for actual roasting operation. Iron oxide is also a colouring
substance. Fluorite melts at almost the same temperature i.e., 13600C and becomes a
free-flowing low-viscosity liquid getting mingled with the BaS, and hence is
considered deleterious. Strontium sulphate or SrSO4 (celestite) which is often
associated with barytes because of their isomorphous relationship has similar thermal
properties as barytes and also reacts with carbon at the same temperature i.e., 13000C
to form strontium sulphide, thus eating up a part of the carbon. Hence it is not
desirable. So far as CaO and MgO are concerned, these are highly reactive at the
roasting temperature and readily forms low-melting aluminosilicates which remain in
molten state mixed up with BaS, hindering operation.
The industries generally prefer barytes with snow-white colour and containing
95% (min) BaSO4; 1.0% (max) Fe2O3 and SrSO4 each; as low as possible Al2O3,
SiO2, CaO, MgO and clayey matter; the other criteria are absence of fluorspar and
particle size 0.85-4.0 mm.
