Earthy
pigments varying from dull yellow to red and brown are commonly called ochres
in painting. Ochres are defined as clays
used to make the earth colours. In pigment terminology, the word ‘ochre’ is
predominantly used as a synonym for yellow ochre. Its colour is given by a
presence of different iron oxy-hydroxides and oxides, mainly goethite and
hematite. Yellow ochre is also termed as gold ochre. The brownish colour is due
to the presence of manganese oxide present in the pigment. Generally the soil
color assessment was made according to the “Munsel soil color chart”.
Yellow
ochre is a commercial iron oxy-hydroxide which is represented as FeO (OH)x.H20.
Since it is Fe3+ compound, the formula is better represented as
FeOl+x(OH)l_2x.H20, x=0 to 0.5. Its formula is also written as . For pigment
analysis XRD, IR, Raman and EPR are the useful techniques. The sample of yellow
ochre obtained from Vempalli mandal, Cuddapah district, Andhra Pradesh, India
is used in the present work. In the present investigation, its chemical
analysis, XRD, TEM, EPR and FTIR techniques and non linear optics are used to
know the composition, crystalline phase and site symmetry of Fe3+ in the
mineral. Read more...............
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