Inorganic pigments and method for preparing same

There are disclosed new greenish-yellow, yellow and orange-yellow pigments which consist substantially of bismuth vanadate of monoclinic structure, bismuth phosphate of monoclinic structure and aluminum phosphate of orthorhombic structure and which, in the case of the yellow and orange-yellow pigments, also comprise a compound based on Bi.sub.2 O.sub.3 and V.sub.2 O.sub.5 the X-rays diffraction pattern of which shows the following peaks: 1.87, 1.88, 2.41, 2.77, 2.80, 3.11, 3.82 and 7.6 A. The new pigments are characterized by a dominant wave length .lambda..sub.D of from 573 to 586 m.mu., and have a molar ratio of Bi.sub.3 O.sub.3 /P.sub.2 O.sub.5 equal to 1, and a Bi.sub.2 O.sub.3 /V.sub.2 05 molar ratio of from 1.39 to 5.59, inclusive, while their molar ratio Al.sub.2 O.sub.3 /V.sub.2 O.sub.5 lies within specified ranges of values. Also disclosed is a method for preparing the new pigments by calcining, in the presence of air, BiPO.sub.4, Al.sub.2 O.sub.3 and V.sub.2 O.sub.5 or compounds which generate Al.sub.2 O.sub.3 and V.sub.2 O.sub.5 during the calcination.

THE PRIOR ART 
The inorganic yellow pigments which are presently most in use are chrome 
yellows (based on lead chromate), cadmium yellows (based on cadmium 
sulphide) and cadmiopone yellows (based on cadmium sulphide and barium 
sulphate). The most diffused orange-yellows are the chrome orange pigments 
(based on basic lead chromate) and the cadmium orange pigments (based on 
cadmium sulphoselenides). Since they contain lead and hexavalent chrome or 
cadmium, these different pigments are suspect with respect to toxicity. 
Alternative pigments based on nickel titanates TiO.sub.2 --NiO--Sb.sub.2 
O.sub.3 (greenish-yellow) and on chromium titanates TiO.sub.2 --Cr.sub.2 
O.sub.3 --Sb.sub.2 O.sub.3 (orange-yellows) are known. These pigments do 
not have, however, properties comparable to those of the above-cited 
traditional pigments as regards color saturation (color intensity), 
colorimetric purity and tinting strength. 
It has also been proposed to use bismuth vanadate BiVO.sub.4 of monoclinic 
crystalline structure as a substitute pigment. In fact, this product shows 
optical and pigmenting properties similar to those of the chrome yellow 
"primerose". However, its cost is too high for it to be commercially 
feasible due to the high price of the raw materials used for its 
preparation: i.e., vanadium and bismuth compounds. Furthermore, it is not 
endowed with high thermal stability. 
THE PRESENT INVENTION 
On object of this invention is to provide new greenish-yellow, yellow and 
orange-yellow pigments free from chromium, lead and cadmium, and that are 
endowed with optical characteristics similar to those of chrome yellow, 
chrome orange, cadmium yellow, cadmiopone yellow and cadmium orange 
pigments. 
Another object of this invention is to provide new pigments based on 
BiVO.sub.4 that are cheaper than the pigment of only BiVO.sub.4, thanks to 
the presence of other components of lower cost and that are endowed with a 
higher thermal stability in respect of the pigment of only BiVO.sub.4. 
Still another object of this invention is to provide a process for 
obtaining the new pigments. 
These and other objects are achieved by the present invention which 
provides the greenish-yellow, yellow and orange-yellow pigments having a 
dominant length wave comprised between 573 and 586 m.mu., and which 
substantially consist of bismuth vanadate of monoclinic crystal structure, 
bismuth phosphate of monoclinic crystal structure and aluminum phosphate 
of orthorhombic crystal structure and, in the case of yellow and orange 
yellow pigments, also of a compound based on Bi.sub.2 O.sub.3 and V.sub.2 
O.sub.5 the X-rays diffractogram of which shows the following peaks: 1.87; 
1.88; 2.41; 2.77; 2.80; 3.11; 3.82 and 7.6 A. 
In these pigments, the molar ratio Bi.sub.2 O.sub.3 /P.sub.2 O.sub.5 is 
equal to 1, while the molar ratio Bi.sub.2 O.sub.3 /V.sub.2 O.sub.5 is 
comprised between 1.39 and 5.59. The molar ratio Al.sub.2 O.sub.3 /V.sub.2 
O.sub.5 is different for the greenish-yellow and for the yellow and 
orange-yellow products, and it varies according to the molar ratio 
Bi.sub.2 O.sub.3 /V.sub.2 O.sub.5 as follows: 
______________________________________ 
Molar Ratio Molar Ratio 
Al.sub.2 O.sub.3 /V.sub.2 O.sub.5 
Al.sub.2 O.sub.3 /V.sub.2 O.sub.5 
of the of the yellow 
Molar Ratio greenish-yellow 
and orange- 
Bi.sub.2 O.sub.3 /V.sub.2 O.sub.5 
products yellow products 
______________________________________ 
5.59 1- 3.1 &gt;3.1- 5.35 
4.19 1- 2.4 &gt;2.4- 4.0 
3.35 1- 2.1 &gt;2.1- 3.2 
2.79 1- 1.8 &gt;1.8- 2.7 
2.39 1- 1.6 &gt;1.6- 2.3 
2.09 1- 1.5 &gt;1.5- 2.0 
1.86 1- 1.4 &gt;1.4- 1.8 
1.67 1- 1.3 &gt;1.3- 1.6 
1.52 1- 1.2 &gt;1.2- 1.45 
1.39 1- 1.18 &gt;1.18- 1.35 
______________________________________ 
In fact, it has been found that pigments consisting of the first three 
compounds of the tabulation and having the molar ratios between the oxides 
Bi.sub.2 O.sub.3, V.sub.2 O.sub.5, Al.sub.2 O.sub.3 and P.sub.2 O.sub.5 
defined above, have a greenish-yellow color corresponding to a dominant 
wave length .lambda..sub.D comprised between 573 and 575 m.mu. (for the 
definition of .lambda..sub.D, see for instance the treatise by A. G. 
Hardy: "Handbook of Colorimetry", Massachussets Institute of Technology, 
Cambridge, Mass., 1936, in particular page 11). Said compounds have 
excellent optical characteristics, that are equal to those of chrome 
yellow, cadmium yellows and cadmiopone yellows, and superior to those of 
nickel titanates, in particular as far as the color saturation (color 
purity), the colorimetric purity and the tinting strength are concerned. 
We have also found that when the fourth compound is present, the pigments 
have a yellow or orange-yellow color corresponding to a .lambda..sub.D of 
575-586 m.mu.. At the same time, in the presence of the fourth compound, 
the molar ratio Al.sub.2 O.sub.3 /V.sub.2 O.sub.5 is higher with respect 
to that of the greenish-yellow pigments, as appears from the foregoing 
table. 
These new yellow and orange-yellow pigments have optical characteristics 
that are superior to those of the chrome titanates, in particular as to 
the color saturation and colorimetric purity. 
These new greenish-yellow, yellow and orange-yellow pigments having a 
dominant wave length comprised between 573 and 586 m.mu., are obtained, 
according to this invention, starting from a mixture of BiPO.sub.4, 
Al.sub.2 O.sub.3 and V.sub.2 O.sub.5 or aluminum and vanadium compounds 
capable of generating Al.sub.2 O.sub.3 and V.sub.2 O.sub.5 during the 
successive calcining phase, the molar ratios between Bi.sub.2 O.sub.3, 
V.sub.2 O.sub.5, Al.sub.2 O.sub.3 and P.sub.2 O.sub.5 in said mixture 
being in agreement with those previously defined herein. 
The mixture is calcined in the presence of air, at temperatures comprised 
between 700.degree. and 1100.degree. C. when greenish-yellow pigments with 
a molar ratio Bi.sub.2 O.sub.3 /V.sub.2 O.sub.5 greater than or equal to 
1.67 are to be obtained; at temperatures comprised between 880.degree. and 
1100.degree. C. when a greenish-yellow pigment with a molar ratio Bi.sub.2 
O.sub.3 /V.sub.2 O.sub.5 lower than 1.67 is desired and at temperatures 
comprised between 900.degree. and 1100.degree. C. when one wishes to 
obtain a yellow or orange-yellow pigment. At the end of the calcining, the 
product is slowly cooled down and is then wet-ground. 
The chemical reactions which presumably are involved may be schematized in 
the following way: 
______________________________________ 
2 BiPO.sub.4 + Al.sub.2 O.sub.3 .fwdarw. Bi.sub.2 O.sub.3 + 2 
(1)O.sub.4 
Bi.sub.2 O.sub.3 + V.sub.2 O.sub.5 .fwdarw. 2 BiVO.sub.4 
(2) 
2 BiPO.sub.4 + Al.sub.2 O.sub.3 + V.sub.2 O.sub.5 .fwdarw. 2 BiVO.sub.4 + 
2 AlPO.sub.4 (3) 
______________________________________ 
In reaction (1), the bismuth phosphate is always in excess with respect to 
Al.sub.2 O.sub.3 : in this way the presence of bismuth phosphate in the 
end product is ensured. 
As already explained, the greenish-yellow pigments contain three 
crystalline phases, evidenced by the diffractometric analysis under 
X-rays: monoclinic bismuth phosphate (in general present in two forms: one 
of monazite type and one called "high temperature form"), orthorhombic 
aluminum phosphate and monoclinic bismuth vanadate. When the molar ratio 
Al.sub.2 O.sub.3 /V.sub.2 O.sub.5 is equal to 1, the reaction (2) occurs 
in a stoichiometric way inasmuch as one mole of Al.sub.2 O.sub.3 frees in 
reaction (1) one mole of Bi.sub.2 O.sub.3 which reacts with one mole of 
V.sub.2 O.sub.5 in reaction (2). If the molar ratio Al.sub.2 O.sub.3 
/V.sub.2 O.sub.5 is greater than 1, it may be assumed that there either 
remains an excess of Al.sub.2 O.sub.3 in the end product or that Al.sub.2 
O.sub.3 reacts completely according to reaction (1) wherefore there is an 
excess of Bi.sub.2 O.sub.3 in reaction (2). Under X-rays examination, 
there is found no presence either of Al.sub.2 O.sub.3 or of Bi.sub.2 
O.sub.3. However, it cannot be excluded that, in the end product, there 
may be small quantities of Al.sub.2 O.sub.3 or Bi.sub.2 O.sub.3. 
One may also assume that there is formed a solid solution of Bi.sub.2 
O.sub.3 in BiVO.sub.4. 
In the case of yellow and orange-yellow pigments, on the contrary, the 
greater excess of Al.sub.2 O.sub.3 with respect to V.sub.2 O.sub.5, gives 
place to the formation of the fourth crystalline phase mentioned above; as 
could be deduced from the tests carried out on the system Bi.sub.2 O.sub.3 
--V.sub.2 O.sub.5, it is the question of a new compound based on Bi.sub.2 
O.sub.3 and V.sub.2 O.sub.5, whose peaks of diffraction under X-rays 
examination of greatest intensity are the following: 
______________________________________ 
interplanar distance 
in A relative intensity 
______________________________________ 
1.87 very low 
1.88 very low 
2.41 very low 
2.77 low 
2.80 low 
3.11 high 
3.82 very low 
7.6 low 
______________________________________ 
The determination was carried out on a Siemens diffractometer using a 
CuK.alpha. radiation. 
The percentual composition by weight of the pigments in Bi.sub.2 O.sub.3, 
V.sub.2 O.sub.5, Al.sub.2 O.sub.3 and P.sub.2 O.sub.5 in relation to the 
molar ratio Bi.sub.2 O.sub.3 /V.sub.2 O.sub.5 may be calculated on the 
basis of the molar ratios between the various oxides defined previously. 
These percentual compositions are reported, for the greenish-yellow 
pigments in Table I while in Table II they are recorded for the yellow and 
orange-yellow pigments. The usefulness of the two columns on the right end 
of the table are explained infra. 
TABLE I 
__________________________________________________________________________ 
Greenish-yellow Pigments 
% by weight 
% by weight 
Molar Molar % Bi.sub.2 O.sub.3 by 
V.sub.2 O.sub.5 % by 
Al.sub.2 O.sub.3 % by 
P.sub.2 O.sub.5 % by 
of V on 
of Al on 
ratio ratio weight 
weight 
weight weight 
starting 
starting 
Bi.sub.2 O.sub.3 /V.sub.2 O.sub.5 
Al.sub.2 O.sub.3 /V.sub.2 O.sub.5 
on the total 
on the total 
on the total 
on the total 
BiPO.sub.4 
BiPO.sub.4 
__________________________________________________________________________ 
5.59 1-3.1 70.74-66.84 
4.93-4.67 
2.76-8.12 
21.55-20.36 
3 1.59-4.93 
4.19 1-2.4 68.97-65.65 
6.42-6.11 
3.60-8.23 
21.01-20.00 
4 2.12-5.09 
3.35 1-2.1 67.28-64.17 
7.83-7.47 
4.39-8.81 
20.49-19.55 
5 2.65-5.57 
2.79 1-1.8 65.67-63.08 
9.18-8.81 
5.14-8.90 
20.01-19.21 
6 3.18-5.72 
2.39 1-1.6 64.14-61.95 
10.45-10.10 
5.86-9.07 
19.54-18.87 
7 3.71-5.94 
2.09 1-1.5 62.68-60.69 
11.68-11.31 
6.54-9.52 
19.09-18.49 
8 4.24-6.36 
1.86 1-1.4 61.28-59.56 
12.84-12.48 
7.20-9.81 
18.67-18.14 
9 4.77-6.68 
1.67 1-1.3 59.95-58.57 
13.96-13.64 
7.82-9.95 
18.26-17.84 
10 5.30-6.89 
1.52 1-1.2 58.67-57.69 
15.03-14.78 
8.42-9.96 
17.87-17.57 
11 5.83-7.00 
1.39 1-1.18 
57.45-56.52 
16.05-15.80 
9.00-10.47 
17.50-17.22 
12 6.36-7.51 
__________________________________________________________________________ 
TABLE II 
__________________________________________________________________________ 
Yellow and Orange-yellow Pigments 
% by weight 
% by weight 
Molar Molar % Bi.sub.2 O.sub.3 by 
V.sub.2 O.sub.5 % by 
Al.sub.2 O.sub.3 % by 
P.sub.2 O.sub.5 % by 
of V on 
of Al on 
ratio ratio weight weight weight weight starting 
starting 
Bi.sub.2 O.sub.3 /V.sub.2 O.sub.5 
Al.sub.2 O.sub.3 /V.sub.2 O.sub.5 
on the total 
on the total 
on the total 
on the total 
BiPO.sub.4 
BiPO.sub.4 
__________________________________________________________________________ 
5.59 &gt;3.1-5.35 
&lt;66.84-63.12 
&lt;4.67-4.41 
&gt;8.12-13.23 
&lt;20.36-19.23 
3 &gt;4.93-8.50 
4.19 &gt;2.4-4.0 
&lt;65.65-62.21 
&lt;6.11-5.79 
&gt;8.23-13.03 
&lt;20.00-18.95 
4 &gt;5.09-8.50 
3.35 &gt;2.1-3.2 
&lt;64.17-61.32 
&lt;7.47-7.14 
&gt;8.81-12.85 
&lt;19.55-18.68 
5 &gt;5.57-8.50 
2.79 &gt;1.8-2.7 
&lt;63.08-60.46 
&lt;8.81-8.44 
&gt;8.90-12.67 
&lt;19.21-18.41 
6 &gt;5.72-8.50 
2.39 &gt;1.6-2.3 
&lt;61.95-59.62 
&lt;10.10-9.71 
&gt;9.07-12.49 
&lt;18.87-18.16 
7 &gt;5.94-8.50 
2.09 &gt;1.5-2.0 
&lt;60.69-58.80 
&lt;11.31-10.95 
&gt;9.52-12.32 
&lt;18.49-17.91 
8 &gt;6.36-8.50 
1.86 &gt;1.4-1.8 
&lt;59.56-58.01 
&lt;12.48-12.15 
&gt;9.81-12.15 
&lt;18.14-17.67 
9 &gt;6.68-8.50 
1.67 &gt;1.3-1.6 
&lt;58.57-57.23 
&lt;13.64-13.33 
&gt;9.95-11.99 
&lt;17.84-17.43 
10 &lt;6.89-8.50 
1.52 &gt;1.2-1.45 
&lt;57.69-56.48 
&lt;14.78-14.47 
&gt;9.96-11.83 
&lt;17.57-17.20 
11 &gt;7.00-8.50 
1.39 &gt;1.18-1.35 
&lt;56.52-55.75 
&lt;15.80-15.58 
&gt;10.47-11.68 
&lt;17.22-16.98 
12 &gt;7.51-8.50 
__________________________________________________________________________ 
For the greenish-yellow pigments there may be determined the Al.sub.2 
O.sub.3 /V.sub.2 O.sub.5 molar ratios (and consequently also the ponderal 
ratios of the various oxides) that correspond to values of the Bi.sub.2 
O.sub.3 /V.sub.2 O.sub.5 molar ratios not reported in Table I, by 
proceeding in the following manner: while the minimum molar ratio Al.sub.2 
O.sub.3 /V.sub.2 O.sub.5 is always equal to 1, the maximum molar ratio may 
be calculated on the basis of the following empirical equation obtained on 
the basis of the gathered experimental data: 
##EQU1## 
For the yellow and orange-yellow pigments, the molar ratios Al.sub.2 
O.sub.3 /V.sub.2 O.sub.5 that correspond to values of the molar ratio 
Bi.sub.2 O.sub.3 /V.sub.2 O.sub.5, not reported in Table II, are 
determined by proceeding in the following way: the minimum Al.sub.2 
O.sub.3 /V.sub.2 O.sub.5 molar ratio is obviously greater than the maximum 
molar ratio Al.sub.2 O.sub.3 /V.sub.2 O.sub.5 of the greenish-yellow 
pigment having the same value of the molar ratio Bi.sub.2 O.sub.3 /V.sub.2 
O.sub.5. The maximum Al.sub.2 O.sub.3 /V.sub.2 O.sub.5 molar ratio is that 
corresponding to the molar ratio Al.sub.2 O.sub.3 /Bi.sub.2 O.sub.3 
=0.958, whatever the value of molar ratio Bi.sub.2 O.sub.3 /V.sub.2 
O.sub.5 is. Considering equation (1) it will be seen, in fact, that the 
molar ratio Al.sub.2 O.sub.3 /Bi.sub.2 O.sub.3 must be inferior to 1, 
otherwise the end product would not contain BiPO.sub.4. 
As already stated, in all the pigments according to this invention, the 
molar ratio Bi.sub.2 O.sub.3 /V.sub.2 O.sub.5 is comprised between 1.39 
and 5.59. A molar ratio greater than 5.59 gives pigments with an 
insufficient color saturation, while a molar ratio lower than 1.39 gives 
more expensive pigments due to their high content of V.sub.2 O.sub.5 and 
which do not have better chromatic characteristics. The pigments that have 
a molar ratio comprised between 1.39 and 2.79, are presently preferred 
because they are endowed with a high color saturation and with a high 
tinting strength. Particularly presently preferred pigments are those 
having a molar ratio comprised between 1.39 and 1.67. 
In the greenish-yellow pigments, the molar ratio Al.sub.2 O.sub.3 /V.sub.2 
O.sub.5 shall not be lower than 1; molar ratios that are near the unit but 
less than unit (0.8-0.9) do not ensure products having the desired optical 
characteristics, while molar ratios decidedly below unit (i.e.: less than 
0.8) yield brownish-green pigments. Vice versa, the variation of the molar 
ratio Al.sub.2 O.sub.3 /V.sub.2 O.sub.5 within the limits of the invention 
does not introduce appreciable variations in the optical characteristics 
of the greenish-yellow pigments. 
In the case of the yellow and orange-yellow pigments, at equal Bi.sub.2 
O.sub.3 /V.sub.2 O.sub.5 molar ratio and calcining temperature, to 
increasing molar ratios of Al.sub.2 O.sub.3 /V.sub.2 O.sub.5, there 
correspond products that are progressively more orange, that is, to 
products having a growing .lambda..sub.D. 
The proportion of the various crystalline phases in the pigments of this 
invention depend on the molar ratios of the oxides. 
In the greenish-yellow pigments, on reducing the molar ratio Bi.sub.2 
O.sub.3 /V.sub.2 O.sub.5 and on increasing the molar ratio Al.sub.2 
O.sub.3 /V.sub.2 O.sub.5, there are respectively increased the quantities 
of BiVO.sub.4 and AlPO.sub.4 to the detriment of the BiPO.sub.4. 
In the yellow and orange-yellow pigments, on increasing the molar ratio 
Al.sub.2 O.sub.3 /V.sub.2 O.sub.5, the following variations occur: the 
fourth compound based on Bi.sub.2 O.sub.3 and V.sub.2 O.sub.5 becomes more 
abundant, while the quantity of BiVO.sub.4 drops; also the quantity of 
BiPO.sub.4 is reduced while the quantity of AlPO.sub.4 is increased. 
The starting bismuth phosphate may be, for instance, of a monoclinic or 
hexagonal crystal structure. It is preferred to use a product which has 
particles with a particle size comprised between 0.2 and 1 micron. 
The starting vanadium compounds, for instance, may be: V.sub.2 O.sub.5, 
VO.sub.2, V.sub.2 O.sub.4 or NH.sub.4 VO.sub.3, while the aluminum 
compounds may be, for instance, Al.sub.2 O.sub.3, Al(OH).sub.3, Al.sub.2 
(SO.sub.4).sub.3 or Al(NO.sub.3).sub.3. 
The proportion of the three starting compounds is chosen in consideration 
of the characteristics of the pigment to be obtained, keeping in mind the 
molar ratios between Bi.sub.2 O.sub.3, V.sub.2 O.sub.5, Al.sub.2 O.sub.3 
and P.sub.2 O.sub.5 as previously defined. The last two columns of the 
Tables I and II, respectively, report the percentage by weight of vanadium 
with respect to the starting BiPO.sub.4 corresponding to each molar ratio 
Bi.sub.2 O.sub.3 /V.sub.2 O.sub.5, and the percentage by weight of the 
aluminum with respect to the starting BiPO.sub.4 corresponding to the 
molar ratios Al.sub.2 O.sub.3 /V.sub.2 O.sub.5 for each value of the molar 
ratio Bi.sub.2 O.sub.3 /V.sub.2 O.sub.5. 
The starting compounds are preferably wet mixed so as to ensure a good 
homogenization. 
The mixture is subsequently dried, for instance at between 100.degree. and 
130.degree. C., then homogenized, for instance, in a mechanical mortar and 
finally calcined at the temperatures defined above. The use of 
temperatures lower than the minimum values indicated herein results in the 
formation of products with non-homogeneous colors, while operating at 
temperatures exceeding the indicated maximum value (1100.degree. C.) 
results in sintered products. 
In the case of greenish-yellow pigments having a Bi.sub.2 O.sub.3 /V.sub.2 
O.sub.5 molar ratio greater than or equal to 1.67, it is preferred to 
calcine at temperatures comprised between 850.degree. and 1000.degree. C. 
For the pigments having a molar ratio smaller than 1.67 and for the yellow 
and orange-yellow pigments it is preferable to operate at temperatures 
comprised between 900.degree. and 1000.degree. C. In the case of the 
yellow and orange-yellow pigments, in general it is noticed that on 
increasing the calcining temperature there are obtained products with a 
growing .lambda..sub.D. 
The calcining is carried out in the presence of air under static 
conditions, or preferably in a rotary furnace for ensuring a better 
homogenization of the reactants. The duration of the calcining operation 
is generally comprised between 1 and 3 hours. 
At the end of the calcining, the products are allowed to slowly cool down, 
for instance to temperatures comprised between 200.degree. C. and room 
temperature in a time comprised between 3 to 24 hours. Successively, the 
products are then removed from the oven, cooled down, if required, to room 
temperature and then wet-ground. It has been found that the wet-ground 
products show optical characteristics superior to those of corresponding 
products that have been dry-ground. The wet-grinding may be carried out, 
for instance, in ball mills, microsphere mills or sand mills. In general, 
the calcined product is subjected to dry-crushing before being wet-ground. 
At the end of the wet grinding, the product is filtered, washed and dried, 
for instance at 100.degree.-130.degree. C., and is then dry-ground, for 
instance in an automatic mortar. 
The pigments thus obtained consist, in general, of particles of dimensions 
comprised between 0.5 and 3 micron and, on a sieve of 325 mesh, usually 
leave behind a maximum residue of 0.5% by weight. 
Such pigments find their application in such fields in which colored 
inorganic pigments are commonly used, that is, in particular as colored 
pigments for paints and plastic materials. 
Due to the high temperature at which the new pigments are prepared, and 
which impart a high thermic stability to them, they can be used in 
application fields where high working temperatures are required, 
application fields in which the use of pigments of only BiVO.sub.4 or of 
chrome yellows is precluded.