Identification of Synthetic Organic Pigments by Characteristic Colour Reactions
Nicoline Kalsbeek
The purpose of this study was to develop a method for the identification of synthetic organic
pigments on the basis of colour reactions with four different chemical reagents: concentrated
sulphuric acid, concentrated nitric acid, a mixture of concentrated sulphuric acid and concentrated
nitric acid and, finally, a solution of potassium iodate in concentrated sulphuric acid. Colour
reactions for a large number of synthetic organic pigments in the form of pure powdered material
were recorded. It is shown that the colour reactions observed are in overall agreement with the
chemical grouping of pigments for instance, naphthol AS pigments or quinacridone pigments), i.e.,
pigments belonging to the same pigment group yield similar colour reactions. This fact is the basis
for the construction of flow charts for identification. Tests performed on a number of artists' colours
containing pigments in various binding media show that the binding media do not noticeably affect
the colour reactions observed, making the flow charts useful for the identification of actual artists'
pigments. A fifth chemical reagent, namely a saturated solution of potassium hydroxide in 96%
ethanol, was tested and abandoned as a result of its instability, which resulted in severe problems
with reproducibility, especially for certain pigment groups, for example the acetoacetarylide
pigments.
INTRODUCTION
Until the beginning of the twentieth century the use of synthetic organic pigments as artists'
pigments was limited. The common pigments encountered were inorganic pigments (naturally
occurring or synthesized) and organic pigments derived from plants and insects. Development of
early synthetic organic pigments started in the nineteenth century, but it is the twentieth century that
is characterized by an immense expansion in the synthetic organic pigments available. Pigments are
coloured molecules that are insoluble in the applied binding media, in contrast to dyes that are
dissolved therein. Frequently the same molecules can be used in dyes as well as pigments. Soluble
dyes can be converted to insoluble pigments, for example by forming insoluble metal salts (lakes)
from soluble acidic groups present in the dye molecule [1, p. 1]. Synthetic organic pigments can be
assigned to pigment groups based on chemical structure. Accordingly, pigment groups contain pigments of similar chemical structure are expected to react chemically in a similar way. Examples are
diarylide-, quinacridone- and benzimidazolone pigments. Historical and chemical reviews of
synthetic organic pigments are numerous [1-5].
Early pigments (inorganic as well as organic) can be identified by relatively simple and quick
methods or by very complicated, time-consuming but more accurate analytical methods. Analytical
methods that are not too time-consuming include polarizing light microscopy [6], microchemical
tests [6—12], infrared spectroscopy (IR) [13-15] and thin layer chromatography (TLC) [16-19],
The simple identification of modern synthetic organic pigments is often impeded by the
complicated large chemical structures found in these pigments; Chromatographic methods,
however, have proved very suitable. TLC and pyrolysis-gas chromatography (PyGC) [20] have
been used successfully in the identification of synthetic organic pigments. High performance liquid
chromatography (HPLC), a method that is used extensively for the identification of dyes from
various insects [21—23], is an obvious method for the analysis of synthetic organic pigments as
well. HPLC and PyGC are, however, often very time-consuming.
In forensic laboratories a microchemical procedure for the analysis of paints based on colour
reactions with various chemical reagents was applied as early as the 1950s [24]. Castle stated that
three reagents (concentrated sulphuric acid, concentrated nitric acid and saturated potassium
hydroxide in 96% ethanol) were sufficient to distinguish the most important pigments encountered
in paints [8]. About 20 organic pigments in paints were investigated [8, pp 181—182]. Also, very
importantly, Castle established that the binding media in most cases did not influence the colour
reactions observed. De Keijzer utilized an analysis based on the same three reagents on a number of
historically important synthetic organic pigments [9-12]. Schweppe based the assignment of
unknown synthetic organic pigments to specific pigment groups on characteristic colour reactions
with various chemical reagents [7]. The four reagents used in this study are inspired by the work of
Schweppe, and a number of pigment groups not treated in [7] are included. Strauss goes beyond the
assignment of pigments to certain pigment groups [19]. By the use of TLC she can distinguish
individual pigments within specific pigment groups.
The main purpose of the present study is to identify organic synthetic pigments by colour reactions
based on the chemical grouping of pigments. The assumption is that pigments belonging to the
same pigment group behave in a similar way chemically and therefore react with the test reagents
under study to yield similar colours, i.e., a number of characteristic colour reactions could
determine a pigment to belong to a specific pigment group. If the colour reactions obtained are
compiled in flow charts, these can be used to assign unknown pigments to specific pigment groups.
The flow charts will be based on an investigation of a large number of synthetic organic pigments,
and test whether the above hypothesis is confirmed. The effect of binding media is investigated by
testing a number of artists' pigments in different binding media.
EXPERIMENTAL
Pigments
Ninety-one synthetic organic pigments were examined. All pigments were purchased commercially.
No selection among available pigments was performed; as many different pigments that could be
provided were included in the investigation. The pigments were in the form of
pure powders with no binding media and used directly from the jar; no cleaning treatment or
purification procedures were employed. As a control for the influence of binding media on the
colour reactions, 12 different artists' colours in various binding media were tested. Ten of these
contained a single pigment; the remaining two contained a mixture of two different pigments. The
chemical classification of pigment groups and the distribution of the 91 available pigments into
these groups are given along with approximate first occurrence (Table 1) as well as details about the
pigments studied (Table 2). Table 3 contains data about the Winsor & Newton artists' colours
investigated.
Equipment
The microchemical colour reactions were carried out using a Carl Zeiss Jena stereomicroscope with
10x magnification oculars and adjustable magnifications between 0.63 and 4. Two light sources
were employed: a Schott-Mainz KL150B T 1.25 A lamp with an Osram HLX 64634 Xenophot, 15
V, 150 W bulb used in direct connection with the stereomicroscope and the fluorescent tube in the
ceiling, which was a Philips TLD 58W/94.
Chemical reagents
Concentrated (98 w/w%) sulphuric acid (H2SO4), concentrated (65 w/w%) nitric acid (HNO3), a
mixture of concentrated H2SO4 and concentrated HNO3 in the volume ratio 400:15 (called
H2SO4/HNO3) and a solution of potassium iodate (KIO3 ) in concentrated H2SO4 (2 g KIO3 in 300
mL concentrated H2SO4) (called KIO3 in H2SO4) were employed as the four test reagents
throughout the study.
Procedure
One or a few pigment grains were placed on a welled microscope slide and a drop of test reagent
was added. All observations for the four test reagents were made with the Schott-Mainz lamp. Any
colour change of the solution was observed immediately after adding the test reagent. Significant
changes in colour of solution within the first minute were observed as well.
For the pure concentrated acids (H2SO4 and HNO3) additional observations were registered. Any
colour changes of solution and pigment were observed immediately after adding the test reagent
and again after three minutes. The colours observed from the chemical
Table 1 The organic pigments investigated divided into pigment groups in accordance with [1, p.
4-11]. Number of pigments tested in each pigment group, approximate year of first occurrence and
the most important pigments in each pigment group are given [1, 12, 19, 25-28],
The Colour Index (C.I.) [26]pigment names Pigment Yellow (PY), Pigment Orange (PO), Pigment
Red (PR), Pigment Blue (PB), Pigment Violet (PV) and Pigment Green (PG) followed by the
pigment number are used throughout the paper, i.e. Pigment Yellow 1 (Hansa Yellow) is
abbreviated PY1.
reactions in both light sources were described using the Natural Colour System (NCS) [30]. The
gradual changes, if any, in colour over the three-minute period were described. Likewise possible
dissolution of the pigment was observed; the particular mode of dissolution was noted as well as
possible formation of bubbles. When not stated otherwise, the colours that were registered in the
chemical tests were the same in the two illuminants. In several cases metamerism (a phenomenon
where colours are perceived differently dependent on the light source) was observed; when present
both colours were noted. All reactions and changes observed
for pigments and solutions during the three-minute period from reaction with concentrated H2SO4
and concentrated HNO3 are available from the author or on www.kons.dk.
Great care must be taken to use clean glassware. Small impurities such as, for example, slight
remains of water on the surface of the microscope slide may greatly affect the colour reaction. The
violet dioxazine pigment PV23 on reaction with H2SO4 provides a good example. The green colour
obtained on proper performance of the test changes radically to bluish green with violet areas in the
presence of small amounts of water.
Table 2 Pigments tested listed according to C.I. pigment name, C.I. number, pigment group,
supplier and commercial pigment name
Table 2 Continued
Table 2 Continued
Examples of some common names for the pigments investigated are also given (the list is not
exhaustive). Dates in [12, 25] are based on very thorough studies of patents available.
The artists' colours were in the form of pastes in tubes. Each artists' colour was painted on to a sheet
of white paper and allowed to air dry completely for approximately three to 14 days depending on
the artists' colour. Small samples of the dried paint were taken with a scalpel and tested as described
for pure pigment powders. Colour reactions were observed immediately and during the first minute.
RESULTS
The colours resulting immediately from reaction of pigment with the test reagents are given in
Table 4. For some pigments the colour of reaction with the test reagent changes continuously over
the first minute. For colour changes this fast, the change in colour is noted in Table 4, e.g., 'yellow
from violet': in this case, the solution first appears violet and then rapidly changes to
Table 3 The tested artists' colours with pigments in different binding media
All artists' colours are from Winsor & Newton. AOC = Artists' Oil Colours, AAC = Artists' Acrylic
Colours, AWC = Artists' Water Colours and Winton = Winton Oil Colours. The binding medium in
water colours is usually gum arable [29, p. 425]. Several pigments are obtained in more than one
binding medium and two artists' colours contain two pigments.
yellow. For some pigments it is difficult to distinguish whether the first colour observed (violet in
the example) is the colour of the pigment on reaction with the test reagent or indeed the colour of
the solution. The second colour (yellow in the example) is straightforward: the colour found for the
solution. In all cases the final colour is used in the flow charts in Figure 1 (yellow in the example).
In Table 4 and Table 5 these colours are underlined. From Table 4 it is clear that for many pigment
groups, on reaction with the mixed test reagents H2SO4/ HNO3 and KIO3 in H2SO4, more than one
colour of solution is observed. The colour change may be very rapid or it may take up to one
minute. Results for the 12 artists' colours studied are given in Table 6. Special observations for
specific pigment groups are listed below.
Diazopyrazolone pigments
The colour of solution for the pigment PR38 with KIO3 in H2SO4 (Table 4) changes from violet
through blue to the final colour, green. The two other pigments in this pigment group change
directly from pink to green.
Blue phthalocyanine pigments
With HNO3 the solution for all seven pigments is clear and colourless. For PB15:2, PB15:3 and
PB15:4, violet rings around the pigment grains are observed for the first 30—60 seconds. After one
minute the violet colour has
disappeared. The solutions for all pigments in the pigment group turn olive green with
H2SO4/HNO3 and KIO3 in H2SO4, as found for H2SO4, but unlike the olive green solution from
H2SO4 the solutions from the two mixed test reagents change from olive green to yellow.
Diketo-pyrrolo pyrrole pigments
For PO73, PR254 and PR255 an interesting phenomenon is observed from reaction with H2SO4,
namely a total change in colour of solutions when they are observed against dark backgrounds
(black instead of white). The colours listed in Table 4 for PO73 (pink), PR254 (bright carmine red)
and PR255 (bright orange) are observed against a white background; all three colours appear very
bright, almost fluorescent. The corresponding colours observed against a black background are
orange (PO73), yellow (PR254) and bright green (PR255).
Triarylcarbonium pigments
Peculiar leaflike structures are observed when treating PB61 with HNO3. PB61 behaves quite
differently in this test reagent compared to the other two pigments. The pigment turns black and
dissolves vigorously and immediately. The insoluble remains form leaflike structures.
Table 4 Colours of the solutions for all pigments with the four test reagents. Underlined colours
are used in the flow charts in Figure 1. The colour observed immediately after application of the
test reagent is given. If the colour reaction is slightly impeded, the colour observed within the first
minute is given instead.
Table 4 Continued
Table 4 Continued
1
When nothing else is stated the phthalocyanines are Cu-phthalocyanines. ± S indicates if the
crystal polymorph is structurally stabilised. The ß-modification is the naturally most stable and the
other modifications (a and e) are more or less willingly converted to the ß-modification. As a
consequence the ß-modification does not need stabilisation. ± NF indicates if the pigment is altered
towards non-flocculative properties [1, p. 443].
2
A peculiar phenomenon is observed for PO73, PR254 and PR255 on reaction with H2SO4,
namely that the colour of the solution is strongly dependent on the colour of the background against
which it is observed. The colours listed in this table are observed against a white background, all
three colours appear very bright and almost fluorescent. The corresponding colours observed
against a black background are orange (PO73), yellow (PR254) and bright green (PR255).
DISCUSSION
Problems with the saturated solution of potassium hydroxide in ethanol
The first three test reagents chosen for this study were concentrated H2SO4, concentrated HNO3 and
a saturated solution of potassium hydroxide (KOH) in 96% ethanol (EtOH). Severe problems arose
with the solution
consisting of saturated KOH in EtOH. It darkened very significantly within a short time (hours).
This is probably caused by the OH"-catalysed oxidation of ethanol to acetaldehyde (ethanal) and a
further aldol condensation resulting in brown high molecular weight compounds [31, pp 471—472].
Probably because of these reactions, in several cases it appeared impossible to reproduce results
with this test reagent. Using solutions
(a)
Figure 1 Flow charts for all pigment groups or individual pigments that react with concentrated
H2SO4 to yield yellow, greenish yellow (Figure 1a), orange, brownish yellow (Figure 1b), violet,
pink (Figure 1c), bright carmine red, red (Figure Id), colourless (Figure 1e), olive green, green
(Figure 1f) or blue (Figure Ig) solutions. Colour codes from Table 7 are used for the flow charts.
Five colour codes represent pigment groups or individual pigments that are not resolved by the
flow charts; these pigment groups and individual pigments are italicized.
(b)
(c)
Figure 1 Continued
(c) continued
(d)
Figure 1 Continued
(e)
(f)
(g)
Figure 1 Continued
that had been prepared 10-15 minutes earlier, changes in the colour reactions for some pigment
groups were observed. It is important to note that this irreproduci-bility occurred at a time when a
darkening of the test reagent was not yet visible. This is illustrated by the aceto-acetarylide
pigments that yielded a yellow to orange solution with a freshly prepared test reagent of saturated
KOH in EtOH and yielded a strongly dark blue to violet solution with a similar solution that was
prepared about 15 minutes before use. These difficulties resulted in rejection of the test reagent,
saturated KOH in EtOH.
The individual pigment groups
Colours of solution after reaction with the four test reagents for all individual pigments are given in
Table 4. The colours for the solutions are observed immediately after application of the test
reagents. In some cases the colour change is slightly impeded, and colour changes occurring within
the first minute are included.
From Table 4 it is clear that for many of the pigment groups investigated the colour reactions with
the four test reagents for individual pigments in the pigment group are indeed identical or similar.
This is the fundamental basis for the flow charts in Figure 1. If the flow charts were to be based on
individual pigments instead of pigment groups, they would only be valid for the 91 tested pigments.
Table 5 contains results for the individual pigment groups, i.e., colour changes observed for the
pigment group as a whole, and possible exceptions are listed separately. In some pigment groups,
for instance the biacetoacetarylide pigments, the pigments in the group behave so differently that
the pigment group cannot be treated as a group based on colour reactions and they are listed
individually. Some pigment groups, for instance the benzimidazolone pigments, were subdivided
according to their colour reactions. The subdivision of pigment groups is based on the colours of the
solutions observed in this study and not on structural considerations.
Table 5 Colours of the solutions after reaction with the four test reagents (from Table 4) arranged
according to pigment groups. Possible exceptions from the colour reactions found for each pigment
group are listed separately. Underlined colours are used in the flow charts in Figure 1.
Table 6 Results from tests on a number of artists' colours in different binding media. In each case
the results for the pure pigments are listed as well. In ca of artists' colours containing two pigments
the one of the two pigments not described previously in the Table is listed under the column with
pure pigments. Colours in bold differ from the colours observed for the pure pigment with the same
test reagent.
1
For many of the colour reactions where It is stated that the colour is 'orange from violet' or 'yellow
from violet', it may be the pigment that turns violet yielding an orange or yellow solution by
reaction. It can be very hard to tell if one is observing the colour change of pigment or solution.
What in effect will be seen are both the colours violet and orange or yellow respectively.
Schweppe gives a broad account of the qualitative analysis of organic pigments [7]. The pigments
were divided in pigment groups and examined by a wide range of methods, including the resulting
colour of the solutions when pigments were treated with a range of chemical reagents. Among other
chemical reagents, Schweppe used concentrated H2SO4, a mixture of concentrated H2SO4 and
concentrated HNO3, and KIO3 in concentrated H2SO4, while concentrated HNO3 was not applied [7,
p. 16]. Evaporation from alcoholic solutions of KOH and NaOH was also employed.
Schweppe analysed a large number of pigments [7, pp 16-18]. Pigment groups analysed by
Schweppe that are also tested in the present study include the acetoacetarylide pigments, ß-naphthol
pigments, ß-naphthol lake pigments, naphthol AS pigments, BONA lake pigments, diarylide
pigments, diazopyrazolone pigments, phthalocyanine pigments, perinone pigments, quinacridone
pigments as well as anthraquinone and anthraquinoide pigments.
The results that are found in the present study when applying H2SO4 are in very good agreement
with the results found in [7], Schweppe reports results for only two pigment groups tested with
KIO3 in H2SO4. With this test reagent, agreement with the present study is observed for the
acetoacetarylide pigments, while several discrepancies are noted for the diarylide pigments.
De Keijzer has analysed a number of historically important pigments prepared as polished surfaces
of paint cross-sections [9-12]. A drop of the test reagent was placed on the polished surface and the
change in colour of the pigment surface was noted. De Keijzer registered the colour change in
pigment as well as possible dissolution of pigment with possible accompanying change of colour of
solution during a 15-minute period, applying concentrated H2SO4, concentrated HNO3, and a
saturated solution of KOH in EtOH. Castle investigated a large number of pigments mainly
encountered in nitrocellulose alkyds, but pigments in acrylics and poly-urethanes were also
analysed [8]. The analyses were performed on small paint flakes including binding media placed
directly under the microscope, using the same chemical reagents as in [9—12] and monitoring
possible reactions for 15 minutes. Castle did not observe colour differences of pigments in different
binding media with H2SO4 and HNO3 [8, p. 180]. In the saturated KOH in EtOH, changes in
reaction pattern were observed for some binding media but the colours of reaction were unaffected.
The same invariance of reaction in relation to binding media was found in [17] where TLC analysis
was performed. Milovanovic et al. observed problems only for the phthalocyanine pigments,
leading to a proposed extraction procedure for the removal of binding media for this pigment group
[17].
Agreement between results found in [8] and [9-12] is almost perfect for those pigments investigated
in both studies; however, several major discrepancies are observed in comparison with the present
study. In several cases, comparing results from the present study with [8—12] reveals the fact that
the colour observed in solution in the present study corresponds to the colour of the pigment grains
after application of the test reagent in [8-12]. This fact may depend on the different ways of
performing the test. Major discrepancies are observed for PY16, PR112, PR168 and PG7 in H2SO4;
PR3 and PV19 in HNO3; and PG10 in H2SO4 as well as in HNO3.
An interesting feature concerning the blue phthalocyanine pigments is observed. When these are
treated with HNO3, a violet colour in the solution is observed for some, but not all, pigments in the
pigment group. As pure pigments, PB15 [8-12] and PB16 [12] are found to yield purple colours
with HNO3 In the present study, colourless solutions are found for both pigments. Violet rings
around pigment grains are formed for PB15:2, PB15:3 and PB15:4 in the present study, and results
for these pigments are not reported in [8-12]. For PB15:1 in artists' acrylic colours investigated in
the present study, the paint flake changes colour to violet while the solution stays colourless (the
pure pigment PB15:1 also yields a colourless solution). For the blue phthalocyanine pigments it
appears to be more or less random whether a colourless solution or a violet solution is observed
with HNO3
Effect of binding media on the colour reactions
If the microchemical colour reactions presented in this study are to obtain practical application, it is
very important that they are independent of the binding medium in which the pigments are
employed. The investigation of pigments in different binding media is not exhaustive; it is
performed to get an idea if and how different binding media affect colour reactions with the four
test reagents.
Generally, colour reactions for artists' colours are not as clear as for pure pigments. Typically, the
colour from reaction with the test reagent is observed as a thin rim around the flake of artists'
colour. Commonly, the colour reactions are delayed for artists' colours compared to pure pigments,
due to the presence of binding medium probably leaving the pigment particles less accessible. So a
slight reaction period is recommended during testing.
It appears that the colour reactions are most easily observed when very small, thin flakes are taken
from the paint layer; if the samples are taken by very fine careful scraping of the surface, it seems
that less binding medium is present in the sample. This favours a correct determination of the colour
of reaction and hence the pigment.
Colour reactions for individual artists' colours (Table 6), representing oil colours, acrylic colours
and water colours, show an overall good agreement with colour reactions for pure pigments. Minor
discrepancies are observed for Cadmium Yellow Pale hue (PY1), Scarlet Lake (PR188, AWC),
Viridian hue (PG7, Winton) and Dioxazine Purple (PV23, Winton). These discrepancies could be
explained either by a slower reaction in artists' colours than for pure pigments or by very weak
colour reactions making decision between related colours difficult, for instance the distinction
between red and orange. For PY1, the greenish yellow colour initially observed with KIO3 in H2SO4
changes to red for the pure pigment, while for the artists' colour the change to red is not observed.
The same phenomenon is seen for PV23 in H2SO4. These are examples of slowed-down reactions.
For PR188 and PG7, slightly different colours — pale greenish yellow instead of orange (KIO3 in
H2SO4) and pale orange instead of red (H2SO4) - are observed respectively. The colours formed for
the artists' colours were rather weak, which probably accounts for the different appearances.
Permanent Rose (PV19) in both binding media (AOC and AWC) and Vermilion hue (PV19 and
PR3) show major discrepancies with several of the test reagents (Table 6). In several cases the
solutions from reaction with the artists' colours are colourless or very weakly coloured while the
pure pigments yield very strong colours, i.e., violet or bright carmine red. This indicates either a
selective protection of the pigments in the artists' colours from different test reagents or that the
artists' colours in question contain different or extra pigments than those declared.
In addition to pigment(s) and binding medium, artists' colours frequently contain different sorts of
filler material or extenders that might be suspected of participating in the colour reactions. Common
extenders include chalk (calcium carbonate, CaCO3), talc (magnesium silicate, Mg3Si4O10(OH)2),
silica (silicon dioxide, SiO2), china clay (aluminium silicate, mainly kaolinite, Al2Si2O5(OH)4),
baryte (barium sulphate, BaSO4) and mica" (mainly muscovite, KAl2(Si3Al)O10(OH,F)2) [4]. The
reaction of all of these materials with all four test reagents results in colourless solutions. In the case
of chalk the reaction is vigorous, with the formation of carbon
dioxide (CO2) bubbles from the reaction of carbonate with acid. So the discrepancies in colour
reactions observed between pure pigments and artists' colours cannot be attributed to filler materials
in common use.
Cadmium Red hue (PR188 and PR170) contains two naphthol AS pigments. These pigments (as
pure pigments) have identical colour reactions in H2SO4, HNO3 and H2SO4/HNO3. In KIO3 in
H2SO4 the colours are orange (PR188) and yellow (PR170). In agreement with this, Cadmium Red
hue in the first three test reagents yields the same colours as the pure pigments, and in KIO3 in
H2SO4 orange is observed as found for PR188.
From the above discussion it is concluded that the characteristic colour reactions of synthetic
organic pigments are mainly unaffected by possible binding media, in agreement with [8, 17]. This
is the basis for the applicability of the flow charts developed.
Classification of pigments in flow charts for identification
The flow charts in Figure 1 for the identification of synthetic organic pigments are constructed from
Table 7 and based on data in Table 5. Figure 1 reveals a very good resolution of pigment groups and
individual pigments (the exceptions from the groups). In the few cases where pigment groups or
individual pigments yield the same colours with all four test reagents, criteria like intrinsic colour of
pigments, year of first occurrence and pigments most frequently used on the market can be
employed. When distinction between pigments within a pigment group is needed the same criteria
are used.
Once an unknown pigment is determined to belong to a specific pigment group, very often only one
or a limited number of pigments in the pigment group have been extensively used commercially.
Combining knowledge of important pigments with the intrinsic colour of the unknown pigment and
perhaps reference pigments, it is possible with very simple experimental means to determine
synthetic organic pigments. Results from these simple analyses can be confirmed, for instance, by
IR or HPLC.
In Table 7 the observed colours of solutions for reaction with H2SO4 are divided into seven groups:
(1) Yellow, greenish yellow; (2) Orange, brownish yellow; (3) Violet, pink; (4) Bright carmine red,
red; (5) Colourless; (6) Olive green, green; and (7) Blue. Observed colours for HNO3 are
analogously divided into six groups: (a) Colourless; (b) Yellow, greenish yellow; (c) Orange; (d)
Red, brownish red; (e) Violet,
Table 7 Colour changes observed for the pigments and pigment groups studied
Symbols 1-7 for concentrated H2SO4, a-f for concentrated HNO3, I-VIII for concentrated H2SO4 /
concentrated HNO3 and a-n for KIO3 in concentrated H2SO4 are used to describe the colours of the
solutions for the investigated pigment groups. In some pigment groups one or several pigments
behave differently from their pigment group; these exceptions are listed separately. The resulting
four combination colour codes i.e. 1,a,l,a for yellow, greenish yellow (H2SO4), colourless (HNO3),
yellow, greenish yellow (H2SO4/HNO3) and red (KIO3 in H2SO4) are used to construct the flow
charts in Figure 1.
Colour codes: H2SO4 1: Yellow, greenish yellow, 2: Orange, brownish yellow, 3: Violet, pink, 4:
Bright carmine red, red, 5: Colourless, 6: Olive green, green, 7: Blue.
HN03a: Colourless, b: Yellow, greenish yellow, c: Orange, d: Red, brownish red, e: Violet, pink, f:
Blue.
H2SO4/HNO31: Yellow, greenish yellow, II: Orange, III: Brown, IV: Reddish violet, violet, pink,
V: Bright carmine red, red, VI: Blue, VII: Green, bluish green, VIII: Colourless.
KIO3 in H2SO4 α: Red, ß.' Yellow, greenish yellow, γ. Orange, δ; Green, brownish green, ε: Blue, λ:
Brown, brownish yellow, μ: Violet, π: Colourless.
pink; and (f) Blue. For H2SO4 /HNO eight groups are as follows: (I) Yellow, greenish yellow; (II)
Orange; (III) Brown; (IV) Reddish violet, violet, pink; (V) Bright carmine red, red; (VI) Blue; (VII)
Green, bluish green; and (VIII) Colourless. For KIO3 in H2SO4 eight groups are: (α) Red; (ß)
Yellow, greenish yellow; (γ) Orange; (δ) Green, brownish green; (ε) Blue; (λ) Brown, brownish
yellow; (μ) Violet; and (π) Colourless.
These symbols are used for the colours in Table 5 and result in the assignments in Table 7. Each
pigment group with a capital letter is now characterized by a four-symbol colour code; for example,
for pigment group B, the pyra-zolone lake pigments, the colour code is l,b,I,ß, which means that for
this pigment group the colours from reaction are 'yellow, greenish yellow' with all four test
reagents. As can be seen from Table 7, pigments that react differently from the pigment group (the
exceptions) accordingly have colour codes different from the pigment group.
The colour codes from Table 7 were used to construct the flow charts in Figure 1. Each flow chart
represents one of the seven colours from reaction with H2SO4. According to the subsequent colour
reactions with the three remaining test reagents, the flow chart is followed until a pigment group is
identified. It is important to keep in mind that the flow charts have been constructed using colour
reactions performed on pure pigments, meaning non-degraded pigments containing no binding
media. When analysing unknown material, one may very easily encounter cases where it is difficult
to choose whether a particular reaction colour is red or orange, for instance. As a result of this,
when a certain pigment or pigment group is identified, it is very important to set up all possible
control measures to check the plausibility of the result.
An obvious first control measure, if a single pigment is identified from Figure 1, is the intrinsic
colour of the pigment: does the colour of the pigment found employing the flow charts match the
colour of the pigment under investigation? If a pigment group is identified, does the pigment group
contain pigments that match the colour of the pigment under investigation? If these questions
cannot be answered positively, one must return to the start of the test.
Next control measure: as can be seen from Figure 1, several pigment groups and individual
pigments can be identified from reaction with H2SO4 and HNO3 alone or in combination with
H2SO4 /HNO3. If the identification was made using only two or three test reagents — e.g., in Figure
la, PR169 that is identified with H2SO4 and HNO3 or PB1 that is identified with the additional
mixture H2SO4/HNO3 - it is strongly recommended to make the third and fourth test with the
remaining test reagents, to make sure that these last colour reactions also comply with the pigments
identified. If this is the case, facts like time of first occurrence of the pigments should be compared
to data from the object investigated.
Pigment groups and individual pigments not resolved by the flow charts
As can be seen from the flow charts in Figure 1, the resolution of pigment groups or individual
pigments is very good when applying the four chosen test reagents. In differentiating between
unresolved pairs, the intrinsic colour of the pigments and the colours of solution given in Table 4
are studied. Pigment groups yielding the same colour in the flow charts may very well differ when
details are studied in Table 4, i.e., one pigment group may have a yellow solution with a certain test
reagent while the other is yellow from violet. For both pigment groups the colour yellow will
appear in Table 4. So in fact the two pigment groups do differ but this is not discernible from the
flow charts in Figure 1, reflecting the way they are produced.
Generally, criteria like approximate year of first occurrence of the pigments, intrinsic colour of the
pigments and a closer look at Table 4 can help distinguish between pigments with the same colour
codes.
Only five cases of unresolved pigments or pigment groups are found (Figure 1 and Table 8), i.e.,
pigment groups and pigments that do not have unique colour codes. The intrinsic colour of
pigments and year of first occurrence of specific pigment groups can in some cases be used to
differentiate between pigment groups that are not resolved by colour codes. Table 8 reveals that
naphthol AS pigments and diazo condensation pigments, both with colour code 3,a,V,ß, can
potentially be separated by years of first occurrence, since these are relatively different, ß-naphthol
pigments and naphthol AS pigments with colour code 3,a,V,γ and perylene pigments and
diazopyrazolone pigments (PR38) with colour code 3,a,VII,δ may be separated using a combination
of pigment colours found within the pigment group and year of first occurrence. Problems arise
especially in the attempt to separate perinone pigments and benzimidazolone pigments (PY194),
both with colour code 2,a,I,γ, since colours found in both pigment groups are similar, as are the
years of first occurrence. The same problem arises for the isoindoline pigments, quinophthalone
pigments and isoindolinone pigments (PO61) with colour code l,a,I,ß. Here the only potential
difference between pigment groups is the uncertain first year of occurrence for the quinophthalone
pigments. In these last two situations it must be accepted that it may be necessary to include other
means of analysis, for example IR, TLC or HPLC.
Test of the method: identification of unknown pigments by using the flow charts
The purpose of the experiment was to identify six unknown pigments (two yellows, two oranges
and two reds) by use of the flow charts (Figure 1). The six unknown pigments called Yellow1,
Yellow2, Orange1, Orange2, Red1 and Red2 were chosen among the 91 pigments tested (i.e., pure
and without binding media) and tested as described previously in this paper. It is recommended to
identify the colours with all four test reagents before using the flow charts in Figure 1.
Yellow1 yields 'yellow, greenish yellow' solutions with all four test reagents. From Figure la this
leads immediately to pigment group B containing the pyrazolone lake pigments. In this case it can
be stated for the unknown pigment that it belongs to pigment group B. Since there are no
differences within the pigment group (at least not in the three tested pigments) the precise pigment
cannot be identified. Ten yellow pyrazolone lakes are listed in [1, p. 224]. Yellow1 was indeed
PY191, a pyrazolone lake (Table 4), illustrating a correct identification with the flow charts.
Yellow2 yields orange (H2SO4), orange (HNO3), violet (H2SO4/HNO3) and red (KIO3 in H2SO4)
solutions. The first two colours lead directly to PY97 in Figure lb, i.e., from Table 4, a pigment with
the same colour reactions as PY97. There is no guarantee that the unknown pigment is PY97, but
since the 91 tested pigments represent a large number of the most used pigments, PY97 is likely.
Yellow2 was indeed the acetoacetarylide pigment PY97.
Orange 1 yields yellow solutions with H2SO4, H2SO4/ HNO3 and KIO3 in H2SO4 and a colourless
solution with HNO3. From Figure la these four colours lead to pigment group M (isoindoline
pigments), pigment group T (quinophthalone pigments) or PO61 (an isoindolinone pigment). From
Table 8, both pigment groups contain pigments with orange shades; it is therefore not possible to
determine Omngel more precisely than that it is an isoindoline pigment, a quinophthalone pigment
or PO61 pigment. Only two orange isoindoline pigments, PO66 and PO69 [1, p. 414] are listed. No
specific orange quinophthalone pigments are listed in [1, p. 540]. If the unknown pigment sample
were taken from a dated object, it might be possible to use the year of first occurrence for identification. For example, if the object were dated to 1930, the quinophthalone pigment group would be
the obvious choice (Table 8). Orange 1 was the orange isoindo-linone pigment PO61.
Orange2 yields bright carmine red (H2SO4), colourless (HNO3), yellow from green (H2SO4/HNO3)
and green (KIO3 in H2SO4) solutions. From Figure Id these four colours lead to the pigment group J
that contains the diazopyrazolone pigments. There are two orange pigments, PO13 and PO34, in
this pigment group [1, p. 267] and Orange2 was PO13.
Red1 yields bright orange changing to green against a black background (H2SO4), pale brownish red
(HNO3), yellow (H2SO4/HNO3) and orange (KIO3 in H2SO4) solutions. Using Figure lb, the first
two colours lead directly to the red diketo-pyrrolo pyrrole pigment PR255. Redi was therefore
identified as a red pigment with the same colour reactions as PR255. As for Yellow2 there is no
guarantee that the unknown pigment is PR255 - but it is likely. Redi was indeed PR255.
Red2 yields violet (H2SO4), colourless (HNO3), green from bluish violet (H2SO4/HNO3) and blue
(KIO3 in H2SO4) solutions. From Figure lc, when using all four colours, an unknown pigment
would be identified as a red pigment with the same colour reactions as PR149. As with Yellow2 and
Red1 there is no guarantee that the unknown pigment is PR149 - but it is likely. Red2 was correctly
identified as the perylene pigment PR149.
A very successful identification of six unknown pigments was possible using the flow charts
developed. It was established that the colour reactions were generally independent of binding
media, i.e., artists' colours showed similar colour reactions to those found for pure pigments. This
indicates that the flow charts proposed in this paper are as suitable for artists' colours with pigments
in various binding media as they are for the pure pigments on which they are based. The
experiments were performed on a number of artists' oil colours, artists' acrylic colours and artists'
water colours and confirmed by Castle [8], studying colour reactions analogous to those in the
present study and in [17] applying TLC.
The question may arise whether the observed colour reactions used for identification will also occur
for cured painted material, i.e., aged and worn painted material where a significant or total curing of
the binding media has occurred. Problems in pigment identification arising from cured or aged
binding media and/or pigments are not unique to this proposed method of identification; the
problem is general for other methods of identification.
Table 8 A list of the pigment groups and individual pigments that were not resolved by the flow
charts in Figure 1. Five colour codes are shared by more than one pigment group or individual
pigment. Differentiation may be obtained by criteria like colour shades found in specific pigment
groups and approximate year of first occurrence of pigment groups.
The question has to be studied in detail in order to propose relevant answers. It was demonstrated
that in some cases the binding media impeded the colour reactions of the pigments, thus affording
to some extent a sort of 'protection' to the test reagent. In other cases the 'protection' causes very
weak colour reactions, making precise colour assignments difficult. It is certainly a possibility that
an even higher degree of protection could be provided by well-cured binding media. Subjects like
these will need to be addressed in further studies.
CONCLUSION
This study has confirmed that colour reactions with the four test reagents - concentrated H2SO4,
concentrated HNO3 a mixture of concentrated H2SO4 and concentrated HNO3, and a solution of
KIO3 in concentrated H2SO4 — are similar for pigments within pigment groups. Several exceptions
do, however, occur and are included in the results. Generally, binding media do not affect the colour
reactions observed. The few observed discrepancies can most likely be attributed to the possible
presence of more than one pigment in the artists' colours or the delay of colour reaction caused by
the protective properties of the binding media on pigment grains.
Flow charts for the identification of synthetic organic pigments were developed on the basis of the
colour reactions obtained and tested successfully in the correct identification of six unknown
yellow, orange and red pigments.
Keeping the above notes about cured paint in mind, the flow charts are recommended as a useful
tool for the simple and non-time-consuming identification of synthetic organic pigments in general.
ACKNOWLEDGEMENTS
Conservator Knud Bo Botfeldt, School of Conservation, Copenhagen, is warmly thanked for
constructive criticism and review of the manuscript. His encouragement during this work has been
very valuable. Tech. Lie. Jan Jörn Hansen, School of Conservation, Copenhagen, is gratefully
thanked for constructive criticism and review of the manuscript. I owe special thanks to the editor
Dr Marie-Claude Corbeil, Canadian Conservation Institute, Ottawa and the two anonymous
referees for important improvements of the manuscript. Clariant Specialkemikalier A/S, Naverland
8, 2600 Glostrup, Denmark, are gratefully acknowledged for their free supply of a large number of
pigments.
MATERIALS
Pigments PY1, PY183, PR53:3, PR48:1, PR48:2, PR48:3, PR48:4, PR52:2, PR57:1, PR63:1,
PY153, PY185, PB15, PB15:6, PB16, PR178, PV29, PR169, PB1, PY138 and PB60: BASF
Aktiengesellschaft, Marketing Welt Pigmente, 67056 Ludwigshafen, Germany.
Pigments PY74, PY14 and PR202: Ciba Speciality Chemicals, Basel, Switzerland.
Pigments PY73, PY191, PO5, PR4, PR53:1, PR2, PR112, PR146, PR188, PY151, PY181, PY194,
PO36, PY12, PY83, PY13, PY127, PO13, PO34, PR38, PY16, PY155, PR242, PR.262, PY139,
PB15:1, PB15:3, PG7, PR149, PR179, PO43, PV23, PB61 and PR168: kindly donated by Clariant
Specialkemikalier A/S, Naverland 8, 2600 Glostrup, Denmark (Danish division of Clariant GmbH,
Division Pigments & Additives, BU Pigments, 65926 Frankfurt am Main, Germany).
Pigments PY97, PY111, PO38, PR12, PR170, PR185, PV32, PB15:2 and PB15:4: Hoechst
Aktiengesellschaft, 6230 Frankfurt am Main 80, Germany.
Pigments PY3, PY6, PR3, PR9, PY154, PO60, PR144, PR166, PY129, PG10, PY109, PY110,
PY173, PO61, PG36, PR122, PV19, PO73, PR254, PR255, PR264, PR83, PV5:1 and PO51:
Kremer Pigmente, Dr Georg Kremer, Dipl.-Chemiker, Farbmühle, 88317 Aichstetten / AUgäu,
Germany.
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AUTHOR
NICOLINE KALSBEEK received her PhD in 1993 from the University of Copenhagen, Denmark, on
studies of organic compounds applying single crystal X-ray diffraction data and solid state NMRspectroscopy. Since 1993 she has worked at the School of Conservation, Royal Academy of Fine
Arts, Copenhagen, with a special interest in SEM analysis and the identification and properties of
organic and inorganic pigments. Address: Royal Academy of Fine Arts, School of Conservation,
Esplanaden 34, 1263 Copenhagen K, Denmark. Email: nk@kons.dk
Resume — Le but de cette étude est de développer une méthode d'identification des pigments
organiques synthétiques sur la base de réactions colorées en employant quatre réactifs chimiques :
l'acide sulfurique concentré, l'acide nitrique concentré, un mélange de ces deux acides, et enfin une
solution d'iodate de potassium dans l'acide sulfurique concentré. On a documenté les réactions
colorées d'un grand nombre de pigments organiques synthétiques sous forme de poudres. Il
apparaît que les réactions colorées observées sont en total accord avec les groupements chimiques
des pigments (par exemple, les pigments naphtol ou quinacridone), c'est-à-dire que les pigments
appartenant à un même groupe produisent des réactions colorées similaires. Cette observation
constitue la base de la construction de diagrammes pour l'identification. Les tests effectués sur un
certain nombre de couleurs d'artistes contenant des pigments dans différents liants montrent que
ces derniers n'affectent pas notablement les réactions colorées observées, ce qui rend les
diagrammes mis au point utiles pour l'identification des pigments dans ces peintures. Un cinquième
réactif chimique — une solution d'hydroxyde de potassium à 96 % dans l'éthanol - a été testé et
abandonné en raison de son instabilité. Cette instabilité donnait lieu à de sérieux problèmes de
reproductibilité, en particulier pour certains groupes de pigments, comme par exemple les pigments
acétoacétaryliques.
Zusammenfassung — Das Ziel dieser Studie ist es, auf der Basis von Farbreaktionen mit vier
chemischen Reagenzien eine Methode zur Identifizierung synthetischer organischer Pigmente zu
entwickeln. Konzentrierte Schwefelsäure, konzentrierte Salpetersäure, eine Mischung von
konzentrierter Schwefelsäure und konzentrierter Salpetersäure sowie eine Lösung von Kaliumiodat
in konzentrierter Schwefelsäure werden in der Studie verwendet. Farbreaktionen für eine große
Anzahl von synthetischen organischen Pigmenten, in ihrer Fonn als pulvriger Reinstoff, werden
beschrieben. Es kann dabei gezeigt werden, daß die stattfindenden Reaktionen mit der chemischen
Eingruppierung der Pigmente übereinstimmen (z.B. Naphtol AS oder Quinacridon Pigmente), dies
meint, daß Pigmente aus der gleichen chemischen Gruppe ähnliche Farbreaktionen zeigen. Diese
Tatsache bildet die Basis für die Entwicklung von Flussdiagrammen für die Identifizierung. Die
Untersuchungen an einer Reihe von Künstlerfarben mit Pigmenten in verschiedenen
Bindemittelsystemen zeigen, daß die Bindemittel die Farbreaktionen kaum merklich stören, weshalb
die Methode zur Identifizierung von Künstlerpigmenten tauglich ist. Ein fünftes Reagenz, eine
gesättigte Lösung von Natriumhydroxid in 96% Ethanol, wurde wegen ihrer Instabilität verworfen.
Diese Instabilität bereitete große Probleme bezüglich der Reproduzierbarkeit der Untersuchung,
insbesondere bei bestimmten Pigmentgruppen wie den Acetoacetaryl Pigmenten.
Resumen — La intención de este estudio es desarrollar un método para la identificación de
pigmentos sintéticos orgánicos en base a las reacciones de color producidas con cuatro reactivos
químicos diferentes. En este estudio se empleó ácido sulfúrico concentrado, ácido nítrico
concentrado, una mezcla de ácido sulfúrico y nítrico, y finalmente una disolución de iodato
potásico en ácido sulfúrico concentrado. Se obtuvieron reacciones de color con un gran número de
pigmentos sintéticos orgánicos utilizados en polvo. Se evidenció que las reacciones de color
observadas se encuentran en total afinidad con el agrupamiento químico de los diferentes
pigmentos (por ejemplo, pigmentos tipo naftol AS o pigmentos de quinacridona); esto significa que
los pigmentos pertenecientes al mismo grupo químico muestran reacciones de color semejantes.
Este hecho es la base para la elaboración de cartas de referencia para la identificación. Las
pruebas llevadas a cabo en varias pinturas para artistas que contienen pigmentos en varios
aglutinantes diferentes indican que estos últimos no afectaban a las reacciones de color
observadas, mostrando que las mencionadas cartas de referencia podían ser empleadas para la
identificación de los propios colores para artistas. Se probó también un quinto reactivo químico,
una disolución saturada de hídróxido de potasio en etanol (96%), aunque fue desechado por su
inestabilidad. Esta inestabilidad se manifestó en forma de serios problemas a la hora de reproducir
las reacciones, especialmente en ciertos grupos de pigmentos, por ejemplo el los pigmentos de
aceto acetarilido.