Copper Verdigris
by Blaire Molitor, 2002
Abstract
The purpose of my research project was to test the hypothesis of Dr. Scott, of the Getty Institute in
California, who stated that the urine of different people, when mixed with copper, would produce
different colors of copper verdigris. In order to test this, I obtained the urine samples of eight
different individuals. By placing the urine over salt coated copper shavings and incubating this
solution I was able to produce copper verdigris. By comparing the copper verdigris to the color
spectrum chart I was able to determine the wavelength (color) of each sample. The wavelengths
ranged from 510 nm to 578 nm, which is the blue-green range. Since each sample had a different
wavelength, I was able to correctly prove Dr. Scott's hypothesis.
Purpose
Dr. Scott at the Getty Institute hypothesized that different people’s urine produces different colors
of copper verdigris. I made copper verdigris from different people’s urine to test if this is true.
Background
Verdigris, by definition, is the greenish patina that forms on copper after long exposure to the
atmosphere. It is a copper carbonate created on copper surfaces by the action of carbonic acid in
the atmosphere. In chemistry, the term is applied to acetates of copper formed by reaction of
copper with acetic acid. The acetates themselves are powdery or minutely crystalline substances
that vary from green to blue in color (1). What is incredibly interesting and unique about copper
verdigris is its origin.
Verdigris was ultimately a woman's work, making it differ from other scientific experiments and
occurrences in ancient times (2). Not only is verdigris used as pigment in paints, as a mordant in
dyeing, and as a fungicide and mildew preventive (1), scientists also study it rigorously across the
country. It is through devoted experiments by individuals such as Dr. David A. Scott at the Getty
Institute in California that scientists are beginning to understand how to conserve old paintings,
bronze pieces, and other artifacts that have corroded (3).
Verdigris is the oldest of all the manufactured copper greens. It dates back to ancient times where
people would use the corroded copper as pigments and for medicines. The first-century AD
historian Pliny the Elder actually recorded a recipe for making copper verdigris in his renown
Natural History: "There is…a verdigris…made by grinding up in a mortar of true Cyprian copper
with a pestle of the same metal equal weights of alum and salt or soda with the very strongest
vinegar…The mixture is ground up until it becomes a green color." Furthermore, a Greek
manuscript by Theophilus listed a recipe for verdigris. His recipe, entitled viride salsum, is made
"by sprinkling common salt over copper plates brushed with honey and placing them over vinegar
in a closed container." Also, a 17th Century manuscript, "Ricitte Per Far Ogni di Colori gives this
recipe:"To make verdigris, take pieces of copper anointed with purified honey, and fasten them to
the cover of a well-glazed pot, which must be full of hot vinegar and made with strong wine. Then
cover it and place it in a warm situation for four to five weeks. When you uncover it, remove the
color that you find on the piece of copper…" (4). Although all these recipes contain some similar
components, it is apparent that they all have very individual characteristics. That is why Dr. Scott
dedicates his time to testing the numerous different ways copper verdigris can be made.
As stated previously, copper verdigris was a woman's art. It was the women of Montpeiller,
France who controlled the verdigris manufacture from the13th to the 20th centuries. It was
through their use of alchemical knowledge that they turned the synthesis of copper verdigris into a
highly profitable business (3). In fact, women in this position could earn over 3,000 livres a year;
this is 12 times the amount an average craftsman earned at this time (2). This verdigris was then
used to paint interiors of French and Dutch homes. It was also mixed with oils to color and
preserve woods, to tint paper, and to illuminate stamps (3). What a scientist who is studying
verdigris must do is infer the recipes of the French women. Since the secrets of its cultivation
merely passed from mother to daughter, one must imagine the resources that were available at the
time (2). That is exactly what Dr. Scott is presently doing.
Numerous methods have been tested to make copper verdigris. Among these is placing copper
strips in a variety of different liquids including old-wine vinegar, urine, oils, and honey. Each
liquid produces different copper (II) acetate compounds. Their shades vary from light green to
pale blue. The formulas for each of these different shades are shown in Table 1 (3):
Table 1: Chemistry of Copper Verdigris
Color
Formula
Light green
Cu(CH3COO)2·[Cu(OH)2]3·2HOH
Blue-green
Cu(CH3COO)2·[Cu(OH)2]4·3HOH
Blue
Cu(CH3COO)2·[Cu(OH)2]2
Pale blue
Cu(CH3COO)2·Cu(OH)2·5HOH
The color, name, formula, and crystal structures of the different shades of verdigris are shown in
Table 2 (4):
Table 2: Crystal Structures of Cooper Verdigris
Color
Name
Formula
Crystal
Structure
pale green
nantokite
CuCl
cubic
vitreous green
atacamite
Cu2(OH)3Cl
orthorhombic
pale green
paratacamite
Cu2(OH)3Cl
rhombohedral
pale bluish green
botallackite
Cu2(OH)3Cl
monoclinic
pale green
clinoatacamite
Cu2(OH)3Cl
monoclinic
light green
anarakite
(CuZn2)2(OH)3Cl
rhombohedral
It is because so many different forms of verdigris are possible that I decided to test one of Dr.
Scott's hypotheses: Does the urine of different people produce different copper verdigris? I was
excited to continue the tradition of women making verdigris (unfortunately I won't be making a
profit). With everything considered, verdigris is not only a fascinating scientific discovery, but also
a useful and exhilarating one as well.
Procedure
Purpose Part 1: To see which way of making copper verdigris produces the fastest results.
Procedure Part 1:
1. I obtained four jars. I placed copper shavings in two of the jars and metal copper strips in
the other two jars. I added 20.0 mL of vinegar to each. I covered one of the jars filled with
the copper shavings with parafilm and left the other with shavings uncovered. I then
covered one of the jars with the copper metal strip with parafilm and left the other with the
strip uncovered.
2. I mixed 6.00 g of ammonium chloride with H20 and coated a sample of copper shavings
with this solution. I placed the copper shavings covered in the solution in a jar and covered
it with parafilm.
3. After having each jar sit for two days, I used a filtrate to filter out the liquid and placed the
samples in separate petri dishes.
4. I tested the filtrated liquids at different wavelengths on the spectrophotometer to determine
where the percent transmittance is the highest. I recorded this data and then graphed it.
Purpose Part 2: To make copper verdigris with different people's urine to test if different urine
produces different color copper verdigris.
Procedure Part 2:
1. I cut strips of copper shavings (enough to fill eight jars), coated all of them in a mixture of
ammonium chloride and H20, and then placed them each in eight different 50.0 mL
beakers.
2. I tested and recorded the pH of the urine samples of eight different people using a pH meter.
I graphed the pH in order to see the difference between urines.
3. I then poured 30.0 mL of urine (a different person’s for each jar) over the copper shavings.
I covered each jar with parafilm, labeled them, and incubated them at 32°C.
4. Once the copper corroded (48 hours), I used a filtrate to filter the copper verdigris.
5. I transferred the filtrated liquid to test tubes. After having let this liquid sit for 24 hours I
poured off the top layer of each sample.
6. I removed 1.00 mL of each sample and mixed each one separately with 20.0 mL of H20 in
order to dilute the sample so it could be tested on the spectrophotometer.
7. I attempted to test each liquid in the spectrophotometer, but because the liquid was not
clear I was unable to do so. Therefore, I held up the jars to the Spectrum Chart and
recorded which wavelength the color matched. I scanned the spectrum chart and drew
lines where the wavelengths matched, in order to compare the different colors.
8. I poured the rest of the samples (which I didn’t dilute) into eight watch glasses and placed
them under the fume hood to evaporate into crystals.
Graphs and Results
Figure 1: pH of Samples
pH of Samples
7.5
7
pH
6.5
6
5.5
5
4.5
4
0
1
2
3
4
5
6
7
8
9
Sam ple Numbe r
Through this first graph I was able to see that the eight different verdigris samples from different
urines all had a different pH level, ranging from 5.7 to 7.29. The different pH levels automatically
led me to believe that the urine of different people would produce different colors of copper
verdigris.
Figure 2: Color of Samples
I was able to see through Figure 2 that the verdigris samples show a different color of copper
verdigris. The lowest was at 510 nm and the highest was at 578 nm. This graph proved my
hypothesis.
Color of Samples
600
590
Wav elength (nanom eters)
580
570
560
550
540
530
520
510
500
0
1
2
3
4
5
Sample Number
6
7
8
9
Figure 3: Wavelength vs. pH
Wavelength vs. pH
590
580
wavelength (nm)
570
560
550
Wavel ength
540
530
520
510
500
5
5.2 5
5.5
5.7 5
6
6.2 5
6.5
6.7 5
7
7.2 5
7.5
pH leve l
Figure 3 displays the relationship between the wavelength of the samples and the pH
level. The pH of sample 4 was 5.27 and the wavelength was 520 nm. The wavelength was
also 520 nm for sample 7 that had a pH level of 7.29, making a definite relationship between
pH and color hard to conclude. Below is a table comparing the pH levels with the wavelengths
of the verdigris:
Table 1: Comparison of Wavelength and pH
Verdigris from Urine
Wavelength (nm)
pH
1
525
5.92
2
524
6.49
3
578
5.70
4
520
5.27
5
520
5.86
6
510
6.37
7
520
7.29
8
530
6.54
Both Table 1 and Figure 3 show that no direct relationship can be seen between the wavelengths
and the pH levels. For sample 1 the wavelength was 525 nm and the pH was 5.92. For sample 2
the wavelength decreased to 524 nm and the pH rose to 6.49. When the wavelength rose to 578
nm for sample 3, the pH decreased to 5.70. Therefore I am unable to determine if pH has an effect
on the wavelength.
Figure 4: The Color Spectrum
#3: 578nm
#1: 525nm #4,5,6: 520nm
#8: 530nm #6: 510nm
#2: 524nm
Figure 4 displays the wavelength of each different urine sample. From this color spectrum chart,
one can easily see that the samples resulted in a variety of different wavelengths. Sample #6 had
the lowest wavelength, 520 nm. Sample #3 had the highest wavelength, 578 nm. Sample #’s
1,2,4,5, and 6 had the median wavelengths, ranging from 520 nm to 525 nm. Refer to Table 2 to see
the exact numbers. Also, refer to Figures 5 and 6 to see pictures of the final filtered copper
verdigris.
Figure 5: Filtrated Liquid
From Figure 5, it is clearly apparent that each different urine produced verdigris of a different
color. Below are the wavelengths of each sample:
Table 2: Wavelengths of Liquids
Verdigris from Urine
Wavelength (nm)
1
525
2
524
3
578
4
520
5
520
6
510
7
520
8
530
Figure 6: Final Copper Verdigris
Figure 6 shows that the eight samples of copper verdigris are all different colors, proving that
the urine of different people results in different color verdigris. Refer to Table 2 to see the
exact wavelengths for each sample.
Conclusion
I was very content that through my data I was able to prove Dr. Scott's hypothesis. It is clear
that with a range in wavelengths from 510 nm to 578 nm that the urine of different people
produces different colors of copper verdigris. I am not able to understand the direct relation
between pH level and color but that would be an interesting project for someone else to pursue.
Furthermore, it would be exciting to test the urine of the same person on numerous different
days, recording what he or she ate each day and seeing if food had an effect on the color of the
verdigris. There are numerous factors that could be altered. One could let the sampled urine sit
for a certain number of days before pouring it on the copper or he or she could pour it on
immediately after the sample was taken. They could also incubate the samples for a longer
period of time, or not incubate them at all. Because I coated my copper shavings in ammonium
chloride, the filtered liquid was cloudy, therefore it could not be tested on the
spectrophotometer. That is why one could try it without ammonium chloride, so that the final
product would be lucid, then, in turn, you could test it on the spectrophotometer.
In response to my personal research, I encountered several problems that may have resulted in
imperfect data. I did not collect all of the eight urine samples on the same day, meaning that
each urine sample had sat for a different amount of days, and this could or could not have
affected the outcome. Also, since I was unable to use the spectrophotometer, I had to
determine the wavelengths by eye, and it is obvious that my interpretation of where a color
falls on the spectrum chart is not going to be perfectly equivalent to the view of another.
Despite these problems, I feel my research was still a success.
References and Notes
1. Encyclopedia Americana Online (2002); see
http://ea.grolier.com/cgi-bin/build-page?artbaseid=0401890-00$dbname=ea&query=%28
copper%2b%20verdigris%2b520%29%3ATEXT&viwename=go&docid-2747 Accessed
2002 April 5.
2. R. Benhamou, French Historical Studies [online]. 16, 650 (1990). Available from: Gale
resources InfoTrac
http://web4.infotrac.galegroup.com/itw/infomark/132/268/23122675w4/purl=rc2_EAIM_
1_verdigris Accessed 2002 April 5.
3. L. Fruen, Chem News. 13, 14-15 (2001).
4. L. Fruen, Ancient Copper Pigments.
http://realscience.breckschool.org/ipper/fruen/files/Enrichment/files/CopperPigments/Co
pperPigments.html Accessed 2002 April 15.
5. D. Scott, Copper and Bronze in Art: Corrosion, Colorants, Conservation. (Getty
Publications, Los Angeles, CA, 2002, ch1-5.