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The Mystery Behind Sanford McGee's Copper PatinasCaroline Vaughn & Laura LaMonica |
Local artist Sanford McGee uses copper and etches botanicals
into it with weak acid washes of varying dilutions of vinegar (acetic acid)
to produce
beautiful copper patinas. He mists the washes in a rain-like way onto botanicals,
such as oak leaves. The leaves decompose and are peeled off
producing different colors of leaf shapes and etchings on the copper plates.
His art is caused either by the combinations of organic
botanicals, which contain tannic acid, and the acetic acid solutions he uses,
or by the reaction of the copper and the acetic acid.
In our experiment we attempted to find the answer to Sanford’s Mystery by testing the effects of tannic acid and acetic acid with and without the factor of oxygen.
Copper was first used over 10,000 years ago and continued to be the only metal known for the next 5 millennia. Copper today is the most frequently used metal because of its attractive and adaptable properties, which enable it to be often alloyed with zinc to form bronze. Copper is a basic metal that can be found in different crystalline forms called allotropes and can exist in different structural forms in the same solid state.
When copper reacts with air, a patina is formed. The patina formed on copper is copper oxide. Patinas form on the exposed part of the metal, and the process can be accelerated by heat. In a natural patina, neutral copper metal reacts with air to form a pink oxide, Cu2O, called cuprite, which has a copper cation in the +1 oxidation state. Overtime the copper will oxidize further to a blacker oxide, CuO, called tenorite, which has Cu+2 ions. With water present, the tenorite will react with sulfur dioxide and carbon dioxide from the air to form a patina made of brochantite, a hydrated copper sulfate, and malachite and azurite, both hydrated copper carbonates. These three compounds form blue and green colors on the copper surface, which has been fully oxidized to Cu+2 ions.
Copper reacts with most acids to create copper (II) salts that are blue
or blue-green because of the hydrated copper (II) ion. Acetic acid,
better known as vinegar, is a weak acid. Acetic acid’s chemical formula
is CH3CO2H. When acetic acid is added to copper it can form Copper (II)
acetate which is a blue color called “Blue Verdigris” or it can form
Copper (I) acetate. Tannic acid comes from plants and contains glucose
and gallic acid. It is a weak acid and is used primarily in the
commercial staining of wood. Tannic acid’s chemical makeup is C76H52046.
Red oak leaves are very full of tannic acid, which keeps them from
decomposing as quickly as other leaves.
There are many variables that affect patina results. These variables are the composition of the metal, the acids used, the purity of the chemicals and water used, the thoroughness of the plate washing, method of application of acids (hot versus cold) and the amount of time the acids were left on.
The mystery behind Sandford McGee's art is the role that the Oak leaves play on the copper plates. It is the tannic acid that will cause a bigger reaction, i.e. color, as opposed to the vinegar solution or 5% acetic acid.
Project #1
Cut out (2) small shapes from the felt sheet and (2) small shapes from the thin, cotton fabric that will fit onto the copper plates. Then soak (1) felt shape and (1) cotton shape in a 5% tannic acid bath prepared in a plastic tray. Next, soak the other felt and cotton shapes in a vinegar or 5% acetic acid bath also prepared in a plastic tray. Keep them submerged for (5) minutes or until thoroughly wet.
Copper Cleaning Directions: Two of the copper plates must be cleaned with the Putz-Pomade using steel wool to remove the oxide film from the surface. Once the surface is clean from the black residue, Mineral Spirits must be poured onto the surface of the plate and wiped. After wiping, the copper plate will appear to be shiny. After cleaning, the last step is to apply Isopropyl alcohol (rubbing alcohol) to the surface to wipe the oils from the surface caused by the Mineral Spirits.
Place the shapes that were soaked in acetic acid on one copper plate and the ones soaked in tannic acid on the other plate. Place each plate in its own plastic tray and cover. Then place the trays in a dark place where they will not be bothered with or where the sun will not be a factor.
Remove the trays and the plastic after (4) days, rinse under water, pat dry, and observe.
Project #2
Clean a copper plate (see #1) and place in a beaker. Place the beaker on a hot plate, and turn the hot plate to 100 degrees F). Next, use a dropper to make a small puddle of 5% tannic acid on one half of the copper plate and again use the dropper to make a puddle on the other half with the 5% acetic acid.
Cover with a watch glass or flat glass and leave overnight. Observe the next day.
Project #3
This project is conducted to see how effective air is on copper in relation to the two different acids.
Two pieces of cheesecloth are cut and placed in a separate acid bath of acetic and tannic acid, each being 5%. After a new copper plate is cleaned (see #1), each wet cheesecloth piece is placed on its appropriate half of the copper. The copper plate is placed in a tray and covered with plastic wrap. Then it is placed in a dark place away from the sun. Leave in the cabinet for a day, and observe the next day after rinsing and lightly drying.
Project #4
On a clean, copper plate (see #1) (3) Maple leaves are placed on top of one copper plate. On the second plate, (3) Oak leaves are placed on the copper plate. Both plates are placed in separate trays and are sprayed with vinegar (5% acetic acid). They are then covered and placed in a dark place to dry.
They should be checked after 2 days and will probably still be damp, but rinse in water and pat dry.
Project #5 (Enhancement 1)
A cleaned copper plate (see #1) is sectioned off into (5) squares with a metal stylus and numbered from one to five. Then allowing one square to be exposed, the other four squares must be painted over with ground and left to dry for 10-15 minutes. When dry, the whole plate should be dropped into the acid bath with only one square exposed (either 5% sulfuric acid, copper nitrate/nitric acid, cupric sulfate/ammonium chloride, 5% acetic acid, and 5% tannic acid solutions) and completely submerged.
1) Red - semi-matt (for Copper & Copper Plate)
Ingredients
(A) Cupric Sulfate... 25 gm [MORE INFO]
Distilled Water... 1L
(B) Ammonium Chloride... 0.5 gm [MORE INFO]
Process
Boil immersion (A) 15 min.
Boil immersion (A)+(B) 10 min.
Immerse in boiling cupric sulfate solution about 15 min. or until color well developed.
Remove to hot water while ammonium chloride is added to the cupric sulfate solution. Then immerse about 10 min. Remove and wash in hot water. Dry and finish.
Important note: In general, chemically induced finishes are unsuitable for use on articles to be used with food.
2) Dull Pink (for Copper & Copper Plate)
Ingredients
Cupric Nitrate... 1 tsp [MORE INFO]
Nitric Acid 10% ... 100 ml [MORE INFO]
Distilled Water... 1L
Process
Hot immersion - 5 min.
Immerse in hot solution (140-158°F, 60-70°C) that etches surface. Remove after 5 min., wash in warm water and air dry. Wax finish.
After all five squares have been soaked and submerged in their appropriate bath for (1) per square, then the plate is ready to be observed.
REMEMBER: rinse the plate after each submersion, and clean the ground off with mineral spirits and isopropyl acohol (rubbing alcohol).
Project #6 (Enhancement 2)
A Maple leaf (1) should be soaked in a 10% tannic acid bath for (5) minutes while a cotton fabric cutout should be soaked in acetic acid for (5) minutes also. After these are finished soaking, they should be placed evenly apart from each other on a clean copper plate (see #1). Then a Tulip Poplar leaf should be applied to separate the leaf from the cloth. Then vinegar or 5% acetic acid should be sprayed using a spray bottle over the copper plate while inside of a plastic tray. After generously spraying the vinegar over everything, then the tray should be covered with plastic wrap and placed in a dark space to dry. It should only be left to dry for (1) day. Then after one day, the tray should be uncovered and rinsed. Also, it needs to be patted dry and then can be observed.
Project #6 (Enhancement 3)
A hole should be drilled into the favored plates, and then thread a thin, metal wire through each hole and attach to a sturdier piece of metal wire. Hang from a tree or in a room to create your very own copper patina wind chime!
Project 1: (figures 1, 2, 3 and 5)
The felt would not absorb the acid and stick to the surface, while the thin cloth
absorbed it and stuck to the plate. This created very even, precise lines in
both acids with the thin cloth, and blurry outlines with the felt. The acetic
acid thin cloth form turned blue, while the tannic acid thin cloth turned yellow.
The forms soaked in 5% acetic acid made a blue patina around them. The forms
soaked in 5% tannic acid made a black and reddish patina around them.
Project 2: (figures 3 and 6)
Acetic would not stick to the plate, while the tannic acid spread thinner;
5% tannic acid evaporated much faster than the 5% acetic acid. The end result
was the tannic acid turning black around the edges, and the acetic acid appearing
to “clean” the plate.
Project 3: (figure 7)
·The 10% acetic acid turned the cheesecloth on the copper blue almost
immediately. After 2 days the area surrounding the cheesecloth was very blue,
but in the small sections of the plate covered with cheesecloth nothing had
happened.
·The 10% tannic acid turned yellowy after about an hour. 2 days later
there was black on the copper where the cheesecloth had been.
Project 4: (figures 8, 9, 10, 11, and 12)
·The plate with the oak leaves with 5% vinegar solution produced blue,
green and violet patinas around the leaf. The stems etched the copper and,
where the leaf was turned, different shades of red.
·The plate with the maple leaves with 5% vinegar solution produced a
beautiful patina with blues, greens and browns around the leaf. Underneath
the leaf are etched stem markings and even violets
Project 5: (figures 13, 14, and 15)
1) 5% sulfuric acid caused no reaction/color change when submerged
2) The cupric sulfate dissolved quickly in the hot water and ammonium chloride,
turning the mixture blue, and causing a harsh smell. The copper turned a red-semi-matt
finish, just as the mixture said it would, but it was not a drastic change
from the original copper color.
3) The cupric nitrate and nitric acid solution also was blue. After just 5
minutes of submersion it formed a light pink.
4) 5% acetic acid turned blue very fast once the copper plate was submerged
in it. The next day nothing had happened, but the ground covering the rest
of the plate had turned blue-ish and was very hard to remove.
5) 5% tannic acid was left in the bath for four days, took off the shiny luster
of the copper but, then the luster came back
Project 6: (figure 16 and 17)
Our creative project of a maple leaf soaked in tannic acid, tulip poplar soaked
in acetic acid and cloth shape soaked in acetic acid produced blue patinas
around the acetic acid and the botanicals etched the copper along the stems.
The cloth soaked in acetic acid turned blue and the leaf soaked in tannic
acid turned more yellow.
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The 5% acetic acid and the 5% tannic acid had similar reactions with the copper, but the acetic was bluer and the tannic acid was blacker. When the plates were submerged in the acids with out air there was no reaction proving the necessity of air as the main factor of oxidation in a patina. Our project greatly differed from our original plan which was to just test the different acids and recreate Sanford McGee’s original artwork in order to find which element played the biggest role in his art. We ended up not only testing both acids on copper but also testing them with different types of cutouts and leaves, as well as with and without air. Our hypothesis was very wrong, the acetic acid caused much more color than the tannic acid, but the tannins in the leaves did etch the plates, creating a beautiful effect. Did we solve the mystery? Yes and No. We found out that the main factor is his patinas is air but we are not sure what the reaction exactly is. If we were to redo the experiment we would use red oak leaves that are very full of tannic acid and decompose slowly while still holding the acid in place on the plate. This way we could have let the patinas develop much longer, and possibly gotten much different results. The oxide coating of copper is dissolved by acids, and comes back once exposed to air for a period of time, and thus where our forms and leaves were on the copper plates already started changing to back to duller copper tones.
http://www.sciencecompany.com/patinas/patinaformulas.htm#6
http://www.finishing.com/261/74.shtml
http://encyclopedia.jrank.org/SUS_TAV/TANNIN_or_TANNIC_ACID.html
Special thanks to Sanford McGee for creating the art behind our project and for offering us valuable information without which this project would not be possible. Also, thanks to Prof. John Bordley and his wife Carolyn Fitz who helped us in a countless number of ways such as figuring out the actual science occurring on the copper and even helping us with the idea for our project even when there were considerable doubts that we might not find any answers at all! The Chemistry Dept. as a whole is responsible for helping us arrive at our conclusion and understanding our results!
figure 8 
