Introduction

In this experiment, I will study the effects of different chemical solutions on copper and note how the environment influences the patina process.

Background

A patina is a coating (sometimes of copper carbonate) on articles of copper or copper alloys (including bronze and brass). As it is a naturally occurring reaction, patinas have existed ever since the discovery of copper. According to Dr. Kirven, the patina is nothing more than a bond between copper and salt. Therefore, the patina is not actually the copper plate, rather the chemical reaction which appears on the surface of the copper. The presence of salt explains the crusty film that forms on top of the copper plate. A natural patina forms after long exposure to a moist atmosphere or burial in the earth. The patina process is an oxidation process, whether naturally or artificially created. Unfortunately, neither Dr. Bordley nor Dr. Kirven could show me a half-reaction for the chemicals that I used, which would have enabled me to write an oxidation/reduction reaction. Although commonly green, due to the application of differing chemicals, the patina can take on a wide diversity of colors and consistencies; it may be red, brown, black, blue, or gray, or it may be smooth, glossy, or crusty.

There are two ways to make a patina, both hot and cold methods. Hot patination is the more popular method since the reaction is quicker and the chemical solutions bond better to the hot metal surface. Despite its popularity, I opted instead for the cold patina method, which does not require a blowtorch or kiln. Cold patination is the more traditional process. With cold patinas, the bite is not as strong as the warm patina, the patina process takes longer (usually at least one week), and requires many applications of the solution. The cold patinas are also not as durable and the patina can easily flake off or change.  

Procedure

In order to run the experiment, I first had to decide which chemicals I wanted to use. While this should appear easy, the actual preparation proved far more time consuming. At first, I wanted to use acidic and basic solutions, the contrast would then be between the reaction of an acid and a base on copper. After trying to prepare the chemical solution, Dr Bordley and I realized that this experiment would not work. We were using nitric acid (HNO₃) and trying to achieve differing pH levels. The problem was that we used sodium hydroxide (NaOH) to make the solution more basic. Hence, we were using both a strong acid and a strong base and the resulting solutions were radical. We could not slowly alter the pH of the solution. It was either high or low with little variance. After talking with Dr. Kirven, I scrapped the initial proposal and instead looked to use different chemicals (gallic acid and acetic acid) to patina the copper. Dr. Kirven noted that acetic acid would play the role of a normal acid. Acetic acid has a chemical makeup of C₂H₄O₂ (or CH₃COOH) and is obtained in the destructive distillation of wood. The acetic acid exhibits a very pungent order that is unmistakable. Meanwhile, the gallic acid would have a slightly different reaction due to its chemical composition. Gallic acid or Trihydroxybenzoic acid   has a chemical structure of C₇H₆O₅. It is obtained by alkaline or acid hydrolysis of the tannins from nutgalls. Thus, gallic acid has a chemical structure very similar to the tannins founds in oak leaves. Local artist, Sanford McGee, uses these oak leaves in his art, which involves copper patinas. After determining which chemicals I would use, Dr Kirven and I had to develop a proper mathematical equation for a chemical solution that would give both the acetic and the gallic acids the same molarity. The Chemistry stock room only had gallic acid in a powder form, so we had to find out the solubility of gallic acid (1g per 87 mL of water). With the help of Jeremy Anthony, I created a soluble liter of gallic acid with a molarity of 0.068. Using water to dilute the solution, we made an acetic acid of the same molarity and quantity.

On April the 13th, I began the project using Dr. Spaccarelli's house as the site of the experiment. At his house, I maintained four testing sites. One site in a forest; one site in a windowless barn; one site in a field; and a final site with a copper plate submerged in 100 mL of solution of the chemical. All these assorted sites were chosen because of the significant role the natural elements factor into the patina process. Sunlight and rain greatly enhance and quicken the patina process and I was interested in noting the effects of the different environments on the patina process. Twice a day for two weeks, I biked out to Dr. Spaccarelli's house and applied the acetic and gallic acids to different copper samples. The spraying usually occurred early in the morning, as the sun rose and then again in the evening around dusk. I had four samples at each site, two copper plates for the acetic acid and two copper plates for the gallic acid. The samples were sprayed liberally (roughly 10 mL) so that the entire sample was coated and then some solution excess pooled on the surface of the sample and could soak into the copper throughout the course of the day. I took observations and noted the changes in the copper samples. After the two week period, I collected the samples and submitted them to a colorimeter test.             

Materials:

Observations and Data

Hypothesis:     

My initial hypothesis was that both the acetic and the gallic solutions would radically transform the copper samples. I also predicted that the patina in the barn would be retarded (relative to the other samples) by its lack of exposure to light. Due to its constant immersion in the solution, I thought the samples in water would also react strongly.

During the testing, in every case, I found that the Gallic Acid reacted first. Within several days, the Gallic acid caused the copper to patina. I noted that when it did not rain, the salt residue remained on the samples. When there was rain, the salt deposits on the samples were washed away. Needless to say, the barn samples contained a salt residue at the end of the experiment as there was no rain interference. In the water samples, the liquid immediately assumed the color of the patina. The acetic acid produced a transparent light blue water. The gallic acid produced an opaque dark brown water

Gallic Acid Samples

Acetic Acid

Clearly, the gallic acid produced the most instances of solid patination, although scratching at the surface can easily remove the patina. I really find the water samples the most stunning with the burgundy color. The acetic acid reacted the best in water, scratching at the surface did not make the patina go away as easily as the gallic sample. Otherwise the patina was minimal at best. The field produced a faint cloudy patina, but the reaction was not noteworthy. 

L*a*b* Results

(L* defines lightness; a* denotes red/green attributes; b* denotes yellow/blue attributes)

In the gallic acid, the darkest patina goes to the water samples. The forest acetic acids were the darkest of the acetic acids, however even the darkest acetic acid was no where near the lightest gallic sample.

Conclusions

Initially, I had predicted a radical patination with both the gallic and the acetic acid; this assumption was clearly proven fallacious. While the gallic did react, the acetic acid produced very little tangible results. I did not expect this, nor did I expect that the gallic acid would patina so quickly. Before the experiment I had no clue what color patina would emerge from the process and the gallic acid was a nice patina. I especially noted the interesting potential of the gallic by looking on the back of the samples, while the dark sections are still present, other sections form a rich brown color, which is now coveted by Sanford McGee. I was right in thinking that the barn would react the least, although it was interesting to note the salt deposits as they were elsewhere washed away. Obviously, we can conclude that sunlight is a huge key to the patina process and allows the cooper plates to undergo a more radical transformation. I was also correct in assuming that the samples submerged in water would react strongly.

If I did this experiment again, I would make sure that the gallic acid sample did not darken my acetic acid samples as happened in the forest. Furthermore, I would probably not use the acetic acid. Acetic acid will react on copper, Sanford has assured me of this, nevertheless, the dilute version of acetic which I used was not concentrated enough to produce a meaningful reaction.

Enhancement:

Disappointed with the paltry results of the acetic acid, I was bored and looking for an enhancement. I was talking with an artist named Fehl Canon, an old Sewanee alum who now resides in Washington D.C., and related my experiment to him. He commented that Andy Warhol had once urinated on copper and used the resulting patina for some relief painting. Needless to say, when Andy Warhol urinated on the copper, it would turn a lovely blue-green color. I tried....and I agree.

Other References

Budavari, Susan, ed. 1996. The Merck Index 12 th ed. NJ: Merck & Co., Inc.

Copper Development Association. 2004. www.copper.org

Young, Ronald D. 2000. Contemporary Patination. CA: Sculpt Nouveau.

Acknowledgements

The author of this study would like to thank Dr. Bordley, Dr. Kirven, Dr. Spaccarelli, Sanford McGee, Fehl Canon, and Jeremy Anthony for their invaluable help with this project.