Patinas on Copper
Being able to choose from a wide variety of project topics was not easy due
to the fact that we have covered such a large array of content throughout the
semester. Thus I choose to examine two correlating topics that have sparked
the most interest to me, metals and their reactions to elements. Luckily, patination
combines both these topics and ultimately results in beauty. If you have ever
seen the Statue of Liberty, you have seen a patina. But you needn’t go
as far as New York to see patinas and the beauty they offer. They occur naturally
and artificially in places like gutters, roofs, furniture and nearly any metal
that is exposed to nature or exposed to the hands of an artist or scientist.
A patina is simply a film on a surface where compounds have reacted with the
air, thus forming acids that react with the metal. The colors of patinas are
dependent on the concentration level of active chemicals. Artists, who use patinas
in their art, perform patination with knowledge of the elements that form
the composition of the patina, for these elements determine the color of the
patina. Copper’s outer layer, for example,
develops a blue-green patina on its surface when patination occurs. It is
no coincidence that when found in nature, copper (II) oxide is blue-green.
But when artificially created, the shades of blue and green can be altered. In my project, I examined the effects of various patina formulas on the color formed on copper plates. My experiment is informative to both scientists, in the way it deals with the variables involved with the formation of patinas, and artists, in the way it correlates color of patinas with the formulas used to produce them.
Knowing little about the art of patination, I was advised by Dr. Bordley with ideas about patinas and perhaps more importantly, he loaned me a book by Ronald Young titled, Contemporary Patination. This book gave me the idea to examine the effects of different patina formulas on copper. After reading Ronald Young’s book, Contemporary Patination, I learned there are many techniques for applying patinas to metals. Facing a limited budget and lack of experience with toxic fumes, I chose to perform, what Young calls, the “Cold Patina Process.” Essentially, the only difference between the cold patina process and the hot patina process is the hot temperatures in the hot patina process, which cause the reaction to take less time. The variables in this project are the two different types of patina formulas. One formula is a vinegar-based formula consisting of 14.5 parts Ammonium Chloride, 14.5 parts Sodium Chloride, 64 parts Vinegar to 1 part Ammonia. This formula is called Light-Blue Green, and it will be recognized in the project as “LBG”. The second variable formula is Powder Blue – recognized throughout this experiment as “PB” – consisting of 33 parts water to 1 part Cupric Chloride and 6 parts Ammonium Chloride to 1 part Cupric Chloride. The constants in this experiment were the copper plates, the cleansing techniques of the copper plates, and the application device of the patina formulas. The varied results of these formulas provided me with the answer to which formula produced the richest patina. Although the human eye can determine which formula produces the richest patina, I used a colorimeter to confirm the distinctive patinas. This richness will be determined through the patina that yields certain greatest L*a*b* values.
|Pictured Below: PB patina formula (left) and LBG formula (right)|
3 copper plates
3 Natural Bristle Brushes
2 Containers and 2 tubs for patina formulas
After obtaining the materials, I cut the plates in half several times to provide more plates if an error occured and also for opportunities to enhance. After that, clean the plates. The cleansing will be done in an acid bath (white tub shown in the top of the picture below). This bath will be made with 10 parts nitric acid to 1 part distilled water [Presumably this should be 1 part nitric acid and 10 parts water JLB]. The plates are submerged for 90 seconds. The effects of the acid bath are clear immediately. I choose to use a nitric acid solution because it gives the copper a glimmer that is quite appealing to the eye.
During this interval, bubbles form on the plates, these bubbles must be removed with one of the natural bristle brushes to ensure no unwanted variables, such as altered texture of copper plate, for experimentation.
After the plates are clean, it is time to create the formulas that will later be used for patination. The PB formula will be produced first for a reason explained later. To produce the PB recipe, which is provided in Contemporary Patination, you simply add a reduced ratio of Cupric Chloride, Ammonium Chloride, and water. After weighing out 42.56 g of Ammonium Chloride and 7.094 g of Cupric Chloride, combine the two chlorides and then mix with 237 mL of water and stir until the chlorides have dissolved. This will form a clear, green solution. After finishing this formula, begin with the production of the LBG formula. Weigh out another reduced ratio, except this time using 200 mL of Vinegar, 45.4 g of Ammonium Chloride and 45 g of Sodium Chloride. Do not apply the 3.125 mL of Ammonia to this solution until immediately before the application onto the copper. This is because once the Ammonia is applied, the chemicals react quickly and expire before the application onto the copper plates.
*Picture of copper plates before the application of the patina formulas
Apply the LBG patina formula to the copper plates with a natural bristle brush. After sweeping patina formulas with the wet bristles over the plates, get the second, dry brush and uniformly spread the formula. Once the LBG formula has been applied to the respected copper plates, wash and dry the brushes. After that, apply the PB formula in the same manner to the copper plates as done earlier with the LBG formula. The PB formula can remain idle for any time necessary before the application, and still have the same effect as if it was applied immediately. This particular PB formula will be reused throughout the several applications of the formula. In contrast, the LBG formula must be made again for every application.
Let the formulas dry and observe the patination.
After studying the components of each patina formula, I predicted that if the LBG formula and PB formula are applied to copper plates, then the LBG formula will produce a deeper, richer patina than the PB formula. I will provide scientific results for this through the data from a colorimeter. The reason for my hypothesis is that the LBG formula contains higher concentrations of active chemicals.
Powder Blue (examples 1, 2)
|Powder Blue (examples 3, 4)|
*examples 1 and 3 were tested first, followed by examples 2 and 4 (24 hours after application)
Observations: About 90-100 seconds after the application of the PB formula, long streaks appear on the copper plates and the plates begin to discolor. Although there is no green visible to the human eye yet, there clearly is a distinction between where the patina will be richest on the plate and the certain spots that will lack the patina's richness. After about 25 minutes, the PB's patina starts to display subtle shades of green. The patination's color change seems to stabilize after about the 25 minutes mark. Checking back after 24 hours, I noticed a darker shade of green. There were splotches on these copper plates though, which I attribute to water accidentally splashing out of the sink, which was located only feet away from where my experiment was taking place. The PB offers a very uniform patination but lacks the richness that the LBG patina has.
|Light Blue Green|
|LBG (examples 1, 2)||LBG (examples 3, 4)|
*examples 1 and 3 were tested first, followed by examples 2 and 4 (24 hours after application)
The LBG formula offered an exciting patination. The LBG patina took about 7.5 minutes to display a color change. These changes came in the form of splatter marks that would expand over a period of about 10 minutes after the 7.5 minutes mark. At this point, the patination begins to form a green color. This color starts as a very soft, light green and seems to sprawl out over the splatter marks where it will form a deeper, richer, violet-green color on the edges of the splatter marks. It is not until the next day though, that the colors from the LBG formula have reached its potential richness in green.
There is a major difference between the patina formed on examples 2 and 4 from the patinas formed on examples 1 and 3. This is because I overestimated the amount of time the Ammonium Chloride would react with the other chemicals, thus not providing a very concentrated patina. Where the patinas did end up forming in examples 2 and 4 provided for the richest colors and the highest L* values of all the copper plates.
The data acquired from the colorimeter, and the judgments that can be made by the human eye, support my hypothesis that the LBG formula does in fact produce a richer patina on copper if applied properly. Due to the higher concentration of active chemicals, the LBG formula creates a richer patination. These values are not only displayed through the L* coordinates of the colorimeter, which signifies higher lightness, but also consistent values in the a* coordinates which, through its lower numbers, signify a color more closely representing green, the color of copper when found in nature. The main alteration I applied during this experiment was the application process of the patina formulas. As seen in picture three above, paint rollers were first used for the application of patina formulas. After this application did not thoroughly create a fast, distinctive reaction between the copper surface and the formulas, I choose to use natural bristle brushes, which worked much more efficiently. The manner in which I conducted the application of the LBG formula also changed as I gained knowledge about the Ammonium Hydroxide's reactivity. After seeing little reaction between the copper and the LBG formula, I became more attentive towards the manner in which the LBG application must be carried out; thus applying the formula immediately after the Ammonium Hydroxide was added to the solution. In regards to my original plan for this experiment, my final project changed quite a bit. I not only decided to shift away from testing the reactions of various metals with a constant patina formula, but I also added a more scientific element to this project by deciding to judge the patination by a colorimeter other than just an eye. This made my findings more credible. Overall, these changes greatly enhanced my project. I would highly recommend this project to someone else, with applying a couple alterations -- perhaps using the same patina formula makeups but altering the concentrations of the chlorides -- examining the differences in patina richness.
My enhancement for this project was to determine the effects of cleaning the copper plates with the patination process. This hardly affected the patination, which is clearly displayed through the colorimeter results:
Cleaned Copper Plate L*= 71.66 a*= -1.72 b*= -8.35
Uncleaned Copper Plate L*= 72.08 a*= 1.56 b*= -8.87
I choose to apply the LBG formula to these plates because they produce the richest patinas with the copper -- a fact revealed above. There was little difference between the L*a*b* coordinates of the cleaned and uncleaned copper plates, which suggests one of two things; either the copper picks up grime through the air between cleansing and application of the patina formula or the grime does not affect the copper's patination whatsoever.
These links were helpful to my experiment:
Young, Ronald D. Contemporary Patination. Escondido, CA: Scuplt Nouveau, 2000.
Sincere thanks to Prof. Fitz for holding back any frustration she might have had while helping me and to Dr. Bordley who got the wheels turning on my project.