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Patina application: quality as a function of both time and solution concentrationRichard Palmer |
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Introduction
My purpose in this undertaking was threefold: to gain an understanding of artificial patina application; to apply a green patina to bronze through cold bathes using a Cupric Nitrate solution; and finally, to measure the quality of that patina as a function of both time and solution concentration. I hypothesized that greater bath time would better produce a patina and that greater solution concentration would quicken the patination process. Further, a caveat: over time, because the process is a chemical bath, the use of greater solution concentrations will diminish the amount of control an artist/experimenter has over the outcome.
Background
A patina is the change of an object’s surface as a result
from oxidation over a period of time.
Normally this process is a result of natural aging producing
a brown, green, or reddish coloration.
Artificial patinas may be achieved by
applying acids or electrolytes to the surface of an object (in this case,
bronze.)
Artists working with metals in sculpture often attempt
to achieve patinas on objects of copper or bronze. This produces pieces
that looked aged or antique.
Cupric Nitrate (Cu(NO3)2) is
a water soluble reagent commonly used in cold process patina formulas to
produce a green coloration to a metal’s surface.
There are three main methods of patina application:
1) Hot patinas are applied to heated bronze with a brush or spray. This is regarded as
the fastest, most controlled process.
2) Cold patinas are applied to
bronze at room temperature with brush, spray, or through dipping. This
process requires stronger chemicals than hot patina application and is
less controlled.
3) Buried patinas are applied with brush or spray to bronze which is then wrapped and sealed in a container and
left to slowly color. This process is the slowest and least controlled.
Procedure
Brass
turnings were cleaned using Isopropyl alcohol, then weighed
into groups of 6g to be used in the bathes.
Cupric
Nitrate ground into powder using mortar and pestle.
1st bath:
quality as a function of time.
n 6 6g samples of brass placed into tray
n Cupric Nitrate solution made using 4.5g Cupric Nitrate into 100mL of water.
n Samples placed into bath for set times over 3-51/2 hours.
Sample
1: 3hr.
Sample 2:
3 ½ hr.
Sample
3: 4hr.
Sample 4:
4 ½ hr.
Sample
5: 5hr.
Sample 6:
5 ½ hr.
2nd bath:
quality as a function of time.
n 5 6g samples of brass placed into tray.
n Cupric Nitrate solution made using 4.5g Cupric Nitrate into 100mL of water.
n Samples placed into bath for set times over 8-12 hours.
Sample
1: 8hr
Sample
2: 9hr.
Sample
3: 10hr.
Sample
4: 11hr.
Sample
5: 12 hr.
3rd bath:
quality as a function of solution concentration.
n 3 6g samples of brass placed into bathes for 8hr.
n 3 Cupric Nitrate solutions made
Sample
1: 4.5g/100mL
Sample
2: 9g/100mL
Sample
3: 13.5g/100mL
Observations
made before, during, and after.
After
bathes, all samples were dried on paper towel sheets and
allowed to sit for 1hr before being re-weighed and placed into vials and
capped for observation.
Materials:
n Brass turnings
n Cupric Nitrate
n 8 plastic trays for bathes
n Plastic tweezers
n Glass stirring rod
n 100mL glass beaker.
n Isopropyl alcohol
n Paper towels
n Stopwatch
Observations and Data
Quality as a function of time: 3-5 ½ hr. bath:
Oxidation occurred producing brown coloration, though no distinct green coloration was achieved. Very little marked difference between ½ hour increments Some differences occurred in coloration over hour increments, though changes were not significant.
Quality as a function of time 8-12 hr. bath
n Again, green was not achieved through the oxidation of the bronze pieces.
n Significant change was made to individual pieces from samples 1 and 5, though no significant homogenous change was produced.
n Samples showed marked change in coloration turning from gold to brown, to a brown/silver over time.
Quality as a function of solution concentration- 8hr. bath
4.5g/100mL
concentration
n As with previous sample under same condition oxidation produced a brown coloration on the surface of the bronze pieces.
9g/100mL
concentration
n Resembles closely the coloration of the 4.5g/100mL 11 hr. sample
n Though not homogenous in coloration, many pieces show a silver coloration due to oxidation.
13.5g/100mL
concentration
n Sample shows perhaps the most progress toward a green coloration of all samples- the coloration resembles that of the previous sample, but a more homogenous coloration was achieved.
Results
Oxidation
occurred producing a brown to brown-silver patina on pieces allowed to soak in
the Cupric Nitrate bathes for prolonged periods of time. Also,
similar results were produced in samples soaked in bathes of higher concentration.
No
significant differences in sample weight change after bathes as
a result of oxidation.
Results
inconclusive as to quality of affected change due lack of green patina
formation.
Conclusions
The hypotheses proved to be correct in that greater
bath time and greater solution concentration did affect more change to
the bronze samples, though results proved to be inconclusive as
to the quality of that change.
The extended bathing process as well as testing bronze
samples in higher concentration seem to be a step in the right direction-
due to the relative weakness of the 4.5g/100mL Cupric Nitrate solution,
it is apparent that longer time periods within the bath are required for
the green patina to be formed. Further, a stronger solution may be appropriate
though extended periods with said stronger concentration may limit control
over the outcome of the patina.
With time, a cold bath may prove to be useful for formulation
of patina, though current results are inconclusive. Further testing still
needs to be done to determine this possibility.
The original project plan was designed to
test many different patina formulas, but after consideration, exploration
of a single formulas development, possibilities, and quality proved to
be a worthwhile task to illuminate the complexities of the patination process.
Links
http://w3.gsa.gov/web/p/hptp.nsf/0/afd01917bcb02a75852565c50054b7ac?OpenDocument
http://www.whiteriverfoundry.com/bronze_patina_basics.htm
Provides a basic understanding of bronze patinas.
http://www.goldreverre.com/technique/chemicalpatina.html
A general discussion of chemical patination processes.