Abstract

Monomolecular anti-tarnish and lubricant layer was utilized in the coining industry for more than ten years, saving millions of dollars each year. However, the benzotriazole (BTA) that formed the monolayer on cupronickel did not react with silver. In recent years, many chemicals were screened for evaluating the bonding strength on silver surfaces. Finally, a mono-octadecanethiol (ODT) film was solidly bonded with silver surfaces in the laboratory utilizing an ultrasonic bath. To apply this technology for industry, the process must be tested in a real production environment, since many unexpected issues needed to be solved just like applying BTA on cupronickel years ago. In this work, several bench tests were conducted on the silver production line at Sunshine Mint. The results illustrated that the mono-ODT film could be applied on silver surfaces with different processes and parameters according to the production conditions. The processes are simple, reliable, and low cost. Based on the test data, the process can soon be fully implemented on the production line.

1 Introduction

In 2010, a mono-benzotriazole (BTA) film was first applied on cupronickel coin blank surfaces at the United States Mint production lines after four years of research work in the laboratory. Since then, the processes were continuously used to save millions of dollars due to the low coefficient of friction on the interfaces between the die and coin blanks [13]. The residual film left on the coins protected them from tarnishing, as well. Based on these successful data and the thermodynamic model, the process was also used for the silver coin production. However, the BTA does not react with silver or gold, and it cannot form a self-assembled monomolecular layer film on their surfaces. After several years of screening different chemicals, the octadecanethiol had been proven as the best candidate. The results are consistence with many scientists’ findings in their research works on octadecanethiol (ODT) film [413]. There were many papers discussing the alkyl thiol application on silver and gold surfaces. Laibinis et al. [14] used a reflective infrared spectrum to measure the structure of the mono-ODT film on a silver surface. Similarly, Anderson and Gatin [15] used the FT-IR spectrum to measure the mono-ODT film and found the wave numbers of the absorbance peaks were around 2850 cm−1 and 2920 cm−1. In our earlier works [16], an advanced silver anti-tarnish fluid was developed. With the fluid, a mono-ODT film was formed on the silver surface within an ultrasonic bath. Similar to Anderson's work, the intensity of the absorbance peaks of CH3 and CH2 in the molecules was used to estimate the film thickness. In the FT-IR absorbance spectrum, the peak intensity at 2920 cm−1 is about 0.012 units. In the earlier work “Optimization of Lubricant Film Formation on Blanks,” a multiply alkylated cyclopentane formed a uniform monomolecular layer on a silicon wafer by a special freeze–evaporation process. The film thickness was confirmed using X-ray photoelectron spectroscopy. The same film was measured by FT-IR absorbance spectrum with an 85 deg grazing angle. Thus, the intensity of the peak in the FT-IR spectrum was calibrated. 0.001 unit in the absorbance is equivalent to 4 nm. Since the measured peak was 0.012 unit. The film thickness is around 40–50 nm. In this work, we assume the film thickness is the same as that formed in earlier work, since the test conditions and used chemicals are the same.

So far, all the mono-ODT film discussed above was formed in laboratories. Two critical issues needed to be overcome for normal production.

First, the process must be simplified. As Burieigh et al. [17] summarized, the film deposition had four steps:

  • Cleaning the surface—the chemicals used were different by the authors.

  • Etching the surface—the chemicals used were also different by the authors.

  • Appling the ODT on the surface—no method was found how to control the layers of the ODT molecules.

  • Rinsing the surface.

For production, the first three steps were combined into one. In the ultrasonic bath tank, the advanced silver anti-tarnish solution worked as a cleaner, etcher, and ODT molecule supplier. As discussed in earlier work, extra layer remover (ELR) was an important chemical. In the aqueous solution, when molecules tangentially moved on a surface, ELR also had the chance to collide with the silver surface and some residual chemicals on it. Depending on the bonding energy, some thin oxide layer and physically absorbed chemicals reacted with ELR and were dissolved in the water. The bonding between sulfur and silver was strong. Once they reacted, ELR could not open the bond. As a result of dynamic competition between the film reaction and desorption, only a mono-ODT reacted film stayed on the surface after the bath.

Second, the time to form the film must be short. In the papers aforementioned, the film was formed in hours or even days. Most of the films were deposited in either an aqueous solution or an organic solution. The vapor deposition process was another way to coat an ODT film on a silver surface that was mentioned by Love et al. [18]. Lu et al. [19] used a silver sheet as an electrode to increase the deposition rate in an aqueous solution. The deposition time varied from a few seconds to 1 day. The film properties might change with the deposition time. In most cases, ultrasonic bathing is used to clean the contaminated surfaces. In this work, the chemical reaction rate between ODT and silver surface was enhanced by the collapsing of ultrasonic cavities. The molecular frictional movement by the pressure waves increased the surface entropy (collision chances). Therefore, a closely packed mono-ODT film could be formed in a few minutes. The short film formation time made the process possible for the production.

In the above papers, many state-of-the-art instruments, such as XPS, EIS, FT-IR, AFM, etc. were used to detect the ODT film. As well, different techniques were used to evaluate the anti-tarnish property of the film. Liang et al. [20] used electrochemical measurements in 0.5 mol/l NaCl solution containing 10 mol/l Na2S to evaluate the film and it needed 48 h to complete the tests. Jalal Uddin et al. [21] used electrochemical impedance spectroscopy (EIS) to assess the coverage of the film. A 2% K2S solution was used to evaluate the thiol film on the silver surface in Nineva et al.'s work [22]. Grissom et al. used 6 ppm concentration of H2S in a chamber to tarnish the silver surface [23]. The colors of the samples were used to evaluate the surface protection after a 24 h test.

In this work, on the production line, it was impossible to use a sophisticated instrument to measure the film in time. But we must find a method to ensure or prove that the mono-ODT film is really existing and protecting the silver surface. For that purpose, a rapid, simple, safe, reliable, and portable accelerated sulfurization chamber was made. Coins both with and without the mono-ODT film are tested in the chamber for 1 h. By comparing the colors and reflectivity of the coins before and after sulfurization tests, the anti-tarnish properties of the film can be evaluated.

2 Experimental Procedures

Silver anti-tarnish bench tests were conducted at the Sunshine Mint production line. Different from the work at the laboratory, the time efficiency for production must be considered. How to apply the tribology principle proven in the lab to practical everyday production may be more difficult, as too many factors may affect the results. The process must be simple and cost-effective; otherwise, the process will be cost-prohibitive and not commercially viable in a competitive marketplace.

The solution obtained in our earlier work “Advanced Anti-tarnish Fluid for Silver” was used in the production tests. The blanks were ultrasonically bathed in the same two-dimensional, two-frequency ultrasonic tank. The ODT molecules in the advanced silver anti-tarnish fluid form a monomolecular film on the silver surface in the ultrasonic bath. The fluid is an emulsified ODT aqueous solution. The chemicals were discussed in that work. The detailed concentration of the chemicals is omitted since its potential commercial application. The blanks were put into a special rack. When the blanks were placed in the rack, there were only six small contact points to keep the part in position. With a little movement, the fluid was able to flow into the contact points. Thus, the whole blank would be sufficiently coated.

Since the melting point of ODT is 33 °C, to avoid solid phase appearing, the ultrasonic bath temperature was set at 50 °C, which matched the hot water temperature at the Sunshine Mint production floor. The total amount of the solution is 3.65 liters. The ultrasonic bath time was 5 min. After bathing, the blanks or coins were rinsed in 50 °C water, then they were sprayed with deionized (DI) water and towel dried.

An accelerated sulfurization chamber was constructed. The H2S gas or sulfur atom in water vapor was generated from the cut hardboiled egg. The method was adopted from Selwyn on the Canadian government website [24]. Since there is an average of 75 mg sulfur in each egg after boiling [25], once the egg is cut, the H2S is released in the vapor. Although humans do not notice the sulfur in the vapor when they boil eggs, the silver surfaces are very sensitive to it. When vapor from the boiled egg is condensed on the silver surfaces, they are almost immediately sulfurized. It is the easiest and the safest way to get the H2S gas. Several 25 mm long and 12 mm width silver sheets were repeatedly sulfurized in the chamber. The results of the surface tarnishing were very consistent. Thus, this accelerated sulfurization test was used to evaluate all the silver anti-tarnishing properties in the bench test on the production line. The temperature, humidity, and hydrogen sulfide gas concentration were measured. A turnkey sulfurization chamber is built and will be used in production based on the data obtained in the bench tests.

The chamber is a glass container. The coins are set on a perforated stainless-steel sheet. The cut hardboiled egg is put under the sheet. A glass is mounted on the lid as a window. Above the glass, there is a camera mounted that can shoot time-lapse photos. After a 1 h test, the photos that are shot every five seconds can be edited into a video that vividly records the color change during the test. There is also a small fan to blow the fresh air in. On the other side of the glass, there are a few small holes to let the vapor out. In this way, the moisture level in the chamber can be controlled. Currently, the vapor is controlled manually. When too many condensed water drops appear on the coins’ surfaces, the fan is turned on. When all the water drops are gone, the fan is turned off.

Before the accelerated sulfurization chamber was used on the production line, it was first tested in the laboratory to check the repeatability and reliability with different materials. For example, the United State Mint silver “Quarter Dollar” coins were sulfurized with and without a mono-ODT film. Silver quarters are composed of 90% silver and 10% copper, a common alloy known as 900 Coin Silver. One Quarter Dollar coin was coated with a mono-ODT in the ultrasonic bath, and the other was an original one just picked up from the capsule. After a 1 h sulfurization test, the coin with the film almost kept the initial color. Without the film, the coin surface was completely sulfurized as shown in Fig. 1. The test was repeated, this time, the positions of the tested coins in the sulfurization chamber were exchanged. The comparison photos in Fig. 2 proved that the position in the chamber did not affect the tarnishing results. The temperature, humidity, and concentration of H2S were measured and recorded. A turnkey sulfurization chamber is built according to the data and will be used soon on the production line.

Fig. 1
(a) A US Mint silver proof quarter coin was ultrasonically bathed in ODT solution for 5 min (right) that was compared with an uncoated one (left) before sulfurization test in the laboratory and (b) after the 1 h sulfurization, the uncoated coin was completely tarnished, but the one with the film kept its original color
Fig. 1
(a) A US Mint silver proof quarter coin was ultrasonically bathed in ODT solution for 5 min (right) that was compared with an uncoated one (left) before sulfurization test in the laboratory and (b) after the 1 h sulfurization, the uncoated coin was completely tarnished, but the one with the film kept its original color
Close modal
Fig. 2
The test in Fig. 1 was repeated. The coin positions in the sulfurization chamber were exchanged. Coated coin (left) and uncoated one (right). (a) Before the sulfurization test and (b) after the sulfurization test.
Fig. 2
The test in Fig. 1 was repeated. The coin positions in the sulfurization chamber were exchanged. Coated coin (left) and uncoated one (right). (a) Before the sulfurization test and (b) after the sulfurization test.
Close modal

3 Results

3.1 Form a Mono-Octadecanethiol Film on Product Coins by Ultrasonic Bath.

As the sulfurization process was stable and reliable. This process was directly used on the production line. At Sunshine Mint, the stamped coins were directly picked up from the press, and then ultrasonic bathed in the advantage silver anti-tarnishing solution. The parameters were the same as those in the laboratory.

The 1 h coin sulfurization test process was recorded by the video. The video is uploaded on YouTube1. In the video, the moisture formed by the hardboiled egg was condensed on the surface of the coins. It was blown away by the air from the small fan. The coins with and without the monomolecular ODT film before and after the sulfurization test are shown in Fig. 3. With the protective film, the color did not change.

Fig. 3
(a) The coins were ultrasonically bathed for 5 min in ODT solution on the production line. One of them (right) was compared with an uncoated one (left) before the sulfurization test and (b) after the 1 h sulfurization test, the uncoated one was completely tarnished, with the ODT film the coin kept its original color
Fig. 3
(a) The coins were ultrasonically bathed for 5 min in ODT solution on the production line. One of them (right) was compared with an uncoated one (left) before the sulfurization test and (b) after the 1 h sulfurization test, the uncoated one was completely tarnished, with the ODT film the coin kept its original color
Close modal

It is very difficult to prove a film is a monomolecular layer, even with state-of-the-art instruments. However, during the coin stamping process, whether a film was a monolayer can be easily identified. If there is a second molecular layer physically absorbed on the first chemically reacted layer, during the stamping, this layer may be transferred to the die surface under the high pressure and shear stress. As the striking continues, layer after layer, the molecules are accumulated on the die surface very fast. After a few strikes, the surface morphology change on the stamped coins can be identified by operators. They must stop the coining and use white paper to clean the die surfaces. The white paper was blackened when the accumulated layers were wiped off. When this happened, the detail of the coating process must be adjusted, until no black color was found on die surfaces. It meant that only a mono-ODT layer was allowed on the blanks. For example, in the beginning, the rinse DI water used from the tank was not heated. The temperature was about 30 °C. Once the samples were pulled out of the ultrasonic tank and immersed in the rinse water, small amounts of residual ODT immediately solidarized on the surface. The melt temperature of ODT was 33 °C. These molecules could not be seen by eyes on the blanks, but they caused the die surface to be blackened during coining. Once a heater was added to the DI water supply, the problem was solved.

3.2 Form a Mono-Octadecanethiol Film on Production Blanks by Burnishing With Media.

In the aforementioned test, the blanks were coated in the ultrasonic bath. As stated above, it was time-consuming to put the blanks into the rack. On the production line, a more efficient process should be used. In an earlier paper, the burnishing process was used to apply benzotriazole on cupronickel material. The mechanisms of ultrasonic cavity collapsing and the stainless-steel beads sliding are the same. The surface shearing stresses move the molecules physically absorbed on the surface tangentially. The movement created many more collision chances between the ODT molecules and silver atoms exponentially, and so did the chemical reaction. The new chemically reacted silver-octadecanethiol molecules could be formed on a large number of blanks in a short time. Since the mechanical scars created during burnishing could be ironed down by the stamping, the burnishing process provided a more efficient way for the sake of production. 500 ml of the fluid was used to burnish 200 blanks in a small bowl burnishing machine. The burnishing time was 5 min. After burnishing, the blanks were rinsed for another 4 min, and then towel dried. One burnished blank and one normal production blank were tested in the sulfurization chamber. After the test, the burnished blank kept its initial condition, but the production one was completely tarnished.

The coated blanks were directly sent to the press for stamping. Due to the 18 carbons in the hydrocarbon chain, the coefficient of friction on the blank surface was reduced. However, the silver is a solid lubricant. Whether the friction difference (with/without the film) affected the die life or the coin quality could not be evaluated over the course of a few hundred strikes. The friction effect will be discussed when the process is setup on the production line and 1000 coins are stamped. At this time, only the surface color changes were compared to prove the mono-ODT layer truly existed. Although the mono-ODT coated blanks did not change color after the sulfurization test, the coins stamped with the coated blank showed light yellowness after the 1 h test.

3.3 Form a Mono-Octadecanethiol Film by Burnishing Blanks and Ultrasonic Bathing Coins.

The tarnishing of the coins stamped with ODT fluid burnished blanks indicated that the mono-ODT film on the blanks only partially protected the surface. The suspected reason was the surface area increasing during the stamping. For a good mono-ODT film to protect the surface, the molecules must be closely packed so that the H2S could not penetrate through it. During the stamping, there were two kinds of surface area increases. One increment was from the metal plastically flowing from center to edge which caused the diameter to be enlarged. The other one was from the relief filling of the embossed images which caused the flat surface to become a dome.

The surface area was increased, but the number of molecules that reacted with the surface was not increased. The mono film was not closely packed anymore. H2S or S atoms in the vapor had a chance to penetrate the loosely packed mono-ODT film and tarnish the silver surface. Thus, the surface was partially damaged.

This problem could be solved by an additional ultrasonic bath after the blanks were stamped into coins. When the loosely packed mono-ODT film was further ultrasonically bathed in the ODT solution again, the collapsing of the cavities dynamically moved the molecules, and some holes could be refilled by the extra ODT molecules during the bath. Thus, the damaged closely packed film was restored. This kind of film might be the most reliable anti-tarnish film for silver coins.

To prove this concept, the stamped coins with the burnished blanks were further ultrasonically bathed in ODT solution for 1.5 min, rinsed in 50 °C DI water, and towel dried. Three coins were tested in the accelerated sulfurization chamber together.

  1. The blank was burnished and the coin was ultrasonically bathed.

  2. The blank was burnished only, but the coin was not ultrasonically bathed.

  3. The blank was not burnished and the coin was not ultrasonically bathed.

Since the initial window for the sulfurization chamber was designed only for comparing two coins, for the three coins test, it could only expose part of the three coins. Anyway, most areas of the coins were visible and comparable during the sulfurization process as shown in Fig. 4. In the figure, the top coin was stamped with the ODT fluid burnished blank. After stamping, the coin was ultrasonically bathed in ODT solution. The right bottom one was stamped with an ODT fluid burnished blank, but without ultrasonic bath after stamping. The left bottom one is the normal production coin without any ODT protection. The accelerated sulfurization test was also recorded by time-lapse photos and edited to a video posted on YouTube2. To show the whole coin images, after the sulfurization test, the three coins were shot at a 30 deg angle and then compared in one row as shown in Fig. 5.

Fig. 4
Three coins have different treatment histories. (Top) The blank was burnished in the ODT solution for 5 min. After stamping, the coin was ultrasonically bathed in ODT solution for another 5 min. (Right bottom) The blank was burnished in ODT solution for 5 min only before striking. (Left bottom) The normal production coin without any ODT treatment. (a) Before the sulfurization test they were the same and (b) after the sulfurization test, only the coin treated twice kept its original color.
Fig. 4
Three coins have different treatment histories. (Top) The blank was burnished in the ODT solution for 5 min. After stamping, the coin was ultrasonically bathed in ODT solution for another 5 min. (Right bottom) The blank was burnished in ODT solution for 5 min only before striking. (Left bottom) The normal production coin without any ODT treatment. (a) Before the sulfurization test they were the same and (b) after the sulfurization test, only the coin treated twice kept its original color.
Close modal
Fig. 5
The whole coin images in Fig. 4(b)
Fig. 5
The whole coin images in Fig. 4(b)
Close modal

By comparison, the coin stamped by the burnished blank, and ultrasonically bathed had the best anti-tarnishing property. After the sulfurization test, it did not change the initial sheen and color. The coin stamped with only a burnished blank was partially tarnished. Without any ODT film protection, the normal production coin was completely tarnished. The result proved the hypothesis that a loosely packed mono-ODT film could not completely protect a silver surface. The damaged film by surface area increment could be recovered by an additional ultrasonic bath. This process should be implemented in the production line.

3.4 Form a Mono-Octadecanethiol Film in One Minute Ultrasonic Bath.

According to lean production management, the time for each different process on a production line should be equal. The above test suggested that the ultrasonic time for coating a mono-ODT film might be less than 5 min.

In most silver coin production, the stamping is a semi-automatic process. After each strike, the coin is automatically moved out from the striking area and slid into a multiple-column tray. When a column is full, the tray has shifted a distance and the coins slide into the next empty column. When the whole tray is filled with new coins, the operator removes the tray, and a new tray is fed into the position. The time to fill a full tray is about 1–2 min depending on different presses. Therefore, if the ultrasonic bath time can be reduced to 1 min or 2 min, the time to coat a mono-ODT film on all the new coins filled in the tray can be matched with the time to stamp them. Thus, the production will be more efficient.

The rack to hold coins for ultrasonic baths was redesigned. The tray used on a press was modified as a rack. After the tray was fully filled with the coins, a lid would cover the tray. And the tray with the coins covered by a lid was removed from the press together and dipped into the ultrasonic bath directly. Most coin surfaces were exposed to the solution. With this rack, no time would be wasted on coin packaging.

To match the filling time of the rack on the coining press, the 1 min and 2 min ultrasonic baths were tested. With this short time bath, the products could flow smoothly. One batch of the coins was bathed for only 1 min, and the other batch was bathed for 2 min. After the coins were coated with a mono-ODT film, three kinds of coins were tested in a sulfurization chamber.

  1. Normal production of silver coins without mono-ODT film.

  2. The coins were ultrasonically bathed for 1 min in ODT solution.

  3. The coins were ultrasonically bathed for 2 min in ODT solution.

The appearances of the obverse and reverse sides of them before and after the test were inspected. The color changes were similar to the 1.5 min test. From the sulfurization test results, when the coins were ultrasonically bathed in the solution for 1 min, the closely packed mono-ODT film was formed that could protect the silver surface from tarnishing.

The above bench test data show the flexibility of the formation of the mono-ODT film. It can be formed by burnishing or by a very short time ultrasonic bath. Once the film is formed, it can protect the silver surface. Thus, for different production lines, the process can be varied according to the working conditions. This is very important. There are many mints in different countries, and the coining processes also vary from mint to mint. The problem is that all the silver coins produced from the mints around the world are without a protection film. The coins are easily tarnished in the atmosphere. Even in the protective capsules, the edge near the gap of the capsule will tarnish in a short time.

3.5 Form a Mono-Octadecanethiol Film on Gold Products by Ultrasonic Bath.

Gold coins might also become tarnished, giving the appearance of red spots randomly found on the surfaces. Due to the high prices of the gold coin, the buyers always suspected the purity of the material. Many researchers studied the tarnish (red spots) and found that the cause was the tiny quantity of silver sulfide on the gold surface even though the gold purity was higher than 99.99% [2628]. Griesser further indicated that the inclusion of silver embedded in the gold had two phases [29]. At the top, the silver was sulfurized to Ag2S, and beneath the layer, it was pure silver. And the depth of the silver might be as thick as 0.3 0.4 µm. To eliminate the “red spots”, during the production, the gold coin blanks need to be etched in nitric acid for a relatively long time, until the silver inclusions are completely dissolved away. Since on the production line, the mono-ODT film was proven to protect the silver coins from being sulfurized, it should also work for eliminating the “red spots” on gold coins.

The process was the same as mentioned in the experimental procedure. Twenty-four gold bar blanks were ultrasonic bathed in ODT solution for 5 min each batch, rinsed in 50 °C DI water, and toweled dry. After the mono-ODT layer was coated on the blanks, they were directly struck on the press. During the stamping, two phenomena were observed. First, the die surfaces were not blackened. As discussed above, the clean die surface indicated that the ODT coating on the blank surface was monomolecular, there were no extra ODT molecules accumulated on the die surfaces strike by strike. Second, the surface morphology was improved due to the low friction. The silver is a solid lubricant, which means that with or without a mono-lubricant layer between the die and surface, the coin image looked the same. For gold, however, with or without a mono-ODT film, the friction was different. The lower the friction was, the easier the plastic deformation flew, and the smoother the stamped surfaces were. The whole gold bar comparison with/without the film is shown in Fig. 6 and local comparison is shown in Fig. 7. The left one was stamped with coated film, the right one without film. Most fine scratches were ironed down due to the mono-ODT film. Similar to silver coins, evaluating the mono-ODT effect on wear needs 1000 strikes. It can only be accomplished when the process is completely implemented on the production line.

Fig. 6
(Left) Before stamping, the gold blank was ultrasonically bathed in ODT solution for 5 min and (right) normal product blank. After stamping, with the ODT film, the image on the gold surface was smoother.
Fig. 6
(Left) Before stamping, the gold blank was ultrasonically bathed in ODT solution for 5 min and (right) normal product blank. After stamping, with the ODT film, the image on the gold surface was smoother.
Close modal
Fig. 7
Local comparison between the two stamped gold bars shown in Fig. 6
Fig. 7
Local comparison between the two stamped gold bars shown in Fig. 6
Close modal

Half of the stamped gold bars were coated with the mono-ODT layer again. Half of them were kept as stamped. More than ten samples were collected from both batches for the accelerated sulfurization test. After the sulfurization tests, no red spots were showing up on any of the stamped bars. Since the red spots randomly show up, the results of the gold bar sulfurization tests were not sufficient to prove that a mono-ODT layer could prevent the gold surface from tarnishing.

Two defective gold bar blanks were collected from the vault. There were many small red spots on the surface as shown on the left side of Fig. 8. Magnesium was used to react with sulfur and reduce the silver. After the reaction, the red spots on the top of the inclusions disappeared as shown in the middle of Fig. 8. The two blanks then were ultrasonically bathed in the ODT solution. After the blanks were coated with a mono-ODT layer, they were tested in the accelerated sulfurization chamber for 1 h. Since there was a mono-ODT layer, the surface of the tiny silver was protected from tarnishing. The red spots did not show up on the gold blank surfaces as shown on the right side of Fig. 8. The results indicated that the process could be used for gold product lines to replace the nitric etching. As well, the surface quality of the gold products would be improved.

Fig. 8
(Left) The gold bar blank with defects of red spots, (middle) the Ag2S was reduced to Ag, red spots disappeared and (right) the same blank was ultrasonically bathed in ODT solution for 5 min, then sulfurized in the chamber for 1 h, the red spots did not re-appear
Fig. 8
(Left) The gold bar blank with defects of red spots, (middle) the Ag2S was reduced to Ag, red spots disappeared and (right) the same blank was ultrasonically bathed in ODT solution for 5 min, then sulfurized in the chamber for 1 h, the red spots did not re-appear
Close modal

4 Discussion

The mono-octadecanethiol film can be applied on silver or gold surfaces by different methods. To evaluate the film's anti-tarnishing function, seven silver coins (1 oz) from different countries and one US silver proof quarter coin were collected. They were put in the accelerated sulfurization chamber with the Route 66 silver round (1 oz) made by Sunshine Mint as shown in Fig. 9. The color and reflectivity looked the same. The frosted surface and polished surface on the coins have different appearances, but the reflectivity of the coins was almost the same, ranging from 92% to 96%. The blanks for the Route 66 silver rounds were burnished with ODT fluid before stamping and ultrasonically bathed in the solution for 1.5 min after stamping. The surface had the best anti-tarnish mono-ODT film. After 1 h sulfurization test, all the coins changed their color as shown in Fig. 10, and only the Route 66 silver round made by Sunshine Mint kept its initial color without any change. The video of the sulfurization process was uploaded to YouTube3. As seen in the video, there were small water drops condensed on the coin surfaces. Most of the water drops later evaporated but left some watermarks on the coin surface. For the Route 66 round, due to its low surface tension property, no watermark was found on it.

Fig. 9
The silver coins from different Mints around the world. From left top to right bottom, the coins are Australian Mint Kangaroo coin, Poland Mint Germania coin, China Mint Panda coin, US Mint Eagle coin, Sunshine Mint Route 66 round, Austrian Philharmonic coin, US Mint quarter coin, Royal Mint Britannia coin, and New Zealand Turtle coin
Fig. 9
The silver coins from different Mints around the world. From left top to right bottom, the coins are Australian Mint Kangaroo coin, Poland Mint Germania coin, China Mint Panda coin, US Mint Eagle coin, Sunshine Mint Route 66 round, Austrian Philharmonic coin, US Mint quarter coin, Royal Mint Britannia coin, and New Zealand Turtle coin
Close modal
Fig. 10
After the 1 h sulfurization test, all the coins changed their color, except Sunshine Mint Route 66 silver round
Fig. 10
After the 1 h sulfurization test, all the coins changed their color, except Sunshine Mint Route 66 silver round
Close modal

The light reflection of all nine coins before and after the sulfurization test is listed in Table 1. Since we do not know the details of the processes of other mints, and we do not know what chemicals were used in their production, the reflection of the coins varied one by one. The Britannia coin made by the Royal Mint has the worst anti-tarnish protection. The reflectivity of the Route 66 silver round does not change at all.

Table 1

The measured reflectivity (%) of nine coins

Name of the coinBefore firstBefore secondBefore thirdAfter firstAfter secondAfter thirdBefore averageAfter average
Route 66 round94.7794.0795.2995.595.5195.1694.795.4
Chinese Panda96.2896.8696.0782.184.7683.6896.483.5
Australia Kangaroo95.9892.8795.1280.378.6278.4194.779.1
New Zealand Turtle96.4896.8996.7675.566.2374.9596.772.2
Poland Germania93.8596.3894.5869.967.6972.5494.970.0
US Eagle94.394.7892.4968.4865.3666.4693.966.8
Austrian Philharmonic95.1994.9595.7857.5154.8765.7995.359.4
US quarter95.2194.8595.4960.0158.7559.3795.259.4
Royal Mint Britannia93.0491.5193.4156.5157.6661.9692.758.7
Name of the coinBefore firstBefore secondBefore thirdAfter firstAfter secondAfter thirdBefore averageAfter average
Route 66 round94.7794.0795.2995.595.5195.1694.795.4
Chinese Panda96.2896.8696.0782.184.7683.6896.483.5
Australia Kangaroo95.9892.8795.1280.378.6278.4194.779.1
New Zealand Turtle96.4896.8996.7675.566.2374.9596.772.2
Poland Germania93.8596.3894.5869.967.6972.5494.970.0
US Eagle94.394.7892.4968.4865.3666.4693.966.8
Austrian Philharmonic95.1994.9595.7857.5154.8765.7995.359.4
US quarter95.2194.8595.4960.0158.7559.3795.259.4
Royal Mint Britannia93.0491.5193.4156.5157.6661.9692.758.7

The data are plotted in Fig. 11 for easier comparison. The Route 66 silver round has the best anti-tarnishing protection. A specification can be setup from the reflectivity measurement data.

Fig. 11
The change in reflectivity of nine coins before and after the sulfurization test
Fig. 11
The change in reflectivity of nine coins before and after the sulfurization test
Close modal

After a sulfurization test, if the reflectivity is higher than 90%, the products are passed, otherwise, the coins are failed. The silver coins will never have customer complaints about the tarnishing issue.

The surface water contact angles of the coins were measured, and the values are listed in Table 2. The best and worst of water contact angle images are shown in Fig. 12. The ranking is similar to the reflectivity shown in Table 1. The values of the angles explained why after the sulfurization test there were water spots on some of the coin surfaces.

Fig. 12
(Left) The surface contact angle of the tested Route 66 silver round that has the highest contact angle θ, 90.1 deg and (right) the surface contact angle of the tested Royal Mint Britannia that has the lowest one, 52.9 deg
Fig. 12
(Left) The surface contact angle of the tested Route 66 silver round that has the highest contact angle θ, 90.1 deg and (right) the surface contact angle of the tested Royal Mint Britannia that has the lowest one, 52.9 deg
Close modal
Table 2

The values of the contact angles

NameDegree (°)
Route 66 Round90.1
China Panda72.5
Austrian Philharmonic72.2
New Zealand Turtle65.0
Australia Kangaroo59.8
US Eagle57.6
Poland Germania53.1
Royal Mint Britannia52.9
NameDegree (°)
Route 66 Round90.1
China Panda72.5
Austrian Philharmonic72.2
New Zealand Turtle65.0
Australia Kangaroo59.8
US Eagle57.6
Poland Germania53.1
Royal Mint Britannia52.9

The cost to purchase a silver or a gold coin is many times greater than its face value, and in all cases, much greater than its intrinsic value. The problem of the silver coins tarnishing quickly can make the customer very frustrated, especially on high-value collectible pieces. After customers enjoy the initial brilliance outside the capsule, the coins will change their color in less than one month if they forget to immediately put them back into the capsule. Even in the capsule, after years, near the edge, there will be a brown color ring, and for gold coins, there will be red spots, since the H2S can flow into the capsule through the gap. Once the coins have been coated with a mono-ODT film on the production line, the sulfurization will not be a problem anymore. The coins can be exposed to the normal atmosphere outside of the capsules without tarnishing for years.

5 Conclusions

The mono-octadecanethiol film can be applied on silver or gold coins or blanks by burnishing or ultrasonic bath if molecules have a tangential movement along the surfaces. If the closely packed mono-ODT film is formed on the surface, it will protect it from tarnishing. For coining production, if blanks are burnished in the ODT fluid, and after stamping, the coin is further ultrasonically bathed in the ODT solution, the silver surface will not be sulfurized. The bench tests provide sufficient data to implement the process into the production line. After this process, the silver and gold coins will never be tarnished.

Footnotes

Conflict of Interest

There are no conflicts of interest.

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