Abstract
Have you ever wondered if a piece of jewelry is real gold or if it's just some ordinary metal alloy? It turns out that some metals have a unique property; even in small amounts, they can be toxic to some organisms, including algae, molds, fungi, and bacteria, although it often takes many hours to see an effect. Can this phenomena, called the oligodynamic effect be used to tell whether or not the gold or silver in a piece of jewelry is real? Do bacteria react differently to pure, plated, and non-gold jewelry? Find out for yourself in this unique treasure-testing science fair project!Summary
Recommended Project Supplies
Objective
Determine if the antimicrobial properties of metals are a good test for whether or not a piece of jewelry is made out of real gold.
Introduction
Have you ever looked at a piece of gold jewelry and wondered "Is it real?" To answer that question, you might first have to decide on the definition of "real." Gold jewelry is usually described in karats (kt). 24-kt gold is pure gold. While theoretically it is 100% gold, most countries allow a 1% margin for impurities. Pure gold is relatively soft and easily dented, which means that jewelry made from pure gold is easier to accidentally damage. So gold is often mixed with a variety of metals, including copper, zinc, and silver, to make jewelry of different physical strengths, karats, colors, and monetary values. There are also cultural influences over what people think of as "real" gold jewelry. In most Asian countries, "real" gold jewelry is usually 21-24 kt. In the United States, "real" gold jewelry is normally 10 kt, 14 kt, or 18 kt. However, looks are rarely a good way to judge if a piece of jewelry contains a significant proportion of gold. Pieces that are less than 9 kt (the international cutoff for "real" gold jewelry), or that are gold-plated (meaning they have a thin layer of gold put over the surface of a less-expensive metal), or that are even made of brass, can still have the shiny appearance of yellow-gold.
So, how can we differentiate between real gold jewelry and all the others? Because gold is a natural element (number 79, atomic symbol Au on the periodic table of elements), it has specific chemical and physical properties. Jewelers and laboratories that specialize in testing metal use these properties to decide whether a sample is made of gold, and what the level of purity, or karatage, of the gold is. These tests include properties like how the sample scatters light from X-rays, how heavy the ions released from the sample are, and the exact weight of the sample once it is purified.
Figure 1. Looks alone aren't enough to tell which of these pieces of jewelry are gold and which are merely gold-plated, or other metals altogether. From left to right, these pieces are made up of other metal alloys, gold plating, and 24-kt gold.
It turns out that in addition to these physical and chemical properties, gold has a biochemical property: it is toxic to some life forms. Although it is completely harmless to humans and other mammals, gold can be harmful to bacteria, algae, molds, and fungi. Some other metals, including nickel, copper, cadmium, and mercury, also have this property. Collectively, metals capable of causing harm to organisms are often referred to as heavy metals or toxic metals and many of them, like mercury and cadmium, are harmful to humans, too. Usually, it is not the metal that is toxic, but the ions it forms when in contact with water. An ion is an atom or molecule in which the total number of electrons does not equal the number of protons, which gives it a net negative or positive charge. These ions are easily transported into tissues and cells where they interfere with normal functions. The exact mechanism of their interference depends on the type of metal ion, and on the organism under attack. This toxic effect of metal ions, even in very low concentrations, on bacteria is called the oligodynamic effect. The word oligodynamic comes from the Greek words oligos, meaning "few," and dyanmis, meaning "force."
In this science fair project, you will investigate whether the oligodynamic effect of gold on bacteria can be used to determine whether a piece of jewelry is composed of gold, is just gold-plated, or is not gold at all. To do this, you will use a harmless strain of the bacterium Escherichia coli (E. coli). You'll spread the bacteria uniformly across four agar plates: one with no jewelry, to serve as a control, one with gold jewelry, one with gold-plated jewelry, and the last with non-gold jewelry. In the plates containing jewelry, the water in the agar will combine with the metal to form metal ions. Don't worry, it won't harm your jewelry! The amount of metal that is converted into metal ions is so small you wouldn't even be able to measure the loss using the most sensitive of scales. These metal ions will diffuse out from the jewelry into the agar where they can be absorbed by the bacteria. The concentration of metal ions will be highest right next to the jewelry and gradually reduce as you move out in concentric circles from the jewelry. If the metal ions produced have an oligodynamic effect, meaning if they're toxic to the E. coli, the bacteria will not grow where the ions are present. The areas without bacterial growth are referred to as zones of inhibition and are characterized by a clear circle around the jewelry in an otherwise contiguous lawn of bacteria. The larger the zone of inhibition, as measured by the diameter of the clear circle around the jewelry, the greater the oligodynamic effect. Will the different types of jewelry (gold, gold-plated, and non-gold) exhibit different degrees of oligodynamic effect? Can zones of inhibition be used to determine whether a piece of jewelry is "real" gold? Try it and find out for yourself!
Figure 2. This photo shows zones of inhibition, the clear circles surrounding the white disks in what is otherwise a lawn of bacteria, for a Kirby-Bauer disk-diffusion assay. The procedure for this science fair project is an adapted version of that assay. In both cases, the size of each zone of inhibition is measured with a ruler, and the data is recorded in a lab notebook. (D.M. Rollins and S.W. Joseph, 2000.)
Terms and Concepts
- Gold
- Karat (kt)
- Gold-plated
- Brass
- Periodic table of elements
- Biochemical
- Heavy metals
- Toxic metals
- Ion
- Oligodynamic effect
- Escherichia coli (E. coli)
- Positive control
- Zone of inhibition
- Kirby-Bauer disk-diffusion method
Questions
- If a necklace is described as being 14-kt gold, what does this mean?
- What metals is gold commonly mixed with? What effects do these various metals have on the gold?
- What methods are used for telling if an object is made of gold or not?
- Which metals are considered to be toxic to humans?
- How do metal ions form?
- The procedure in this science fair project is an adaptation of the Kirby-Bauer disk-diffusion method. What is this method and for what is it typically used? How is this project's procedure similar, and how does it differ?
Bibliography
Information about gold and the karatage system is available at this website:
- World Gold Council. (n.d.). About Gold Jewellery. Retrieved December 3, 2014.
This resource discusses toxic metals:
- Occupational Safety & Health Administration. (2009, June 2). Toxic Metals. Retrieved November 9, 2009.
An overview of the oligodynamic effect is given here:
- Wikipedia Contributors. (2009, September 15). Oligodynamic Effect. Wikipedia: The Free Encyclopedia. Retrieved November 9, 2009.
This animation from the CCBC Faculty Web shows the complete procedure and use of the Kirby-Bauer disk-diffusion method in a clinical setting. The bacterial growth conditions are different in this project, but the basic concept is the same. The last section covers how to interpret the results of the test, which will be particularly useful for your science fair project.
- CCBC Faculty Web. Antimicrobial Susceptibility Testing by CLSI (NCCLS) Reference Disk Diffusion Method (Kirby-Bauer). Retrieved April 20, 2018.
This website explains the Kirby-Bauer Disk-Diffusion method:
- Rollins, D.M. and Joseph, S.W. (2000, August). Antibiotic Disk Susceptibilities: Kirby-Bauer Disk-Diffusion Method. Retrieved November 9, 2009.
Materials and Equipment 
Recommended Project Supplies
- Neutralizing Bacteria Kit, available from our partner
Home Science Tools. Needed from the kit:
- Nutrient agar plates (12)
- E. coli culture, freeze dried and ready to reconstitute
- Sterile cotton swabs (3)
- Nitrile gloves
- You will also need to gather these items, not included in the kit:
- Permanent marker
- Bunsen burner or, if not available, a candle
- Incubator or, if not available, a warm dry location
- Jewelry (3 pieces). Should all be similar sizes and shapes—the larger the pieces, the easier the results will be to interpret. Flexible link bracelets, necklaces, or anklets work best. The three jewelry types you need are as follows:
- 14-kt gold or higher (1). Gold jewelry sold in the United States is stamped with a karat mark which indicates the karatage; a 14-kt necklace would have a "14K" stamped somewhere on the surface of the necklace.
- Gold-plated (1), sometimes referred to in commercial jewelry descriptions as "gold over"
- Not gold, but some other "gold-colored" metal alloy, like brass (1), sometimes referred to as "gold toned" in commercial jewelry descriptions
- Mild dish detergent
- Clean dish cloth
- Ruler, metric
- Bleach
- Lab notebook
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Experimental Procedure
For health and safety reasons, science fairs regulate what kinds of biological materials can be used in science fair projects. You should check with your science fair's Scientific Review Committee before starting this experiment to make sure your science fair project complies with all local rules. Many science fairs follow Intel® International Science and Engineering Fair (ISEF) regulations. For more information, visit these Science Buddies pages: Project Involving Potentially Hazardous Biological Agents and Scientific Review Committee. You can also visit the webpage ISEF Rules & Guidelines directly.
This science fair project involves the use of a classroom-safe strain of E. coli. While it is not considered a biohazardous or dangerous bacterial strain, it is important to always properly clean and dispose of bacteria and supplies that come in contact with it. See the Bacterial Safety Guidelines for more details on how to handle bacterial cleanup and waste.
Getting the Jewelry Ready
You will need to clean the jewelry before starting this science fair project. This will ensure that the surface of the jewelry is free from contaminating bacteria, and that the metal, gold, or otherwise, is in direct contact with the agar plates, rather than shielded by dust or oil.
- While wearing disposable gloves, wash the jewelry in mild dish detergent with warm water. Rinse it well.
- Gently dry the jewelry with a clean dish cloth and set it aside.
Preparing the Test Plates
- Label four nutrient agar plates with the permanent marker.
- Petri dishes should always be labeled in permanent marker and on the bottom of the plate. Labeling the lid is not sufficient, as the lid is removable and might accidentally get swapped with another plate.
- One of the plates will be a control to make sure your bacterial lawns grow well and evenly. Label this plate: Control.
- The other three plates will be your actual gold jewelry experiment plates—one for each piece of jewelry. Label them:
- Non-gold
- Gold plated
- Gold, and the proper karatage. For example: 14-kt Gold
- To make your bacterial lawns, you will need to work in a sterile environment. Work next to a lighted Bunsen burner, but be careful around the flame. If you have long hair, tie it back. Do not wear flowing sleeves, and avoid reaching across the flame. If a Bunsen burner is not available, work next to a lit candle.
- Take out your E. coli culture and follow the instructions in the kit to reconstitute the dried bacteria. Wait 5 minutes after reconstituting, then mix the gently shake to mix the bacterial suspension one more time.
- Put 3 drops of the E. coli culture on the surface of an agar plate. Try to put the culture in the center of the plate. Use a sterile cotton swab to spread the bacteria around the entire surface of the agar plate. Put the lid on the plate as soon as you are done.
- Repeat for the other three agar agar Refrigerate the remainder of the culture for repeats.
- After you have spread E. coli on the agar plates, wait 5 minutes for the surface of the plates to dry. Remember, keep the plates covered.
- Now you are ready to place the jewelry on the plates.
- Do not add anything to the Control plate.
- Put on a fresh pair of disposable gloves. Add the corresponding piece of jewelry to each of the plates labeled: Gold, Gold plated, and Non-gold.
- Each piece of jewelry should be placed in the middle of the corresponding plate.
- Arrange each piece of jewelry in a manner so that the maximum amount of jewelry surface area is in contact with the agar plate. If the jewelry is made of flexible links, coil it so that it makes a continuous circle on the plate. See Figure 3, below, for a visual example.
- Try to make each piece of jewelry the same size and shape, preferably a circle, on the plate.
Figure 3. Each piece of jewelry should be arranged on its own plate. If possible, coil the jewelry to form a circle with the maximum amount of surface area in contact with the agar plate.
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Re-cover and incubate the plates at 37°C for 48 hours. Make sure to invert the plates (lid-side down, agar-side up) so that any water condensation does not fall onto your bacterial lawn.
- Note: If you do not have access to a 37°C incubator, you can grow the bacteria at room temperature. Keep the plates away from direct sunlight, but in a warm part of the house. For example, you may want to keep them in a plastic bag (to protect them from dust) next to a heating vent or the clothes dryer. The incubation time will be longer than in an incubator. Start checking your control plates after 72–96 hours of growing time.
Measuring Zones of Inhibition
-
After 48 hours of incubation (72–96 hours if you are not using a 37°C incubator), examine your plates (keep the lids on while you do this).
- Do you see a lawn of bacteria on the Control plate? If not, incubate the plates for an additional 24–48 hours until there is a decent bacterial lawn.
- The Control plate should show relatively uniform lawns. If you see dense bacterial growth in some areas and swatches of light or no bacterial growth in other areas, then your bacteria-spreading technique needs improvement. You will need to repeat the experiment again, paying special attention to spreading the E. coli culture across the plates to get reliable data.
- If the metal in the jewelry has an oligodynamic effect, you should see zones of inhibition around the jewelry. The edges of the clear zone should be a relatively uniform distance away from the edges of the metal.
-
Using a ruler, measure, in millimeters (mm), the zones of inhibition.
- If the three pieces of jewelry are all arranged in circles of similar size, measure the diameter of the clear zone.
- If the jewelry is not circular, measure the distance from the edge of the clear zone to the first edge of the jewelry. Make several measurements around the piece of jewelry and average the measurements to get one representative data point.
- Record all your data in a data table in your lab notebook.
Repeating the Experiment
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To ensure that your observations are accurate and repeatable, carry out the experiment two more times.
- If possible, use a new culture each time.
- If a new culture is not available, remove the old E. coli culture from the refrigerator and gently shake tube. Leave tube at 37°C for one hour (or as close to that temperature as possible without exceeding it). Use this culture as your starting point for creating the bacterial lawn.
Analyzing the Data
- Are the sizes of the zones of inhibition consistent across your replicates? Calculate the average and standard deviation for each piece of jewelry you tested. For more information about calculating and interpreting standard deviations consult the Science Buddies guide to Variance & Standard Deviation.
- How do the average zones of inhibition compare for each type of jewelry? Did one type have a larger zone of inhibition than another? Do any of them show an oligodynamic effect?
Bacterial Safety
Bacteria are all around us in our daily lives and the vast majority of them are not harmful. However, for maximum safety, all bacterial cultures should always be treated as potential hazards. This means that proper handling, cleanup, and disposal are necessary. Below are a few important safety reminders.
- Keep your nose and mouth away from tubes, pipettes, or other tools that come in contact with bacterial cultures, in order to avoid ingesting or inhaling any bacteria.
- Make sure to wash your hands thoroughly after handling bacteria.
- Proper Disposal of Bacterial Cultures
- Bacterial cultures, plates, and disposables that are used to manipulate the bacteria should be soaked in a 10% bleach solution (1 part bleach to 9 parts water) for 1–2 hours.
- Use caution when handling the bleach, as it can ruin your clothes if spilled, and any disinfectant can be harmful if splashed in your eyes.
- After bleach treatment is completed, these items can be placed in your normal household garbage.
- Cleaning Your Work Area
- At the end of your experiment, use a disinfectant, such as 70% ethanol, a 10% bleach solution, or a commercial antibacterial kitchen/bath cleaning solution, to thoroughly clean any surfaces you have used.
- Be aware of the possible hazards of disinfectants and use them carefully.
Ask an Expert
Variations
- Do other metals have an oligodynamic effect? Are they greater or smaller than gold's? Devise an experiment to find out.
- If you see a difference between the gold, gold-plated, and non-gold jewelry pieces in the experiment above, try using your observations as a way to determine whether other pieces of jewelry are real gold or not. Hint: consider using a "blind" test method.
- The science fair project above does not distinguish between the metals being bacteriocidal or bacteriostatic. Things that are bacteriocidal kill the bacteria, whereas those that are bacteriostatic temporarily inhibit bacterial growth. If you observe an oligodynamic effect, devise a test to determine if the metal ions are bacteriocidal or bacteriostatic.
Careers
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