Sterling silver is a eutectic alloy of silver having 92.5 percent of silver and 7.5 percent of other metals. These other metals are usually copper, tin, or zinc with lead and cadmium. Whereas, pure silver has 99.9 percent of silver and only 0.01 percent of trace elements. Many people often get confuse whether silver gets rust or not. In this article, I will answer this question in detail.
Does Sterling Silver Rust? No, sterling silver does not rust because there is no iron content in the sterling silver. But it gets tarnish forming a black layer of sulfide or oxide on its outer surface. Sterling silver is an alloy of 92.5% of silver and 7.5% of other metals mainly copper that gets tarnish over time if exposed to air.
Rust is a reddish-brown iron oxide, which forms when iron, reacts with oxygen in the presence of air and moisture. Sterling silver tarnish in a similar way like pure silver does.
2Ag (s) + H2S (Hydrogen sulfide) ————-> Ag2S (Silver sulfide) + H2 (g)
Rust versus Tarnish
Though they are synonyms as per the English dictionary, chemically both are different. Rust forms on iron and its alloys, such as steel.
On the other hand, sterling silver having preferably copper reacts with sulfur to form copper sulfide. It is copper sulfide or oxide which makes silver look rusted, actually which is corroding.
Many people confuse between rust and tarnish because both are forming a blackish-brown layer on top of the metal.
Both have corrosive nature where, like rust, tarnish eats away silver and copper with time. But this process is way too long in comparison to pure silver.
Likewise, white gold also tarnishes over a span of time. Here is the article to check out the reason for the tarnishing of white gold.
Like iron cannot tarnish, silver cannot rust.
Why sterling silver gets tarnish?
To know the core reason for the tarnishing of silver ie; why a thin greyish black layer develops on the surface of silver, the concept of thin-film interference should be studied.
It might be interesting to know that tarnish changes its color with time because of ‘thin-film interference’. When light hits the tarnish, it splits.
Some reflect on the top while some reflect off the silver, under the tarnish. The rate of tarnishing of sterling silver increases if it frequently comes in contact with detergents, cosmetics, and body chemicals as well.
How to prevent tarnish on sterling silver?
At the industrial level, the use of silicon is quite prevalent to prevent the formation of tarnish.
The graphs down below; are showing values of two different concentrations of Ag-CU-Si alloy. The first one has 92.5 percent of pure silver, 7.39 percent of copper, and 0.38 percent of silicon.
Besides this, the second one has 95 percent of pure silver, 4.59 percent of copper, and 0.44 percent of silicon.
- The below graphical representation shows the hardness value of Ag-Cu-Si alloy as a function of silicon content (%S content) and silver content. You can see all the numbers for 92.5 percent alloy are much higher than 95 percent.
- The below graphical representation shows silicon content in the copper-rich phase existing in the network area as a function of the added silicon content.The values for 95 percent allow are higher because of the high amount of silicon present in it.
- The below graphical representation shows the ultimate tensile strength of Ag-Cu-Si alloy as a function of silicon content and silver content.As silicon increases, tensile strength decreases because copper and silicon form an easily breakable network. It also reduces the ductility of the sterling silver.
- The below graphical representation shows how 95 percent alloy reduces its ductility, with an increase in the percent of silicon and silver.
From all the graphs, you can analyze that silicon is a suitable choice as an anti-tarnish element, which also affects the other mechanical properties of sterling silver.
If you are interested to know if stainless steel also rusts or not. here is the article to check out the rusting of stainless steel.
Adding other elements to silver helps, how?
Though copper is easy to tarnish, it is the most preferred choice to form a silver alloy. It is because copper increases the durability of silver to many folds.
The graphical representation down below explains how copper reacts to various mechanical properties at different temperatures and concentrations.
It might interest you, for all properties copper shows similar effects at different concentrations and temperatures.
The graph shows the properties of the annealed silver-copper alloys at room temperature.
How studying mechanical properties can help?
The answer is during the sintering of sterling silver. Sintering is a process where the material is coalesced into a solid mass by heating without undergoing liquefaction.
Sterling silver is a polycrystalline material which needs to be sinter properly to avoid any misalignment of its grains. During this, copper impurity is mixed, where grains of both reduce the occurrence of any defect.
Need for sterling silver
Copper in sterling silver has higher reactivity than the silver, still is in huge demand all over the globe.
Let’s discuss this in a bit of detail.
Pure silver is generally quite soft and not simple to shape efficaciously. Due to this, it is quite tough to give specific shapes and is not a suitable choice for daily jewelry, drink and dining, silverware, and silver-plated items.
Here, sterling silver comes into the picture. The 7.5 percent of copper gives enough strength to the silver, making it a suitable choice for various uses.
From the next time, if you hear someone saying ‘silver-plated’, it means a top coating of sterling silver not pure silver.
Silver alloys have a good ability to join base metals at lower temperatures providing good strength. It makes sterling silver a healthy choice for soldering and brazing purposes, where it reduces surface oxidation and heat distortion to a great extent.
Mechanical Properties of Sterling Silver
From the table, you can see that the tensile strength of sterling silver is high and can give competition to steel (which has a yield strength of 250 MPa) and stainless steel (which has a yield strength of 520 MPa).
Furthermore, the thermal expansion value is pretty low, which makes sterling silver a suitable choice of using along with ceramics.
The specific heat capacity means the amount of heat required to increase the temperature of the metal by 1°C. The same for sterling silver is very high as it is showing 245 J/Kg*K.
It means sterling silver is a suitable choice for high-temperature usage.
The other properties from the table confirm the fact of sterling silver a durable metal and a suitable choice for daily purpose commercial and electrical usage.
Conclusively, the use of sterling silver is everywhere these days because it is durable and easy to maintain than pure silver.
Furthermore, you can note that sterling silver is nothing but an alloy of different metals. Each metal form and alter the mechanical properties of the final metal. Silver being a soft metal, copper is the most suitable choice to form an alloy.
Also, to reduce the tarnishing effect, silicon is used widely at the large-scale but can be cleaned with aluminum and baking soda at home.
Some additional questions for you:
How to clean sterling silver pieces at home?
Sterling silver pieces we use at home can easily be cleaned by keeping it in baking soda, aluminum foil, and boiling water. It follows the same principle of pure silver:
3Ag2S (Silver sulfide) + 2Al (Aluminium) —————> 6Ag (Silver) + Al2S3 (Aluminium sulfide)
Is sterling silver expensive or cheaper than pure silver?
Sterling silver is cheaper than the pure silver and is generally quite an affordable metal to purchase.
Is maintaining sterling silver troublesome?
It depends on which metal has been used with pure silver to form an alloy. If copper and silicon are used, sterling silver does not tarnish quickly.