Platinum is a highly valued precious metal, often associated with luxury and enduring strength. The perception of platinum as “soft” relates to its high malleability and ductility, which measure a metal’s ability to be permanently deformed without breaking. In its pure form, platinum is remarkably easy to shape, allowing it to be hammered into thin sheets or drawn into fine wire. This quality is distinct from hardness, which refers to resistance to scratching or permanent surface deformation. Although pure platinum is technically soft, its practical applications, particularly in jewelry, rely on strengthening it through alloying.
Measuring Platinum’s Physical Properties
Scientists quantify the physical characteristics of metals using specialized scales. Hardness, the resistance to permanent surface change, is commonly assessed using the Mohs scale of mineral hardness. This scale provides a general, comparative measure based on scratch resistance, ranging from 1 (talc) to 10 (diamond). Pure platinum has a Mohs hardness value of approximately 3.5, a moderate score in the mineral spectrum.
For technical precision, metallurgists use indentation tests like the Vickers or Brinell scales. These methods measure the material’s resistance to a non-deformable indenter pressed into its surface, yielding a specific numerical value. Pure, fully annealed platinum has a Vickers Hardness Number (HV) of around 50, which serves as a baseline for comparison with its alloys.
Platinum’s “softness” is better described by its high plasticity, which includes malleability and ductility. Malleability is the capacity of the metal to deform under compressive stress, such as being hammered into a flat plate. Ductility is the ability to stretch under tensile stress, allowing it to be pulled into a thin wire. Platinum is extremely ductile, meaning it can undergo significant plastic deformation without fracturing.
Platinum’s Hardness Compared to Other Precious Metals
Placing platinum’s hardness on a scale with other precious metals helps illustrate its position. Pure platinum, with a Mohs hardness of 3.5, is notably harder than pure gold and pure silver. Both 24-karat gold and fine silver have a Mohs rating between 2.5 and 3, making them vulnerable to scratches and dents in their unalloyed state.
In comparison to palladium, however, pure platinum is softer. Palladium, a member of the platinum group metals, has a Mohs hardness of approximately 4.75. This places platinum between the two extremes of commonly used jewelry metals, being harder than gold but softer than palladium.
The Vickers Hardness Number for pure platinum (around 50 HV) confirms it is more resistant to indentation than pure gold. However, platinum is still considered relatively soft compared to many common industrial metals. For example, some steel alloys have Vickers hardness values several times higher than pure platinum. This difference underscores the necessity of alloying platinum for demanding applications like jewelry.
The Impact of Alloying on Platinum’s Durability
The durability of platinum jewelry, despite the element’s relative softness, is entirely dependent on alloying. Platinum used for fine jewelry is typically 90% or 95% pure, with the remaining percentage composed of other elements. These alloying metals are chosen to drastically increase the overall hardness and strength of the final product.
Common alloying elements include iridium, ruthenium, and cobalt, which substantially elevate the metal’s resistance to wear. For instance, a 95% platinum and 5% ruthenium alloy can reach a Vickers Hardness Number of around 130 HV. A 95% platinum and 5% cobalt alloy often reaches 135 HV, which is more than double the hardness of pure platinum. This significant increase provides the necessary strength to secure gemstones and withstand the stresses of daily life.
The unique way platinum alloys wear contributes to their lasting durability. When a platinum item is scratched, the metal is displaced across the surface rather than worn away and lost. This microscopic movement creates a subtle, matte sheen over time known as a patina. Patina is a distinguishing characteristic of platinum jewelry, and unlike gold, platinum’s unique wear characteristic ensures that the volume and weight of the piece remain constant.