The durability of any material is determined by its resistance to scratching and abrasion, a property known as hardness. Understanding hardness provides a practical measure of a material’s resilience against daily wear and tear. Determining the specific hardness of common glass is important for predicting how it will hold up against other materials it might encounter. This measurement is most often determined by the standardized Mohs scale of mineral hardness.
Understanding the Mohs Scale
The Mohs scale is a qualitative, ordinal system that characterizes the scratch resistance of different materials. Ranging from 1 to 10, the scale is based on the principle that a harder material will visibly scratch a softer one. The test works by attempting to scratch an unknown material with a set of ten reference minerals, each assigned a specific number on the scale.
The scale’s anchors are Talc (1), the softest mineral, and Diamond (10), the hardest naturally occurring mineral. Because the Mohs scale is relative and not linear, the difference in absolute hardness between two consecutive numbers can vary significantly. For instance, the difference between Corundum (9) and Diamond (10) is much greater than the difference between Talc (1) and Gypsum (2).
The Hardness Rating of Common Glass
The hardness of common glass, specifically soda-lime glass, falls within a range on the Mohs scale. Soda-lime glass is the most widely produced type, making up about 90% of manufactured glass found in windows, bottles, and jars. The Mohs hardness rating for typical soda-lime glass is generally between 5 and 7.
The specific hardness for this material is often cited as 5.5, 6, or 6.5. This slight variation is due to minor differences in the glass’s chemical composition, which typically consists of 70–75% silica, 12–15% sodium oxide, and about 10% calcium oxide. The precise ratio of these components, along with trace elements, can slightly alter the material’s final scratch resistance. Specialized glasses, such as heat-strengthened or chemically tempered glass, may exhibit increased mechanical strength, but their Mohs rating remains within the 5 to 7 bracket.
Comparing Glass to Household Materials
The Mohs rating of approximately 5.5 to 7 for common glass has significant practical implications for what materials can cause damage. Any object with a Mohs rating greater than the glass will be capable of leaving a permanent, visible scratch. This explains why common steel tools, which have a hardness of around 5.5, can sometimes scratch glass, especially if the glass is on the softer end of the range.
Materials with a rating significantly above glass, such as Quartz (7) and Topaz (8), easily scratch it. Quartz is a major component of common sand and dust, which explains why simply wiping a dusty window can sometimes result in scratches if sand particles are dragged across the surface. Conversely, materials with a hardness lower than glass, such as the mineral Apatite (5) or a standard copper penny (3.5), will not scratch the glass.