Soapstone is a nonporous metamorphic rock that won’t absorb water, acids, or other liquids, making it ideal for sinks, countertops, and other surfaces in the kitchen or bath. But a common question is: if soapstone is nonporous, why does it seem to absorb mineral oil during treatment? To understand this, we need to take a closer look at how soapstone behaves on the surface, and why it can darken with oil even though it doesn’t actually absorb liquids.
What Is Absorption?
Absorption refers to how much liquid a material can soak into its internal structure. It’s tied directly to porosity—the more porous a material is, the more it absorbs. This is a key concern when choosing materials for surfaces like soapstone countertops, sinks, or other areas where water exposure is common.
To be considered suitable for surface use, international standards require very low absorption rates. For example, ASTM C615 states that natural stones used in construction should absorb no more than 0.4% water. Soapstone easily meets this standard. It’s incredibly dense—around 3.0 tons per cubic meter—which means there’s very little empty space inside the stone for water to fill. In fact, the denser the stone, the less porous it tends to be, and soapstone is both dense and naturally nonporous.
That’s why soapstone doesn’t absorb water, acids, or other liquids. Unlike more porous natural stones, it won’t allow moisture to pass through from one side to the other.
What Is Adsorption?
While absorption refers to a liquid soaking into a material, adsorption is something completely different. Adsorption happens when a substance (like oil) sticks to the surface of a material—not because it’s soaking in, but because of a surface-level attraction between molecules.
So why does soapstone retain mineral oil on its surface if it’s nonporous?
This is where adsorption comes in. Soapstone is made mostly of talc, a mineral that’s naturally hydrophobic, meaning it repels water. In fact, if you shave off some soapstone dust and drop it into water, it floats. That’s because talc doesn’t interact with water molecules, which are polar (water-loving).
However, mineral oil is nonpolar, just like talc. Since similar molecular types attract each other, the oil clings to the surface of the soapstone. This is not absorption, because the oil doesn’t enter the stone—it simply settles on the surface in a thin layer. That’s why applying mineral oil to a soapstone countertop or sink makes the surface look darker and more defined, enhancing the stone’s natural veining through surface-level adsorption.
Soapstone Is Nonporous—And That’s a Good Thing
So, is soapstone porous? Not at all. While it may darken when treated with mineral oil, that’s due to adsorption, not absorption. Soapstone’s dense, nonporous nature makes it an outstanding material for kitchen countertops, bathroom sinks and other high-use surfaces where water and stains are a concern.
If you’re considering soapstone for your next home project, we’re here to help. Contact M Teixeira Soapstone today for a custom quote or stop by one of our showrooms to explore the beauty and performance of real soapstone in person.