Why Does Soapstone Ab(d)sorb Mineral Oil While It’s Non-Porous?

A very common question in the soapstone market for use in countertops is why this rock “absorbs” mineral oil when is supposed to be non-porous? We have two concepts that must be discussed to clarify this question.

Absorption, in the concept of countertop material, is a physical phenomenon where the rock lodges in its pores, a liquid material. It is therefore closely related to its porosity. Absorption and porosity tests are performed by taking 10 samples of  2” x 2” material of equal thickness, oven dried at 175F  for 24 hours so that all the moisture is removed and weighed to determine its dry mass. These bodies are cooled to room temperature and placed on a tray where distilled water is added to 1/3 of its height. After 4 hours, distilled water is added to 2/3 of its height and, after another 8 hours, added water again, covering completely the rocks and leaving them immersed for 40 hours. Water is removed from the surface of the specimens, weighing them to determine their saturated mass and, on a hydrostatic balance, their submerged masses.

With this data, their absorption (α) and porosity (ͷ) are calculated according to the formulas below:

Adsorption is a physical or chemical phenomenon where the surface of the mineral electrostatically attracts another element (physical adsorption) or shares electrons (chemical adsorption). It occurs only on the surface of the mineral and is not related to the “penetration” of the liquid (or gas) into the structure inside the rock.

Well, the ABNT NBR 15844 standard determines that for the use of surface material, natural stones must have water absorption <0.4% and porosity < 1.0%. As porosity and absorption are closely linked, the international standard ASTM C 615 states only that water absorption must be ≤ 0.4%. This is the case of the soapstone!

This fact was already expected because the density of the soapstone used in the building industry is around 3.0 t/m³, a high value for the most common natural stones. It is logical to think that the more porous the material, the less dense it will be, inverse logic is true for soapstone: Very dense and non-porous.

Furthermore, it was to be expected that if the soapstone absorbed water or other liquid this would be drained through its interior passing from one side to another, as with many untreated natural stones.

Well then, why does soapstone retain oil on its surface?

Now we are talking about the concept of adsorption! The mineral talc, main “ingredient” in soapstone, it is hydrophobic, that is, it is water repellent. One quick, easy and interesting test to observe this characteristic is to take a piece of soapstone, scratch it with a knife or file and leave the dust generated fall into a bucket of water. You will see that the soapstone dust will float on the surface of the water.

Back to the adsorption (not absorption, we have to use the right word) of mineral oil by talc, the fact that this mineral is hydrophobic shows us that its molecules have a nonpolar character because they repel and are repelled by the water that has polar characteristics. However, mineral oil also has non-polar and, therefore, is attracted to the surface of the talc mineral, being adsorbed by it.

It is a surface phenomenon and is not related to absorption, but mineral oil adsorption! This explains why soapstone is non-porous and therefore does not absorb any liquid, but it is darkened by mineral oil as it adsorbs it on its surface.