Why Does Soapstone Ab(d)sorb Mineral Oil While It’s Non-Porous?
by Roger Teixeira, M. Teixera Soapstone
A very common question in the soapstone market for use in countertops is why this rock “absorbs” mineral oil when it is supposed to be non-porous. There are two concepts that must be discussed to clarify this question.
Absorption, in the context of countertop materials, is a physical phenomenon in which the rock lodges a liquid material within its pores. It is therefore closely related to porosity. Absorption and porosity tests are performed by taking 10 samples of 2- x 2-inch material of equal thickness. These samples are oven-dried at 175°F for 24 hours so that all moisture is removed, then weighed to determine their dry mass. The specimens are cooled to room temperature and placed on a tray where distilled water is added to one-third of their height. After four hours, distilled water is added to two-thirds of their height, and after another eight hours, water is added again, completely covering the rocks and leaving them immersed for 40 hours.
Water is then removed from the surface of the specimens, which are weighed to determine their saturated mass and, using a hydrostatic balance, their submerged mass. With this data, absorption (α) and porosity (ɸ) are calculated according to the formulas below.
Adsorption is a physical or chemical phenomenon in which the surface of the mineral electrostatically attracts another element (physical adsorption) or shares electrons (chemical adsorption). This process occurs only on the surface of the mineral and is not related to the penetration of a liquid or gas into the internal structure of the rock.
The ABNT NBR 15844 standard determines that, for use as a surface material, natural stones must have water absorption of less than 0.4% and porosity of less than 1.0%. Because porosity and absorption are closely linked, the international standard ASTM C615 states only that water absorption must be ≤ 0.4%. This is the case with soapstone.
This result is expected because the density of soapstone used in the building industry is around 3.0 t/m³, which is a high value compared to most common natural stones. Logically, the more porous a material is, the less dense it will be. The inverse logic applies to soapstone: it is very dense and non-porous.
Furthermore, if soapstone were to absorb water or other liquids, it would be expected that these liquids would drain through its interior, passing from one side to the other, as occurs with many untreated natural stones.
So why does soapstone retain oil on its surface? This brings us to the concept of adsorption. The mineral talc, the main “ingredient” in soapstone, is hydrophobic, meaning it repels water. A quick, simple and illustrative test of this characteristic is to take a piece of soapstone, scratch it with a knife or file, and allow the resulting dust to fall into a bucket of water. The soapstone dust will float on the surface of the water.
Returning to the adsorption (not absorption -- the correct term must be used) of mineral oil by talc, the hydrophobic nature of this mineral shows that its molecules have a nonpolar character, as they repel and are repelled by water, which has polar characteristics. Mineral oil is also nonpolar and is therefore attracted to the surface of the talc mineral, where it is adsorbed. 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.
Looking for a reprint of this article?
From high-res PDFs to custom plaques, order your copy today!
