usenaturalstone.comTest your slabs for hardness, acid resistance, and porosity

Karin Kirk

usenaturalstone.com

All Photos by Karin Kirk

Many of my articles extol the virtues of using simple tests to learn more about the properties of natural stone. So I get a kick out of hearing about customers arriving at stone showrooms armed with lemon slices, pocketknives, glass tiles, and notebooks, ready to perform an impromptu geology lab on the sales floor. It goes without saying that these tests need to be carried out with the blessing and cooperation of the sales staff, but I’m totally in favor of this trend. Similarly, I encourage salespeople, kitchen designers, architects, fabricators, and stone restorers to use diagnostic techniques to learn as much as they can about particular varieties of natural stone. Information gleaned from these tests helps us all.

With that said, let’s break out the tools, roll up our sleeves, and do some stone sleuthing!

 

Test for Hardness

When taking stock of a stone’s properties, it’s wise to start with hardness. This simple, flexible test can tell you a lot about a stone.
If you have a small sample of the stone:

  • Find a sharp, broken edge of the stone. 
  • Place a glass tile on a tabletop, and then press the pointed edge of the stone into the glass. 
  • Drag the stone along the glass, pressing firmly, but no so hard that you risk breaking the glass.
  • Then, inspect the glass. Did the stone leave a true scratch that you can feel? Or did the stone simply ‘draw’ on the glass? 
  • Usually, when the stone is scratching the glass, you can feel and hear it biting in. When the stone is softer than the glass, it almost feels slippery as it glides across the surface. 
Above, left: You can do a simple hardness test with a pocketknife. Here, the harder steel in the knife blade can scratch the marble. Above, right: Hardness test using a glass tile. Here, the sandstone is scratching the glass, leaving a clear mark.

Above, left: You can do a simple hardness test with a pocketknife. Here, the harder steel in the knife blade can scratch the marble.

Above, right: Hardness test using a glass tile. Here, the sandstone is scratching the glass, leaving a clear mark.


If you don’t have a small sample, you can test a full slab:

  • Use the tip of a pocketknife, an awl, or similar pointy metal tool. 
  • Press firmly, and drag the tool back and forth across the surface of the stone. 
  • Most of the time the tool will leave a mark, and you’ll need to determine what the mark is. If it’s a scratch, it will work up  small amounts of powdered rock, and make a visible indentation in the stone.
  • The tool can also rub off on the stone, leaving a silvery or light-colored mark – the key difference is that you can rub this mark off and there will be no indentation left behind.
  • Repeat the test multiple places, particularly if the slab is made up of different minerals or colors.
  • Sealer will have no effect on hardness tests.


What the results mean:

  • Granite, gneiss, sandstone, quartzite will all scratch glass and will not be significantly scratched by a metal tool. 
  • Marble, dolomitic marble, limestone, travertine, onyx, soapstone, and serpentine will not scratch glass. A metal tool will scratch these types of stone.
  • This test can also reveal how a stone responds to being scratched, poked, and prodded. Does it flake apart at the edges? Do grains pop out? Does it chip? Be on the lookout for clues about how the stone tends to break.


Test for Acid Sensitivity 

Use this test to distinguish a potentially mislabeled quartzite from marble, and to get a sense of how a particular stone will respond to common acids.

  • Put several large drops of lemon juice or vinegar on the surface of the stone and leave it there for 5-10 minutes. 
  • Sometimes you can see a faint bubbling or fizzing on an acid-sensitive stone, but you might have to use a magnifier to see it.
  • Then wipe down the stone and look at it at a low angle. Etched areas can look dulled, as if the stone’s polish has been removed.
  • Polished slabs typically will have more noticeable etching than honed or leathered slabs.
  • Sealers do not protect a stone from interactions with acid, so this test can be done on a sealed or unsealed sample.
Etch marks can be seen at a low angle – the etch is the round area in the center of the stone. This is Corteccia “soft quartzite.” There is really no such thing as soft quartzite, and in most cases that term means the stone is marble. The etching test shows that’s the case here, too.

Above: Etch marks can be seen at a low angle – the etch is the round area in the center of the stone. This is Corteccia “soft quartzite.” There is really no such thing as soft quartzite, and in most cases that term means the stone is marble. The etching test shows that’s the case here, too.

Below: Testing for etching / acid sensitivity test, using lemon juice on a quartzite sample.
Testing for etching / acid sensitivity test, using lemon juice on a quartzite sample.


What the results mean:

  • Stones that contain calcite or dolomite will etch from contact with common acids. 
  • Marble, limestone, travertine, and onyx are all made of calcite. Dolomite is a similar mineral that etches somewhat more slowly. Super White is made of dolomite, for example.


Test for Porosity

This test is best done on an unsealed sample so you can assess the stone’s true porosity, but if you want to test the effectiveness of a sealer, you can do the second or third variations of test with a sealed stone.

Above, left: The porosity moisture test shows this White Macaubas quartzite has absorbed water. It will need to be sealed Above, right: Porosity testing in a shallow pan of water. Food coloring can also be used

Above, left: The porosity moisture test shows this White Macaubas quartzite has absorbed water. It will need to be sealed 

Above, right: Porosity testing in a shallow pan of water. Food coloring can also be used 

 

Porosity test with water on marble and limestone. Leave the water in place for 15 minutes, then wipe dry.

Porosity test with water on marble and limestone.
Leave the water in place for 15 minutes, then wipe dry.

If you have a small sample of the stone:

  • Fill a shallow bowl with water and place the stone, edge-wise, in the water so only part of it is submerged in the water.  You can also do this test with some food dye in the water, which makes the results easier to see.
  • Leave for 10-15 minutes, then remove from the water and dry thoroughly. Let the stone air dry for about 10 minutes so that the surface of the stone becomes dry. Then you can evaluate if water worked its way inside the stone.
  • Inspect the stone to see if water has wicked up into the stone above the water line. If so, the stone might be more prone to staining after installation. 
  • Then check to see if the stone looks darker below the water line. If so, it’s moderately porous
  • If the stone has not darkened, then it absorbed little to no water and it has low porosity.


To test a full slab, in the horizontal position:

  • Dribble water onto the surface and leave for 10-15 minutes. 
  • Wipe up the water and dry thor- oughly. Let the stone air dry for about 10 minutes.
  • A darkened area indicates that the stone has absorbed water.


On a full slab, in the vertical position:

This is the least helpful way to gauge porosity, but sometimes it’s the only option.

  • Use a spray bottle to spritz water on the surface of the slab. The water will run off, but you will be able to observe how water interacts with the stone. Does the water leave a darker shadow, or does it bead up and run off entirely? 
  • If you have access to a broken edge of the stone, put water there and see what happens. 
  • Another option is to drape a wet towel on the stone and leave it in place for 10-15 minutes.
After the porosity test, wipe off the stone and let it air dry for around 10 minutes to be sure you are looking at water absorbed into the stone, rather than the wet surface of the stone. In this case, the water clearly left a dark area in the stone, indicating it’s porous. The stone is Lamberti Bianco quartzite.

After the porosity test, wipe off the stone and let it air dry for around 10 minutes to be sure you are looking at water absorbed into the stone, rather than the wet surface of the stone. In this case, the water clearly left a dark area in the stone, indicating it’s porous. The stone is Lamberti Bianco quartzite.


What the results mean:

Porous stones are more likely to stain. Although porosity can be reduced with sealer, it’s still wise to consider a stone’s natural tendency to absorb liquids so that the sealer is not the only defense against staining. Ideally, avoid putting a porous stone in a place where it will have to endure a lot of liquids.

 The more knowledge you have, the more confidence you’ll gain in your decisions. While many variations of natural stone offer gorgeous aesthetics and versatile utility, understanding their differences will help you pinpoint which ones are right for different circumstances. And when you align a stone’s properties with the ways it will be used, the stone itself can take center stage, for many years to come. 


A professional-level stone diagnostics kit can be purchased through MB Stone Pro: https://www.mbstonepro.com/products/stone-id-kit  .


Karin Kirk is a geologist and science educator with over 20 years of experience and brings a different perspective to the stone industry. Karin is a regular contributor to usenaturalstone.com and the Slippery Rock Gazette. Contact her at karinkirk@gmail.com