Page 30 - Demo

30|July 2020
How to Be a Stone Sleuth
Slippery rock Gazette
area in the center of the stone. This is Corteccia “soft quartz- ite.” 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.
Test your slabs for hardness, acid resistance, and porosity
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 custom- ers arriving at stone showrooms armed with lemon slices, pocket- knives, 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 diag- nostic techniques to learn as much as they can about particular vari- eties 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 sam- ple 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
Karin Kirk
usenaturalstone.com All Photos by Karin Kirk
You can do a simple hardness test with a pocketknife. Here, the harder steel in the knife blade can scratch the marble.
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.
If you don’t have a small sam- ple, 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
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.
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.
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.
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Porosity testing in a shallow pan of water. Food coloring can also be used to increase the contrast.
Etch marks can be seen at a low angle – the etch is the round
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 re- spond 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
Hardness test using a glass tile. Here, the sandstone is scratching the glass, leaving a clear mark.
Testing for etching / acid sensitivity test, using lemon juice on a quartzite sample.
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 un- sealed 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.
The porosity moisture test shows this White Macaubas quartzite has absorbed water. It will need to be sealed thoroughly.