posted on September 01, 2008 16:51
As with so much in the natural stone industry, there is a fairly large amount of confusion regarding the actual geologic classification of most commercially available slab materials. The amount of misinformation is astounding and often quite discouraging for the average Sally and Joe Consumer trying to decide on what material would make just the best counter top for their new kitchen. With this article I will try to clarify some of the intricacies of stone classification.
I'll start with a given: Not all commercial granites are true geologic granites. I can already hear you sigh and roll your eyes. I sympathize - science was not my forte either, but take heart, I will try my best to make this entertaining.
In the commercial realm, "granite" gets classified as a hard natural stone which can be polished and requires more aggressive tools and abrasive than what would be used on marble. This is a pretty broad and not very scientific kind of description, which leaves some really wide wiggle room.
True geologic granite gets classified as an igneous rock consisting mainly of quartz, feldspars and mica - much more concise and restrictive. To be quite honest, real geologic granites are not very exciting, from a design perspective. They will have quite a homogeneous grain pattern and can range in color from grays to browns, yellows or pinks. A good example of a real geologic granite would be Georgia Gray (from Elberton, GA and it has a water absorption weight of 0.2%-0.3%). True granites are not reactive to acids, but could be quite absorbent as our example illustrates. This stone would be OK for use as a counter top, but would require sealer. It has been used as cladding for buildings and for monuments and gravestones for many, many years, though.
The rest of the commercial granites can be divided into a couple of broad groups:
Magmatic Rock - formed when magma cooled and crystallized. True granites (like Tropic Brown), syenites (like Ubatuba), gabbros (like Black Absolute), diorites (like Brazilian Black) and charnokites (like Atlantic Green) will fall under this umbrella.
Metamorphic Rock - were formed when one kind of stone i.e. sandstone, got transformed into another kind of material i.e. gneiss. An example of such a stone is Giallo Veneziano (a gneiss from Brazil with a water absorption weight of 0.25%-0.35%). Metaconglomerates (like Verde Marinace), Quartzites (like Almond Mauve), migmatites (like Paradiso Classico), gneisses (like Santa Cecilia) and granulites (like Verde Jewel/Tropical Green) also fall under this group.
What makes the commercial "granites" so appealing is the fact they are so diverse. There are virtually hundreds of different colors and patterns.
And this brings us to the second part of my question: Does it really matter if it is not true geologic granite? In a word: No.
Earlier in my dissertation you might have noticed me mentioning something called the Water Absorption Weight (WAW). This is an indicator of just how absorbent a specific stone may be, the lower the number, the less absorbent the stone.
Following the discussions of natural stone and how they always gravitate to the question of whether a sealer would be required, this number would be a pretty good indicator of how good a stone would stand up to use in a kitchen. Without further ado, I will list a few popular stones, along with their geologic classifications and WAW:
||Water Absorbtion Rate (WAW)
|Black Absolute (gabbro)
|Baltic Brown (granite)
|Santa Cecilia (gneiss0
|Verde Butterfly (charnokite)
|Silver Sea Green (granite)
|Marinace Green (metaconglomerate)
|Kashmire White (granulite)
As you can see from the above list, there are a number of stones that far out-perform true geologic granites with regard to their WAW. There are also a number of stones that absorb a tremendous amount of water (take the Kashmire White for instance). On the other end of the scale, Verde Marinace and Black Absolute are too dense to benefit from the application of a sealer.
Although modern sealer technology has advanced a long way in making stones less absorbent, there are a few materials that should not be considered for use in any high traffic environment. Kashmire White would be a shining example of a stone that cannot be sealed, even with the best sealer.
Testing for absorption issues on granite samples would be as easy as dripping some water on your sample and letting it sit for a while. If it darkens the stone a little, a sealer might help. If the stone immediately becomes darker and maintains the dark spot for some time, stay away! Maintaining a porous stone would be a constant chore.
Etching is another “must-do” test for stones to be used in a kitchen. A lot of stones are chemically inert. Baltic Brown, Verde Butterfly, the REAL Black Absolute, Blue Eyes, the lists go on and on. Some stones react negatively to acids. Blue Bahia (a sodalite-syenite) would be one example. Etches will show up as dull spots on an otherwise shiny surface. Sealers will not prevent etches because etches are chemical reactions and have nothing to do with the absorption rate of the stone in question.
There are two ways to work around this issue. One is to avoid the stone that etched in testing and the other is to hone and enhance the stone. This would still give you a depth of color, but the shine would be absent. Etch marks would still happen but would tend to blend with the honed finish of the slab.
To test for etching, place a wedge of lemon or lime, cut side down, on the sample overnight. Wipe the sample in the morning and hold it at an angle to the light. If there is a rough looking spot where the shine is absent, you have etched the stone. Etching will normally occur where calcium or calcite is present in the make-up of the stone.
The last item to consider is resinated slabs. Resining is a process where resins get impregnated into the stone slabs before they are finished. The remaining surface resin is removed through the polishing process, leaving the resin in the pits and fissures. This serves a few purposes:
1. Consolidation a fissured or flaky slab (Golden Beach would be an example. Without resin, the slab would probably not have been commercially available).
2. Reduce the WAW of a material (Santa Cecilia is a great example here. Even though it is quite an absorbent material, once it is resined, may not require the application of a sealer)
3. Conducive to a superior surface finish (Flaky stones like Verde Butterfly get resined to eliminate surface crystals from flaking off. This provides a smooth finish to the polished slabs)
4. Another side effect of the resining process is enhanced colors. On some stones like Lady's Dream the colors could deepen with the application of the resin.
So what is the bottom line? It does not matter whether the stone you have is “real” granite. The geologic classification has virtually no impact on the performance of the material in a kitchen.
Test the stone and advise your clients accordingly. A well-chosen stone will guarantee a happy, satisfied customer every time.
About the Author: Adriana Pretorius is the owner and operator of the Rocky Hammock Stone Company, Inc. in Chiefland, FL, along with her husband, Casper. Rocky Hammond Stone Company is a small shop specializing in the fabrication of custom natural stone countertops for residential and light commercial applications. They believe in educating their clients (or anybody that would listen, for that matter) about the wonderful product that is natural stone. Adriana can be reached at email@example.com or through their web site at www.1800Granite.net