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ADVICE WANTED!   July 31, 2002
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Granite Characteristics - General

Physical, chemical, technical characteristics of grantite in general

  General, Marble, Granite, Limestone, sandstone, Travertine
  other, bluestone, slate, onyx, engineered stone, manufacture stone, basalt, pebbles, flagstone, lava

Q 2312: I am an artist, and I would like to grind pigments on a slab of granite, with a muller (which could be a smaller block, also made of granite). 
-Are there different kinds of granite?
-Do they all have the same hardness?
-If not, which one is the hardest? And how can I obtain it? Could you also recommend any scientific literature on the subject of grinding. Gadi, July 27,
R2: Dear Gadi: The answers to your questions are as follows:
Yes, there are different kinds of granite.
No, they do not have the same hardness.
One of the hardest is called Absolute Black. Depending upon your needs, you can probably pick up a scrap from most any local fabricator for near nothing, or you can order the stone the stone from him. Finally, I can not recommend any literature on grinding. Mike, USA
R1: Dear Gadi: I will only answer to your first question; after that you can figure out the 
answers to the other two. There's only one kind of granite. The problem is that there are so many stones traded as granite, and that granite are not (approximately the 98% of them!)
Ciao and good luck, Maurizio, USA 
Q 2308: We are considering granite tiles (machinated?) and natural granite for the flooring in some rooms of our house that is now being constructed. Italian tiles/granite are beyond our budget and we can only afford China. Some dealers and installers advise that the China granite tiles 'emit' this powdery substance after some time. How about Chinese natural granite, is it a good material? Thanks, Lisa, Philippines, July 26,
R2: Dear Lisa: "Machinated" granite tiles?? What on earth are those things?! About the mysterious powdery substance that Chinese "granite" tiles would allegedly emit where there is a full moon, which one Chinese "granite" are we talking about here? There are at least a few dozens of them, coming from different parts of China (and it's a big country, you know!). Can they be all affected by the ocean tides?! I'd really love to know that!! Ciao and good luck (with the sort of information you're getting, you'll need a lot of it!) Maurizio, USA
R1: What do you mean by "good"? How do you define "good"? Perhaps the first thing that needs to be clarified is that the lower price of the Chinese granite vs. Italian granite is not at all an indication that the Chinese granite is of "lower quality". We have been importing Chinese granite in various forms - floor tiles and slabs and countertops prefabricated in China. We have installed these materials in some of the renowned hotels and condominium projects in various parts of the US. Conclusion: Chinese granite is a widely accepted material. And by the way, no one among our clients have complained "that the China granite tiles 'emit' this powdery substance after some time". 
Certainly, all this is not to say that all granites are "equal" - regardless of country of origin. As you may have also gleaned from earlier postings on this site, certain stones that come under the broad label of "granite" are not even granite at all - if one were to apply the term in a strict petrographic sense. In my experience in the stone business, the term seems to be broadly applied as a "marketing convenience" to designate stones that are not quite clearly classifiable as marble, limestone or sandstone or slate, etc. In like manner, certain marbles are better classified as limestones and yet they are marketed as marbles. And the market (both sellers and buyers) have tolerated and accepted this "setup" - albeit grudgingly by some quarters. Gene, USA. 
Q 2236: I have looked at many articles in this very useful page, and have not come across a way to tell Marble and Granite apart just by looking at it. To the untrained eye they look very similar, I have also noticed that out of the two sample tiles I picked; one has a mesh type backing glued to the back. Any relation to the type stone it is if a mesh is seen? Hector, July 15,
R2: Dear Hector, granites are generally composed from visible multicolored grains. Typically grain of one color is connected with grains of other colors, e.g. grey quartz is close to pink orthoclase, white plagioclase, black mica. Granite is a mixture of minerals. Marbles are either without visible grains of calcite (limestones), or connected calcite grains of similar color (true marbles). Daniel, Slovakia
R1: Dear Hector: Nobody can teach how to recognize marble from "granite". Usually, the mesh-type backing is typically applied to marble or travertine (they are more fragile than "granite")., Maurizio, USA
Q 2190: I am interested in a honed granite countertop for my kitchen. Can you hone
any granite or does it just come in certain colors (I have seen black and green)? Is this as durable as regular granite? How do you pick out a slab since they are usually shiny? Deborah, July 8,
R1: Dear Deborah: All "granites" can be hone-finished. Dark colosr are not advisable because they represent a maintenance nightmare (they show all sorts of surface soiling!). Durability is not an issue; "granite" -- no matter what stone it is in reality, is always "regular", regardless of the surface finish. Usually hone-finished granite come finished by the factory. Only seldom the fabricator is going to custom hone a polished slab; it takes a very expensive 
piece of equipment that only few fabrication facilities own. Toward the end of this page's left side bar you will find the link to my column, ROCKING THE BOAT. Click on it, and look for the article about selecting a good fabrication facility (and stone). That should help you with 
your decision. Once back on this page, go again at the end of the side bar and click on my guidelines for maintenance of residential stone installation. You do want to treasure those! 
Ciao and good luck, Maurizio, USA
A 2143: I was just wondering if you have a list of commercial stone names that have satisfied this lemon juice test. Jvohl, June 30,
R1: Dear Jvohl: Too long a list, sorry. Ciao and good luck, Maurizio, USA
A 2043: Is there a part of the world that better granite comes from? I heard that granite that is more polished from Italy is better than less polished from India? Can you advise? Gary, June 11,
R1: Dear Gary: God doesn't know much about the geographical definition invented by mankind. "Granites" are good or bad all over. Now, about the finish, yes, it's true, no matter where the stone comes from (Italy buys blocks from all over the world), the slabs and tiles processed in Italy are better than the same "granites" processed somewhere else, with Italian machinery and Italian training.
Why? I don't know ... The "magic touch", maybe!! Maurizio, USA
A 1990: I just got back from looking at granite here locally (NH). As soon as I got home I tested out my Black Galaxy sample, scrubbing at it with the green side of my sponge. My sponge was damp, and it scratched the granite. Not badly, but it was there. I was under the impression that Granite was 2nd hardest next to diamonds...do you know why my sponge would do this? Could it be the stone was sealed and the sealer is what is scratching? Lea, USA, May 31.
R1: Dear Lea: Green sponges can scratch "granite", even in the case of a tough stone as "Black Galaxy". After all, the grits in green sponges are made of silicon carbide, which is the only material below diamond, hardness-wise. So, avoid those types of sponge and use plastic-net scrubbers and the likes instead. Steel-wool would be good, too, but I consider it an overkill anyway. I hope that many other readers see this posting. Hit the link to my daily maintenance guidelines at the end of this page's side bar, and treasure them. (As a matter of fact, I'm thinking of updating them as a consequence of this latest finding). Maurizio, USA
A 1872: After a rather spirited dinner party, we went straight to bed and neglected to notice an area on our granite counter tops where wine had apparently been spilled. Unfortunately, the damage had already been done by the next morning. Any advice on how to repair the etched and pitted finish? It's really not all that bad, but still noticeable to me. Jill, May 15.
R3: Are you sure the surface is granite? Typically it does not etch. As such, you will need to have the area filled and repolished by a professional. Regards, Steven, USA
R2: I don't believe it is etched. Granite is hard to etch. Good luck, Randy, USA.
R1: Dear Jill: If wine etched your stone, it ain't granite. At this point, since we don't know what it is, the best advice I can give you is to get hold of a professional stone refinisher. Maurizio, USA
A 1595: I am doing a report on Granite for my fifth grade science class. I can find a lot of good things on Granite. But I can not find the disadvantages of Granite. Could you help me please. Thank you! Kenny, March 26.
R3: Hi Kenny: I bet you never expected for your question to create an issue where you had to examine a paradigm. This is actually quite exciting and perhaps this will have you embark a career as a scientist.
Thus far you have been given two concepts when you asked what is negative about granite.
Now lets add some parameters to the question. What do you mean when you ask what is negative about granite? There is a concept called context. This is what determines the meaning of a word. For instance, Maurizio talked about "granite" not have a specific meaning in the mercantile sense of making countertop or wall products from it. The negative here has nothing to do with granite It pertains to many other rock formations being included in the generic term granite. Quite fascinating if you think about it. The question we would ask Maurizio in this context is: Are all the other stones called granite inferior to actual granite? His answer would then be a qualified, "It depends". Many of the stone formations are superior to actual granite and many are inferior to actual granite from the perspective of absorbency". Aha!! Weakness #1 Granite is absorbent. So in many settings granite may need to have some impregnator applied or be deemed unsuitable for tha application. This brings us back to the context . Namely, what applications or settings granite should be used in?. 
Weakness # 2 Granite does not have flexural strength. Whoa!! This means that if you need to have a material flex and bend to some sort of dynamic stress granite is not your product. Once again context. What application should granite be used for. Clearly it needs to be used in specific settings with specific criteria.
Weakness #3 Granite is hard to work with or transform into something. Bryan refers to the fact that the ability to cut, shape and polish granite as opposed to other products is much more difficult. This, in this context, is bad, yet, in another context, it may be good.
Misinformation #1 As a scientist we have to evaluate all information given to us. Bryan brings up Radon. So what is that? It is the colorless, radioactive, inert gaseous element formed by the radioactive decay of radium. So what does that have to do with granite? Well, radium has been found in granite foundations (subsoil) all over the world. Because it contains the word radioactive it scares people. However, in many disciplines such as radiotherapy and research it is helpful. Now in the context of a granite sitting around Bryan implies that it is dangerous. Here is where you do your research. The danger he speaks of comes from a minute possibility that you would be in the subterranean basement with no ventilation and you would live there. Well, I know that I would not live in such a setting and practically no one else does. When we review the details Bryan asks us to, we start to understand the paradigm taking place. There is an industry around radon fear. There are people who perpetuate this fear. Even though granite used in the setting of a countertop or wall would never expose us to the hazard Bryan referred to, we still have a vestige of fear remaining. Furthermore, we find that the hazard Bryan refers to needs to have many other things to happen before it could even potentially be a hazard. Namely, no ventilation, quite a lot of radium in the soil, and time (lots and lots of it) for the radium to decay. I hope this helped, Kenny, Regards, Steven, USA
R2: Hi Kenny: Yes it's true most granites used in the stone industry are not true granites, but with such a proliferation of obscure names for igneous rocks, it is usual practice in the industry to call them all granite. Disadvantages, try working it for starters. Also it's weight and wait for comments, but to my mind just never seems to match the luster and delicate color tones found in many marbles (color to those of you in the US) . It also emits radon, a radioactive gas. Normally in such small amounts that it poses no problem. However there have been studies in Aberdeen Scotland (known as the granite city) into the possible health effects of so much granite in one area. Check out the British Geological Survey web site. Bryan, UK.
R1: Dear Kenny: A very quick answer. The disadvantages of "granite" are that there's not even a hint of a serious classification of the different stones trade as granite (approximately 98% of which are not granite), and, consequently, the virtually complete ignorance about stone displayed by the industry authorities and operators. 
No matter how you slice it, ignorance ultimately spells: T-R-O-U-B-L-E all the time! And that, in my book, is a big disadvantage right there! Ciao and good luck, Maurizio, USA
A 1348: We will be remodeling our kitchen on the coast in Florida. We have been receiving conflicting information from three different sources (three separate opinions). I need a definitive answer: CAN YOU PUT A HOT POT ONTO A GRANITE COUNTER TOP.... I am very frustrated. A company that specializes in granite counter tops (and marble) told me that it requires more care than the synthetic tops like Wilsonart and Corian (?). I talked to the designer at Lowes and he told me that it didn't take as much care and that you COULD put hot pots off the stove.
Would you please tell me the straight scoop on granite counter tops?
1. Will granite stain like Wilsonart?
2. Will it crack from heat (example: pots from stove)?
3. Will it scratch easily?
4. What is the average cost per square foot, installed in the south eastern states?
Any help you can give would be most appreciated.  Thank you Faye, Jan 18.
R3: Dear Faye: If it can be of any consolation, the good news is that lack of professionalism is not limited to central Florida, but all over the country and beyond! 
What to do? 
Well, not much for the time being. For starters, it seems to me that the recognized authorities of the stone industry have, as their main goal, to promote the use of stone (to sell more and more of it, that is), but don't care much about business ethics and intelligence. In an environment where there's no official guidance and precise definitions and guidelines, it's hard to expect salespeople to become experts on their own!! 
To have educated people, you need good certified teachers. In this industry, any quack with something to sell can go on stage, grab the microphone, and say that he or she knows everything about stone, and then some!! What else can I say? It's sad, but that's the way it is. Ciao, I'm upset, Maurizio, USA,
R2: Hello Faye, All the answers you have received are confusing. 
Starting from the bottom budget about 75-80 USD a square foot for allowance.
With regards to heat, it is always prudent to test things. Granite started its existence in a flowing state then cooled. The temperatures to melt granite are so much higher than any man made product it doesn't really bear discussion.  I have taken hot pots and put them on my countertop without incident.
I think that if you get a low absorption stone ( use the lemon slice test devised my Maurizio - a good expert on this site) and test each piece of stone your self. All the sales people are telling you things that are correct in specific contexts. The contexts are the following--
1) Many stones get called granite that are not. The swirly juparanos and many of the really light stones are not granite. If you put them (light & swirly) in the kitchen you will have to be very diligent about wiping things up as they occur and use trivets for hot things. Many people do not clean as they go and they forget the trivets. This will lead to problems.
2) Many people are attracted to stones with veins or fractures. Veins and fractures typically are the weakest point in a stone. So back to the stone versus the hot pot question. If you ever touch a piece of stone you will find that it is quite cool. So if you take something really hot and place it on something cool you will have a rapid thermal change in both items. Rapid thermal change causes stress. If the rapid thermal change is near a fracture then it can cause it to crack. So, if there are not any fractures near the point you put the hot pot you should be fine. 
3) On the Mohs scale most kitchen knives fall between a 4 and a 5 for hardness. The granites are usually a 6 or a 7. Therefore you should expect the knfe to dull rather than the stone to cut. But don't trust a sales person, ask for a sample of stone you like. Put hot stuff on it to see if the shine is harmed and the stone is affected, try and scratch it with a knife, put a slice of lemon on it for a minute then take it off. See how fast the stone gets wet. If it greater than two minutes you are in good shape.
Sorry to be longwinded, Regards, Steven, USA,
R1: Dear Faye: 1. I don't know how Wilsonart stains. I do know that "granite" will stain if you select the wrong "granite". Go with my "lemon juice test" - it's very dependable. 
2. Most likely it won't crack from heat (I do that all the time on my own top!), but, if it's a "granite" that need sealing, the heat will damage the sealer. You don't want that! 
3. No, granite doesn't scratch easy at all, but "granite" might (never so easy, anyway). While doing the "lemon juice test" on pieces of scraps try to scratch them with the tip of a knife and be the judge. 
4. I live in NJ, I don't know. Ciao and good luck, Maurizio, USA,
A 1261: I too have been trying to find information (similar to that of A1248) for 3 weeks and have been unsuccessful in locating any information regarding flame retardency or sound absorption of granite. If you have any information on these two areas, or know where I might be able to find information, I would really appreciate it if you could help me in any way. Thanks, Tripti. Dec 14.
A 1248: I am doing a paper on granite for my materials and sources class. I need to know the following on  granite: Pollution Resistance (Air and Chemical Exposure), Flame retardancy & Toxic Factor (smoke), Sound Absorption. Also, if you are using grout to install granite--what type would you use and why would you use it? My paper is due on December 14th.  Any help you could provide would be most appreciated. I will be sure to send the paper to you when I am done. I LOVE YOUR SITE!!!!!!! :-) Nafisa, Dec 10.
R2: I do believe in being as polite to all people who ask questions; but I dislike it when students procrastinate with their projects to the point of asking us to do their research for them. Further I would have to write a treatise to even answer this one. Poorly done. Regards, Steven, USA
R1: I would recommend an epoxy grout or a portland cement based grout. Regards, Steven, USA

A 1142:Do all pieces of granite? If so, to what extent would it be acceptable when purchasing a slab of granite? Do some colors of granite have more pin-holes than other colors? Jane, USA, Oct 13.

R4: Jane, The short answer is yes. Many times in granites the surface is not completely smooth, and some granites are more prone to this than others (those with soft minerals that will abrade out during the polishing process). What has happened in the past is that when the slabs were reviewed a decision was made about whether or not to accept the material. The companies processing the slabs presumed that the pits would not adversely affect the overall aesthetic or performance of the stone (they were not thinking about sealing it or using it --just selling it!). We now see many of these slabs come with resin applied before the polishing phase. This practice decreases the pitting and gives a more consistent gloss. However, the long term effect of this practice has never been studied. Regards, Steven, USA,  
R3: Yes, they do. Considering that the vast majority of stones traded as granite are not granite by a long shot, no rule could be set on how many "pinholes" represent a standard of acceptability. Please realize that, unfortunately, the stone industry worldwide is pretty much unregulated. It seems that consumer protection laws don't apply when it comes to natural stone. Everybody can sell you something under labels that they please and with the (unofficial) grading that they care to disclose. Maurizio, USA,
R2: Dear Jane, Pin-holes are also known as pits and they are most conspicuous when viewing a polished stone surface from an angle so that it is reflecting a light source. They occur in most granite varieties but also in rocks termed granite in the stone trade that compositionally are not true granites. These include the higher grade metamorphic rocks such as the swirly varieties, multicolours, gneisses and schists. Most of the pin-holes or pits to which you refer are the result of the polishing action used in the processing of these stones acting on a soft or abradable mineral. Mostly these soft minerals are platy or flaky and include the micas (such as biotite and muscovite), chlorite, clays, serpentine, talc, and to a lesser extent, the more fibrous amphiboles, anthophyllite and actinolite. When biotite and muscovite are fresh and coarsely crystalline, there is usually little pitting during the polishing process. However, biotite can be easily altered by hydrothermal activity and degraded structurally or chemically, or partially to completely altered to secondary minerals the most common of which is chlorite. Because chlorite is quite soft its surface is susceptible to abrasion during polishing and a small pit results where some of it has been removed. Most of these types of pits tend to be scattered and are easily seen in the reflection described above. This is the general situation for most pits, i.e. where original rock-forming minerals have been altered to secondary minerals that are markedly softer. Other examples of pits are where feldspar cores have been altered to clays (can result in deep, conspicuous pits) or where ferromagnesian minerals such as orthopyroxene and olivine have been altered to clays, serpentine, talc and chlorite. In the high-grade metamorphic rocks (especially those swirly ones and multicolours) the grainsize is usually much finer and the abundance of the micas relative to the harder minerals is much higher. During the polishing process many small flakes can be removed and a surface will result with numerous closely spaced pin-holes. The resultant polished surface is quite "open", and reflectivity is diminished. More importantly, these pin-holes result in elevated fluid absorption and this is often a source of considerable disappointment and unhappiness for many customers. These types of absorptive stones must be religiously sealed and maintained and I would never recommend their use in kitchens where oil spillages could occur. To overcome the pin-hole appearance it has been discovered that certain short-lived substances are applied to the slab surfaces by some suppliers. Pin-holes also occur on a microscopic scale with the effect of a "frosted" or honed appearance. Processors will complain that some schistose rocks won't take a polish. This is simply a mineral orientation effect and incorrect direction of slabbing. Another example of a microscopic pitting can occur where a syenitic green granite from the northern hemisphere is heavily dusted throughout with tiny carbonate crystals. When installed in a city environment these tiny crystals are subject to alteration or to conversion to gypsum with a resultant loss of polish. If you don't like the pin-hole effect you will probably not enjoy the problems that you might encounter with the stone after installation. If you want quality, negligible maintenance, and a retention of original appearance, choose a granite that "closes" well. Many of the fine-grained black granites fall into that category but there are also some classy greys and some greens. They will cost you a bit more but you shouldn't regret it in the long-term. The moral of this story - choose wisely. (Dr.) Hans, Australia,
R1: Dear Jane: The pin holes you are talking about are probably minerals plucked from the surface during polishing. The most likely mineral to form pin holes would be biotite (a black mica). The acceptance level is up to you, depending on the end use of the stone. Lighter coloured granites are likely to have less pinholes, but it is also a function of the quality of the polish. It really depends on what you think is acceptable at the price you are paying. Regards, Jim, Australia,

A 1013: What happens if the granite is heated about 500 deg F? Will it crack? Can you use granite to manufacture dishes or plates? Is it hazardous to health? Cesar, USA. June 20
R2: In as much as it will come as a shock to you, if you heat granite to about 500 deg F it will become hot!
Will it crack? No, it won't. In fact, granite it's the only material on which you can put a pot right off the stove top.
Is it hazardous the health? Big time! especially to your teeth if you try to chew it before swallowing it!
Maurizio, USA Contact

R1: Granite should not be affected by 500F if the heat is evenly applied. Cooling of articles with a change in thickness could be of concern if not done slowly.  There is nothing deleterious in most granites that make it unsafe to eat off.  If "machining" a piece of granite, you would want to know the orientation of the "grain" just as in wood work.  A granite's strength can vary depending on the orientation. Jim Australia