Re: More marble and granite

I got this from a friend who wishes to remain anonomous
I thought some would like to read it, so here it is.

Bill

>
> Hi Bill,
>
> I saw your posting on the DRO list. Since I'm still lurking, I don't answer
> on list. You can share this on the list if you'd like.
>
> The bottom line on machining granite and marble:
>
> Marble is a metamorphic rock consisting of calcitic crystals with different
> combinations of other minerals. Be careful, different people call different
> product marble. True marble contains different mixtures of large, medium
> and fine calcitic crystals with a smaller percentage of quartz and mica.
> Other rocks, that look like marble, like ophicalcites that commonly contain
> other minerals including serpentine, chlorite and anphibole. Marble has a
> wide variety of hardness, tensile strength, fluid and gas absorption,
> elasticity, and breaking loads. Marble's properties change depending on the
> quarry and even specific veins of marble within the same quarry. Marble has
> a Mohs' hardness of about 3 to 4 depending on the type of marble. Steel and
> glass have a Mohs' hardness of about 5.5. So steels and glass appear to be
> a lot harder than marble right ?? Tool steel should have little problem
> cutting marble and not really get dull right ?? No such luck. Marble
> contains quartz. Quartz has a Mohs' hardness rating of 7, twice as hard as
> any steel. The quartz content of marble wears steel tools down to a nub in
> no time flat. The higher the quartz content, the faster your steel tools
> wear. The only thing that cuts marble well is diamond and corundum. Also,
> if you start machining marble, be careful of the grain You'll machine one
> side in one direction and it looks great. No cracks, chips, or flaking. You
> change direction of the table and it starts chipping, flaking and cracking
> all over the place, especially around the veins. Sometimes it just cracks
> along the veins straight through the work piece and falls apart. This is
> due to the stresses induced by the machining vibration. Coolant is another
> problem with marble. Its main binder is calcitic crystals, so marble is
> very porous and absorbs fluids and gases easily. This includes coolant and
> anything in it. Anything other than water will stain the marble. Once the
> marble is stained, it really tough to get out. You don't really get it out,
> you just keep removing material until the contaminated marble has been
> removed. Also, make sure the coolant is NON-ACIDIC the binder is calcium
> crystals. Strong acids on marble looks like a baking soda and vinegar
> experiment. Experience is the best teacher.
>
> Machining granite is a lot more fun. It has a Mohs' hardness rating of 7.
> Granite is composed of the minimum of 50% feldspar (aluminum and silicon
> dioxide a.k.a. glass), 25 - 40% quartz, and 3-10% mica. This is really
> tough on steel tools due to the high quartz content. I only use diamond
> tools for granite. Its the only thing that works half way decently for
> cutting, drilling and machining. By the way, use LOTS of water for milling
> and especially for drilling. Machining against the grain is still a problem
> in granite, but not as bad as marble. This is due to mica content. Mica
> doesn't really cut, it cleaves. I believe its the only mineral with perfect
> cleaving. When the mica cleaves in granite, it leaves little pits in the
> work surface. If you looking for a clean finish, grind and lap the surface.
> You can get it to look like a mirror (good for IR). If you use anything
> other than water as a coolant, it will stain the granite, but not as nearly
> as bad as marble. The fluids get absorbed into the mica and feldspar. Mica
> doesn't really absorb fluids it mica wicks the fluids between the cleaving
> layers. Coolant acidity is not a big problem with granite. Acid effects the
> feldspar, but you need something like concentrated hydrochloric acid (HCl -
> ph = 1) or Hydrobromic acid (HBr -ph = 0). If your using this stuff, your
> shop machines you'll be looking for a new shop. The effect of milder acids
> is over a long period of time depending on the type and concentration of
> the acid.
>
> I'm not trying to discourage you from working with marble or granite. It
> really is great fun to learn to work with this stuff. The next time, I'm
> going to try Corian. Its supposed to be as hard as marble , but without the
> grain issues. Its also has about 1/3 the absorbtion of granite.
>
> Here's a copy of the Mohs' hardness table.
>
> Mohs' Hardness Table
> 1 talc
> 2 gypsum
> 3 calcite
> 4 fluorite
> 5 apatite
> 6 orthoclase
> 7 quartz
> 8 topaz
> 9 corundum
> 10 diamond
>
> Good Luck
>