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Techniques for CollectorsTrimming granite

18th Jan 2007 21:40 UTCBarry Flannery Expert

Hi all,

I'm wondering if anyone has any tips when it comes to trimming granite? I find it always fractures is the most irritating fashion making it nearly impossible to creat an aesthetic specimen. I usually always end up with a big heavy specimen (as in excess matrix) that cannot be trimmed without damage. Anything short of grinding grooves or professional trimmer devices what can I do?

ANY help would be appreciated!

Barry

22nd Jan 2007 15:53 UTCJyrki A

Hi Barry

Hard rock can be removed mechanically by chipping, splitting, wedging, sawing and granite even by burning. One of the many ways should be suitable for the rock at hand.

I run classes of stonework as my winter job and sculpt granite in my spare time. Here are my tips for smallish rocks:

First you need tools.

For chiselling and splitting granite sharp chisel and relatively (even surprisingly) small hammer work best. You need to produce sharp blow to minimal surface area of the rock. Carbide tip tools are good but expensive (+50â‚¬ a piece) and may be hard to find. Steel alloy tools are less expensive and work as long as they are sharp, but they blunt easily. They must be sharpened after every use while tungsten carbide tip stays sharp for long time. The size of the chisel matters. For rocks smaller than â€¦say your head, small chisels are most practical. For minute work long hilti-type nail gun nails are inexpensive and hard enough to bite granite.

To chisel small fragments off the rock point-ended pencil shaped tool is used.

To split the rock wedge shaped point chisel is used moving tool back and forth to create split line in the rock. Split line is first hammered lightly to create sharp shallow groove and then by increasing hammering (chiselling) force along the line until the rock splits at some point. Large stones are actually bent to halves along the line. (not unlike cutting glass with glass knife)

Even with small work tiny splinters of granite take off with considerable speed, so protect your eyes.

The problem with removing matrix from mineral samples is the often pegmatic nature of granite where interesting minerals exist and fractures in the rock. With practice sound, even grained granite splits where you want. But granite breaks along existing fractures more easily than man-made. Pegmati(ti?)c granite tends to break between the mineral grains and along feldspar cleavages.

It might then be better to saw the rock. Cheap angle grinder with eg. 125mm diamond blade works well in granite. Better grinders last longer but with even inexpensive one you can saw great many rocks. Blades vary, some work better on granite than others. They should be marked for dry stone cutting. I use 10-20â‚¬ range diamond blades. With 125mm blade you can make ca. 4cm deep incision in rock surface. With 230mm blade it is 7-8cm, but the corresponding grinder is a bit too clumsy for this kind of work.

When dry-cutting granite, heat spoils the blade easily if it is pressed too hard on the rock or it is not allowed to breath every now and then. Granite dust is harmful and eyes must be protected. It is useful figure out how to fasten small sample for cutting to save fingers.

Sawn surface can, if annoying, be split off, because rock splits easiest near corners where there is least resistance. Incision can be sawn in oblique angle to desired cutting direction and the part then removed by splitting in desired direction to remove saw marks. It is even possible to take a piece from middle of rock surface without marks of sawing.

Because compressive strength of granite is much higher than tensile or flexible strength it is much easier to rip the rock open than press it to pieces. Wedges can be used to do this. They are used to split large rocks, but work as well with smaller rocks. (if nothing else works) Wedges are hammered in aligned boreholes and tightened evenly untill the rock splits along the line. This method is a bit too heavy for mineral samples though mini-wedges for mineral cleaning would be funny to trie.

And last. A bit of practice with just granite, without those minerals would be wise.

Jyrki Autio

25th Jan 2007 22:14 UTCColin Robinson

An interesting observation on the trimming of specimens by Jyrki, a man who works with stone in a professional capacity.

Some years ago, at a famous Weardale location I met a chap who was using a small toffee hammer to trim excess matrix from specimens. He would toss the specimen around in his hand and just tap away. He could easily get the weight down by half and end up with a "natural" looking specimen. We became good mates and I adopted his technique which, I admit, does take a bit of practice. Jyrki is quite right though, small sharp taps to a small area work best. I liken it to flint napping. Our ancient ancestors could make usable tools out of lumps of flint or other rock simply by knocking off the unwanted material. My mate says it works best if you imagine what you want the specimen to look like. Not very scientific but it works.

17th Jun 2007 17:40 UTCRock Currier Expert

Jyrki Autio description of granite trimming is as good as any I have heard and can be applied to other rock types as well. This is a good example of the kind of explanation that should be saved for a special place on mindat, like some of Peter Hass's writing.

Rock

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