Friday, January 11, 2008

How to Polish and shape Petoskey Stones

STEP 1 :
The First step is to have a Petoskey stone. Hopefully you have found some on the beach but if you haven't there are now people selling them unpolished on eBay pretty cheap.
If you have several wet them and pick one that is close to the finished shape you desire. You may be making a pendant to put on a necklace , a paper weight , a small oval to put in a ring blank , a natural shape to drill through and put on a fan-pull, a large natural size to set on your coffee table as a specimen, a flat disc to decorate a box top, or a pair of earrings . You may be making virtually anything at all.
Now look at the "eyes" on the stone, are they nice and round and all facing the same way? If they are oblong then you are looking at the side of the corral and it would be best to cut it so you can polish another end.
If it seems only part of the stone is Petoskey and the rest is just gray then you have a chance of finding Petoskey under the gray. On the other hand you may begin sanding and find that it is just gray or black rock. This is the sediment that the corral "sunk" into as it became fossilized. I like to call it the mud.
Sometimes the Petoskey shapes will be on one side and another fossil on the other. If you see tiny hexagons with no eyes this is the Charlevoix corral, it's scientific name is Favosite.
Some "eyes" seem to be cut in half or smashed. This means the corral did not settle nicely into the sediment. I would keep looking for a better stone.
Sometimes a stone will look good at first but then once you start polishing it there are problems. Maybe it is not very clear and detailed in the rays of the "eye". Maybe it has dark marks all over it, maybe it diminishes in size and turns black-this is the sediment showing up from the back. Whatever you see that you don't like will probably not go away. I usually abandon that stone and go to another. Several times I have spent 20 to 30 minutes polishing this kind of stone hoping it will get better and it never does.
It is for you to decide which stone you like best and wish to shape or polish.


STEP 2 :
Now you must decide if you are going to shape the stone or leave it in it's natural shape. For jewelry I usually shape them. For a fan pull, specimen or paperweight I leave it as close to it"s natural shape as I can, though sometimes to make it's "best" side show I will have to alter how it sits or hangs and so I do shape it a bit.
For necklace pendants I like it to be about an eighth of an inch thick. I think if you sand it down very, very thin then it will break easily. Remember- it is not rock. I love to make a perfect circle, I have also made ovals, diamonds, squares, rectangles, ovals with the corners squares off, tear-drops, octagons, triangles, etc.
For earrings any shape will do and I always like to make the earrings from the same stone, otherwise you may end up with one having a different hue than the other, or different sized eyes, or different clarity.
Rings are very hard because they have to fit into the blank exactly or close to exactly right. Just start with the basic shape and keep sanding down a little at a time.
Paperweights are fun, make them heavy enough to not be pushed around easily and light enough that they are not hard for an elderly person to pick up. I have seen orb paperweights sell for up to $70 on eBay.
A large stone to set on the coffee table as a specimen is also interesting. Some people I know have polished fist-sized stones to put in their fish tank. We have several in our turtle's tank and it does not effect him at all but if you have expensive fish or salt-water fish then you might want to be a little more cautious.


STEP 3 :
I cut my stones with a rotary saw. If you only plan to make a few items then you might jerry-rig your saw and drill as I have done in the photo below.
Cut it to it's proper depth first. Then cut off pieces from the edges to get it close to the shape you want. Once it is close you will start sanding.
When I first experimented with shaping petoskies I used a drill and a sanding pad. As you see in the photo this is very easy to set up with a vice clamp on a work table. A drill with a plastic casing might not like being clamped so see if someone you know has one with a metal casing. I have clamped my plastic ones and not had any problems but I do not want to suggest that you do something and then have it wind up bad. I like the solid sanding pads instead of the ones with a hole and screw in the middle because sometimes the sandpaper would get a wrinkle in it and then I'd have to change the paper to a new one.
You should wear safety glasses and a face mask when you do the cutting and sanding, a lot of dust is produced. Sand your edges and back first, then go to the "face" of the stone.You will do this 3 or four times with a finer and finer grade of sandpaper. If you want to bevel the edges do this with the last 2 sandpaper grades, holding the pendant's edge at a slight angle to the sandpaper.
Now you must sand by hand. The beautiful details only show up after using very high grades of sandpaper and I have not found any to fit a sanding pad. These grades are used wet, I dunk the stone in water and go right to the sandpaper. You want to get a small pool of whitish liquid where you are working. If you do not see the water turn to a whitish liquid then it is too dry and you need to add a few drops of water. Make sure you are doing this sanding on a very flat surface, like a Formica counter top. Just a piece of plywood or the like will be too grooved even though it seems flat to your eyes. For this sandpaper I start with 400, then go to 600, then 800. Between each new sandpaper rinse the stone in water, at this stage the stone can scratch easily and even a grain of sand or debris will scratch it.
To sand large surfaces you can go in circles or back and forth. For sides and bevels I have found that a straight across movement works best. Hold the edge or bevel at the correct angle and drag it across the paper as if you are scratching the paper. Remember you should have a small pool of whitish water on the paper where you are dragging it. If you look at your edges and have lost a sharp bevel or side you can correct it. Go to a lower grade of sandpaper and re-create the nice edge. Sometimes when I do this I have to go to dry sandpaper and then back to wet.


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2 comments:

Cecilia A said...

I was so excited that someone was writing this blog to explain full techniques to finishing a Petoskey stone. I am so sad that it did not go further and include some pictures of the process. Please come back and finish this wonderful blog.

Gigi said...

Do you polish the entire stone flat down to the deeper eyes to make it flat? The eyes on my stone are inset about 1 mm. Thank you for advice!
Gigi