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Pennsylvanian Whetstones

I received some requests for my writeup on Pennsylvanian natural whetstones, so I will post it here in a few installments and add more as I go. This stone hunt was originally geared towards knife polishing, but some have turned out to be great razor hones.



PA Whetstones

For the past few years I have been searching my native Pennsylvania for a suitable whetstone material. We have rocky soil, boulders strewn about the landscape, and plenty of shale and slate outcroppings, and almost all of it is too hard and scratchy. Eventually I discovered a type of stone that performs extremely well as a polisher and sharpener once you know its intricacies. It’s an elusive and sensitive stone.

I’ve been wrestling with these stones for a while trying to figure out how they work. Sometimes they gave amazing finishes, other times they were too scratchy. These aren’t the types of stones that just hand you amazing finishes without due diligence. There are certain stipulations that make these stones a little more sensitive than your average stone, but the rewards are clear as day. As a polisher, the contrast is untouchable by any Japanese stone I have ever used. Cores that are deep mirrors, and claddings that are perfectly opaque. Streaks are completely eliminated with ease, and spot treatments blend in instantly.

These pieces have an unusual characteristic with regards to their hardness. Slurry pours out of them under a diamond plate, making them seem extremely soft. But under a knife blade, they hardly release anything at all. They act hard or soft depending on what you rub on them. Anything with a grit, like the micro protrusions of a diamond plate or sandpaper engages with the stone’s surface and releases a ton of slurry, but anything that is smooth like the bevel of a knife or razor doesn’t catch the surface in the same way and the stone’s particles stay locked in place.

There are a few extremely useful characteristics of these stones. The first is that they give the same level of polish with or without slurry. So, if you accidentally scratch your bevel on water only, you can raise a slurry and take out the scratches without taking any steps backwards. This works quicker on cladding scratches, as the stone is much gentler on the hard core. The second is that they completely eliminate any streaking. No matter where I focused my pressure, or where I did any spot treatments on the finish, the work blends in instantly with the surrounding area. It really is unbelievable.


Some stipulations to keep in mind: (1) They are sensitive to diamond plates. If your plate is too fresh or too aggressive they tend to release larger chunks of grit that can scratch your bevel. I use a PA nagura or the worn out part of my atoma 400 and the slurry is much cleaner. (2) They are really thirsty stones. (3) Most scratches visible on the hagane before you use the PA stone may still be there after, just more mirror and in heavy contrast with an opaque cladding. I used a Nakayama with slurry before going to the PA stone just to clean up lower grit scratches, and this helped a lot with the final finish.

I don’t know how these perform on honyaki, but I would love to find out.

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What have you found is the best means of flattening a working surface?
I may take a hatchet or chisel to the really high spots if I need to, but most of the work is done on a big tile with coarse SiC powder. I then cut them to shape on a wet tile saw if necessary.
 
They bring out a lot of contrast on aogami super steel too. The core looks a little grainy up close, which I guess is the grain of the steel? When I first saw that effect I reset the finish on a fine Nakayama with and without slurry, then redid the PA finish and got similar results.

These seem to perform a lot like uchigumori but with exaggerated effects. Like uchigumori, I think what’s happening here is that the particles themselves are generally coarse but also soft so they resist scratching hard steel. They make the jigane super frosty and more or less buff the hagane.

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Heiji semi-stainless core with stainless clad. This seems to be a coarser example of the PA stones, or at least it’s acting that way with this Heiji. Check out the artifacts that appeared in the core - I'm pretty sure it's not corrosion, so perhaps it's the grain of the steel?

The original finish:
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The PA finish:
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I have been searching for a suitable backing material for the brown PA polishing stones and have come across a black shale that could fit the bill. It is a mid grit stone with a very consistent scratch pattern which I find completely appropriate for day to day edges and finish. On average it can come in thicker pieces which is great for adding mass to a combination stone. It is stronger than the brown stone, but not very strong, which is the only drawback I can think of for using this as a backing.

Seeing that these black pieces are useful in their own right, I may decide to glue them to their own harder backing. Gluing a brown polishing piece to one of these is not out of the realm of possibilities though, like a combo coticule.

The finish is consistent, usually either 1-2k or 3-4k depending on the piece. In some angles you can see the scratch pattern and in other angles you can only see the contrast. The edges feel refined with a little bite - great for the kitchen. Much like the brown PA stones they don’t release much mud on their own under a blade, but will readily release mud under a diamond plate or matching slurry stone. They work best with a slurry, which is inky black in color.

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I have been searching for a suitable backing material for the brown PA polishing stones and have come across a black shale that could fit the bill. It is a mid grit stone with a very consistent scratch pattern which I find completely appropriate for day to day edges and finish. On average it can come in thicker pieces which is great for adding mass to a combination stone. It is stronger than the brown stone, but not very strong, which is the only drawback I can think of for using this as a backing.

Seeing that these black pieces are useful in their own right, I may decide to glue them to their own harder backing. Gluing a brown polishing piece to one of these is not out of the realm of possibilities though, like a combo coticule.

The finish is consistent, usually either 1-2k or 3-4k depending on the piece. In some angles you can see the scratch pattern and in other angles you can only see the contrast. The edges feel refined with a little bite - great for the kitchen. Much like the brown PA stones they don’t release much mud on their own under a blade, but will readily release mud under a diamond plate or matching slurry stone. They work best with a slurry, which is inky black in color.

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Holy smokes that's amazing. The polish is very nice and I can see the scratch pattern in one of the pictures. Much more uniform than most of my coticules.
 
Cool find! and nice write up.
Look's promising.
Holy smokes that's amazing. The polish is very nice and I can see the scratch pattern in one of the pictures. Much more uniform than most of my coticules.
That is a little beauty…I love the blue streak shooting across the brown!

Thanks guys, I am going to seal a few more up in case anyone wants to try them out. They might not be the brown ones above, but the darker shales/slates that you’ll see in later installments to this thread.
 
How are you identifying the brown slates in the wild? Where in PA are you? I grew up just outside Pittsburgh and may get back there again for a visit.
 
I'm game sign me up.

I have found a wild stone that works great for knives but is more mid range. The only bad part is they are way to big to carry down the hill. Spent like two hours trying to find a piece small enough to carry. I finaly found a palm sized piece but it was not easy. Seems like they are only in a football field sized area.
 
How are you identifying the brown slates in the wild? Where in PA are you? I grew up just outside Pittsburgh and may get back there again for a visit.
The brown ones actually look like most other slates/shales out there when they weather unfortunately, no particular characteristics. Just grey. I only know what they are when I cut them open :/

I'm game sign me up.

I have found a wild stone that works great for knives but is more mid range. The only bad part is they are way to big to carry down the hill. Spent like two hours trying to find a piece small enough to carry. I finaly found a palm sized piece but it was not easy. Seems like they are only in a football field sized area.
I've noticed that most of the bigger chunks I find end up being coarser, and the finer stuff usually comes in very small pieces.
 
The brown polishing stones are as rare as hens teeth, and out of the ones I do find a fraction of them are suitable for the job. While searching for them I did find these beautiful examples. They are a greenish grey color and often have a brownish cloudiness. What’s most important though is the polish, and I find it’s really close to the brown stone polish. I would still say they give better contrast than any jnat I have seen.

They feel a little sticky under the blade, especially on water. Build up a mud and the effects happen quick. Not thirsty, really fine, sort of hard but not glassy and very friable under a diamond plate, and they all give an amazing polish.

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I have been searching for a suitable backing material for the brown PA polishing stones and have come across a black shale that could fit the bill. It is a mid grit stone with a very consistent scratch pattern which I find completely appropriate for day to day edges and finish. On average it can come in thicker pieces which is great for adding mass to a combination stone. It is stronger than the brown stone, but not very strong, which is the only drawback I can think of for using this as a backing.

Seeing that these black pieces are useful in their own right, I may decide to glue them to their own harder backing. Gluing a brown polishing piece to one of these is not out of the realm of possibilities though, like a combo coticule.

The finish is consistent, usually either 1-2k or 3-4k depending on the piece. In some angles you can see the scratch pattern and in other angles you can only see the contrast. The edges feel refined with a little bite - great for the kitchen. Much like the brown PA stones they don’t release much mud on their own under a blade, but will readily release mud under a diamond plate or matching slurry stone. They work best with a slurry, which is inky black in color.

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Pretty sure i also have a couple of that stone. Looks very similar anyway. Have you tried out Pennsylvania bluestone? I think it is mined in the northeast (NEPA to the locals) but is available all over the state. I picked up a large piece from the local landscaping supply. Sandstone, feel like it would be coarser then a queer creek. Slow and course with water but fast and fine with slurry. When the weather gets better i want to put a few pieces in my kiln and heat-treat? them. Read somewhere, grinding and honing i think, that heat will change the characteristics of the stone. I have plenty of it so why not.
 
What i have found so far from my collection. Starting on the left.
Random fine sandstone. Slow cutter, burnishes/loads up fast with water. Have not tried it with oil yet. Seems lower mid range.
Second is Pennsylvania blue stone. Sandstone. Lower mid with water, mid mid with slurry. More of a mirror polish on hard steel then a haze.
Third is...no idea. Very hard, gotta finish lapping it to test.
Fourth and fifth are hard grey, i assume shale. Very easy to split but i do not think they are slate. Upper mid, more of a haze the a mirror. The very last one has a spot of green in it, second picture. I do not feel it under a blade tho.
The last 2 are sitting on a piece of newly quarried Pennsylvania slate. Got to cut it up and test it still.
Will post more as i find them in storage. My "fancy" blades are also in storage but i have a "Damascus" chef knife i made many years ago that i will do some test with so the contrast can be seen. It is 1095 and 1018 forge welded steel. Never finished it because final grinding exposed too much 1018 at the apex after heat treatment. But good for testing stones.
 

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