In Pursuit of the Devonshire Oilstone...

[cotedupy]

[Below are copied five posts from B&B; the first three were from January, the final two yesterday. If anybody is from this part of the world I would strongly recommend trying to track some down. Though I don't know exactly where the quarry is, I found these pieces about 3 miles east of Mary Tavy, but you can see the stone used in many old buildings in Tavistock and MT. The Mary Tavy Inn for instance has it in some of the outside garden and carpark dry stone walling. You could probably nab a bit from there tbh!]

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‘13. Devonshire oil-stone is an excellent variety for sharpening all kind of thin-edged broad instruments, as plane-irons, chisels, &c., and deserves to be better known. This stone was first brought into notice by Mr. John Taylor, who met with it in the neighbourhood of Tavistock, and sent a small parcel to London for distribution; but for want of a constant and regular supply, it is entirely out of use here.’

It’s not much to go on is it?

That extract from Richard Knight’s 1836 letter detailing The Principal Stones Used in the Mechanical Arts is about the sum total of the information we have about the so-called ‘Devonshire Oilstone’. And even then Mr. Knight account cannot be assumed to be the most reliable of sources; later on for instance he appears to mix up the Belgian Coticule and German Thuringian hones.

Recently though its existence and identification have occupied the minds of a number of people. Perhaps the most extensive search and research was done about ten years ago by a member on SRP, who unfortunately time and again found that any definite knowledge of the stone has since been lost. Though did discover a single more concrete piece of information, which appears to be a transcription of a presentation given by the same Richard Knight as above:

‘This stone occurs near Huel Friendship Mino, about three miles from Tavistock, in the Devonian slates of that district... it has not, however, become an article of commerce.’

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Here then are the first assumptions I’m going to make... the Devonshire Oilstone is a type of slate occurring around the tiny village of Mary Tavy (where the Jewell Friendship Mine is found). And that it was never quarried specifically as a hone stone.

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My sister lives relatively close to Tavistock and Mary Tavy, and I’ve been there a couple of times recently. Yesterday for instance we went walking around there on a route that took in five or six disused quarries. Though we were a few miles south east of Mary Tavy, due east of Tavistock, at the start of Dartmoor. And by that point the quarries seemed to be for granite, some were unidentifiable, others looked like this:

Though at the very end of our walk I came across something a little more promising. As far as I can see this is basically a small slag heap of local slate that has been used to elevate the old road across Dartmoor:

And in it, pieces of something quite interesting:

A fun geology lesson, and pictures of buildings to follow in part two...

 

[cotedupy]

Now the reason I said I found the stone above particularly interesting, is because I’d already been looking for it...

Many of the UK and Europe’s historic whetstones were quarried originally as a by-product of quarrying for building stones. Go to Leicestershire and you can find houses made from the Charnley Forest Stone, Melynllyn and Idwal paving in Wales. And in Tavistock the buildings are slate, as well as a particular kind of coarse-grained Dolorite known as Tavistock Greenstone or Hurdwick Stone, which probably has no interest for us here.

The slate however does. It comes from a band now known as the Tavy Formation, which is notable for its heterogeneity. If you’d like to read 28 pages about that from a lecture given in Tavistock in 1889, you can do so here. Otherwise I’m going to illustrate it with a picture of a modern house, the outside walls of which are composed almost entirely out of slate:

Which I imagine has almost certainly come from the last remaining slate quarry in the area at Mill Hill, a few miles west of Tavistock. The slates of the Tavy formation are a kaleidoscopic array of purple, red, brown, grey, black, and particularly green. Here’s part of a wall on the other side of the street:

I’ve flattened and tried out some of that green slate before; it’s soft-ish, with low abrasive power, and not that fine for a slate. In general this isn’t a very good whetstone:

But one particular type had caught my eye; a very dark and hard, fine-grained, folliated stone, used in some of the older buildings in Tavistock and especially in Mary Tavy. Though unfortunately if you try asking the local publican if he knows what the floor is made out of you don’t get very far. In fact the bemused reaction is fairly similar to when people find you standing outside their houses touching, and taking pictures of the walls, or doorways:

Anyhow, that’s what this small pile of black slate-like rocks I collected are:

 

[cotedupy]

I say ‘slate-like’ because I’ve made dozens of whetstones from pieces of slate I’ve found in Australia and the UK, and own dozens more actual, recognised slate hones, and this is quite unlike any of them.

It’s right at the top end in terms of fineness of grain, and incomparably harder than any other slate I know. It took me two hours of hard work to get some of this piece to a semblance of flatness:

The surface is still nothing like flat enough to hone a razor on, but it is, just, for a knife. And I tend to try new stones with knives anyway; I use blades from the same knifemaker and the same steel - Aogami 2 - on any stone, and I can get a fairly good idea of grit level and how well the stone will work for knives or razors. I ‘breadknife’ the edge on sandpaper beforehand so that it won’t cut any sort of paper and then go to work on the stone with an atoma 400 surface finish...

This is a very fine stone, and *extraordinarily* hard - a diamond plate will barely slurry it. With a fair amount of pressure it will raise a tiny burr on Aogami 2, though is better on softer steels. Nonetheless after a little while I can restore a knife edge to cleanly drop through cigarette paper and kitchen towel:

Despite that, it’s a pretty naff knife sharpening stone tbh; far too hard and fine, leaving an edge that’s massively over-refined, and basically lacking any kind of grip or teeth. Works marginally quicker with oil than water.

The brightness of the mirror polish on the edge is quite something though, this is rather difficult to take pictures of on my rubbish phone camera, but here goes:

In use and effect the stone reminds me of nothing so much as a black or translucent Arkansas. And it’s so hard that it burnishes in a similar fashion too; after a while it’s really not doing much more to my knife edge in terms of actual abrasion.

I am quite sure though that this would make a very good razor hone, at or nearing the level of a finisher. That is if it weren’t for one aspect of this particular example, I don’t know what these geometric crystalline inclusions are (any geologists out there?), but they feel quite scratchy. And unless they’ve gone by the time I’ve lapped it completely flat I think are going to bugger it up in terms of using for a razor:

Hey ho! I have four or five other pieces to try, so fingers crossed.

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That’s it for now, but I’ll update as and when I manage to get one of them in perfect working order.

I think it’s probably fair to conclude that by now, almost two centuries later, we’ll never know for certain the identity of Mr. Knight’s ‘Devonshire Oilstone’. But I’ll also say that this hard and dark stone of the Tavy formation, whatever it may be - slate or otherwise - is looking like a plausible candidate to me.

 

[cotedupy]

[As I mentioned, these last two posts were written yesterday, several months after the previous. So apologies if they repeat and/or wildly contradict the above.]


So I hadn’t really done much more with these rocks for a while. I knew there was an awful lot of potential, but they’re a complete pita to try to flatten - it takes hours and hours and hours - plus two of three I’d tried looked to have pyrite in them. Though a smaller third piece didn’t seem to, and I finally got round to fully, properly flattening it and trying out earlier today.

The other reason I’d held off slightly is because these are more razor than knife stones. And at the beginning of the year I probably wasn’t confident enough in my razor honing ability and know-how to give a proper verdict to people on the internet who’ve been doing it a lot longer. I am now though, so the below I hope should be a reasonably accurate assessment of the stone.

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But before I get on to that I'm going to talk about what the stone might actually be. Here's how it looks dry, wet, and sides:

And I think we can all agree it looks quite overwhelmingly like a slate. If you asked Plato to think of the slatiest slate he could possibly imagine, this is exactly the kind of stone he would envisage. The colour, texture, fissility; everything about it says: slate.

Except I've changed my mind; I don't think it is a slate any more, for a few reasons: I measured the SG today and it came in @ 2.55, which is too low for a slate, they normally run 2.7 - 2.9 ish. At 2.55 this could be some kind of highly compacted sedimentary stone like a shale, or maybe even a foliated slate-y novaculite, like the Idwal stone. But if it was a novaculite it would slurry white as the driven snow, and this slurries black as night:

So what is it?

Well, the last reason I don't think it's a slate or a novaculite is that when I emailed the Mill Hill Slate Quarry outside Tavistock to ask about it a few months ago they said it was... a shale. Which, frankly, is good enough for me.

 

[cotedupy]

So after all that, and most importantly... what is it like?

TBH it's quite like a trans or black ark, though still a little different in feel. It can cut fast on a rough surface with slurry, or burnished to act slowly because it's so insanely hard (did I mention about the hardness before?). While the first stone I tried could be used on a knife, this one can't: it's too fine and hard.

On a razor, however...

...it is utterly excellent. And I cannot tell you how happy that made me.

I've used quite a wide variety of natural stones, and made many of my own before now. I like to think I'd have quite a good eye for gauging whether something's going to be any good or not. But even still I'd invested a not inconsiderate amount of time, and money spent on abrasive materials, on what until yesterday was really just a hunch. Most natural stones you find and flatten yourself will end up being perfectly serviceable but perhaps not world-beating. This stone however will hold its head with any finisher out there.

When I try a new stone on a razor now; I tend to work from an atoma slurry, gradually diluting, and finishing clean, maybe 100 passes on each side. This technique to my mind should display the true character of the hone, so that the result isn't clouded by any previous finish too much. And if I keep it broadly consistent each time then I should get quite an accurate impression of how things compare to each other.

The stone is quite quick, and surprisingly easy to use. The HHT at the end was as silent and perfect as I've had off any stone on any razor, notably better than the edge on the razor before, and the shave was excellent. The finish is perhaps less 'mellow' than some; this is a stone for people who like it sharp and close.

I couldn't quite believe it tbh, so I tried again on another razor and a kamisori, using nagura slurry.

Same result. If I tried to find fault it would be disingenuous - this is as good a finishing stone as I know.

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As I said above, it is vanishingly unlikely by now that we'll ever know for certain the identity of Richard Knight's 'Devonshire Oilstone', but if I was a betting man I would say this is probably an odds-on contender. Of all the stones I saw and found: this jet-black, mutant shale (?) was the one that caught my eye time and again, and if it isn't the Devonshire stone then the fine folk of Tavistock and Mary Tavy were sitting on not one, but two remarkable hone stones.

Nevertheless we will never know, so I'm not going to refer to it as the DO. Not least because it makes no sense; while many English counties end in 'shire', Devon does not. Perhaps it did in the past, but it doesn't now. So instead I like the name 'Tavy Stone' for the village, and formation it is from.

 

[MichaelS]

Wonderful story and incredible find, thank you!

 

[cotedupy]

Wonderful story and incredible find, thank you!

Fun eh! I was really quite taken aback by just how good it was. The edges are just superb, and it's very fast and easy to use.

I have one more, far larger, piece which I brought back in my hand luggage, though try explaining 2kg of uncut, black stone to airport x-ray teams at every stage of a long haul flight.

The lady at Heathrow took one look at the scan of my bag and just said: "Oh dear. What isn't wrong in here?!" before calling in their head of security. Who spent about five minutes asking me questions, and becoming increasingly baffled, though eventually decided: "Well... I don't personally have a problem with your rock, so I suppose you can take it onboard with you."