Specific Gravity Confusion

[cotedupy]

This is something I find myself explaining or pointing out to people quite a lot, and I'm going to write it out in case it's helpful for anyone, and so that next time I can send a link.

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Specific gravity is a measure of density. It is not a measure of hardness, and it is not a measure of fineness.

The confusion arises because for the stones that it's most often talked about in relation to - Arkansas novaculites - specific gravity does correlate with both both hardness and fineness. However this is not necessarily true for other types of stone, and you cannot compare specific gravities across types of stone and try to draw meaningful conclusions.

A slate might have an SG around 2.80. Is it a harder and finer stone than a translucent Ark at 2.64? No. A coticule might come in at 3.00. Ditto. You can't even use it to compare two different novaculites; a Charnley has an SG around 2.70, and is again is a softer and coarser stone than a trans or black Ark. To look at the specific gravity of something and try to infer characteristics from it, you need to know about both the chemical composition and structure of what you're looking at.

And even when comparing stones of the same type SG does not necessarily have a +ve correlation with fineness. For Idwals and Charnleys; the finer stones tend to have lower specific gravities, closer to that of silica at 2.65. They are purer novaculites than examples with higher SGs. Looking at slates, Thuris, &c. SG is all over the place, it doesn't correlate positively or negatively with the fineness of the grit (ime), or the hardness of the stone, seemingly random. It's a product the composition of initial deposits, and how metamorphic change has affected them, and that's a very complicated equation indeed.

SG is however a useful measurement when trying to determine the identity of an unknown stone. If you have a very fine green-grey hone with an SG of 2.55 - it is not a Thuri, but it could be an Asagi &c. &c.

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That's all really. It's an easy trap to fall into given that traditionally SG is (legitimately) used to look at the fineness of Arkansas stones. But be clear that this equivalence does not hold for other things.

 

[Legion]

All very true.

It is interesting looking at the various SG's and trying to work out why one stone might be this, and another that. What chemicals and minerals are different in a Welsh novaculite, to an America, to a Cretan one, for example.

 

[cotedupy]

All very true.

It is interesting looking at the various SG's and trying to work out why one stone might be this, and another that. What chemicals and minerals are different in a Welsh novaculite, to an America, to a Cretan one, for example.

Yeah it'd be fun if one could do proper chemical analysis of stones and see how it all tied in!

It's particularly interesting as you say when looking at different novaculites. Idwals are kinda half slate, which is presumably why they have quite high SGs. Cretans are interesting because they're quite soft; if they were purer I imagine their SG would be lower, but impurities in the stones probably act in the other way - to push it up, so that coincidentally they have SGs between 2.60 and 2.64. The same as Trans Arks, and yet they're nothing like as hard or fine.

(Hunches obviously, but it would be interesting to see chemical analysis / the opinions of a petrologist).

 

[Empire straights]

+1 on softer llyn Idwals being finer. My soft one is insanely fine and puts a Lazer sharp edge on knives(black ark level) but retains a tiny bit of micro serrations(I feel like my new la lune does this to knives under water only.) I'm going to try to get all my new stones lapped before Monday and I hope this new on is soft too. I have a feeling it will be.

 

[Wid]

I've only considered the specific gravity to be relevant when talking about Arkansas stones. I see SG thrown around a lot on a FB hone forum.

 

[cotedupy]

I've only considered the specific gravity to be relevant when talking about Arkansas stones. I see SG thrown around a lot on a FB hone forum.

This is basically where I am too. Outside of Arkansas stones; trying to interpret SG is at best somewhat confusing and unreliable, at worst it completely misleads people because it's misunderstood.

 

[Alum of Potash]

Sure sounds good on the tongue though. Specific Gravity... Didn't they open up for Led Zep back in '69 or '70?

 

[duke762]

A very misunderstood term when referring to honing. Even when used in the Ark/Novaculite family, where the reading may be of some interest, the variances in stone density make it a moot point for most identifications and relying on it in the Black/Trans family is sketchy at best.

The best method to test, IMHO, is just measure LWH and weigh it. So far I don't think I've seen anyone measure the SG on an oil soaked Washita and gotten an realistic reading. The entrapped oil weight would have to be figured in. New from the factory? Now we have a figure to look at.

Like wise the Trans/Black family. A black Ark will most likely show an elevated SG over a Trans just because of high SG impurities that cause it to be Black. Pure Quartz is SG 2.65. Nice, pure, big crystals that caused the Ouchita mountains to be a strategic resource area during WW2. Used as crystals in radios. Some of the most pure Quartz in the world. But we are talking Novaculite, a slightly different creature than crystalline quartz. A Trans SG much above 2.65 would indicate impurities.

There is that mention of Quartzite on Google and they mentioned a SG of 7.2. I don't see many Quartzite hones, my hunch? They are probably too soft and prone to glazing. SG is really not a thing to rely on but can offer some clue's if limitations of the test are taken into account.

A Trans Ark with an SG in the proper range, is a thing to behold when mastered. I relate a high SG, close to or slightly below 2.65 as being very pure and hard.

Splitting hairs. If your SG is in the ball park, every thing will be fine. I worry much more over my bevels than I ever will the stones SG.

I just use the rock that works the best, haven't been wrong yet.

 

[Legion]

Like wise the Trans/Black family. A black Ark will most likely show an elevated SG over a Trans just because of high SG impurities that cause it to be Black.

This is an interesting point I had not considered before. Dans and other places seem to rate the surgical blacks above a translucent in fineness, but that is often debated. They may be basing the assessment on SG, and whatever makes them black raises the SG...

 

[cotedupy]

All very true!

The best method to test, IMHO, is just measure LWH and weigh it.

You don't use water displacement then? It should be both easier, and more accurate. When I started using displacement measuring I found that my previous ruler measurements tended to be a touch under.

So far I don't think I've seen anyone measure the SG on an oil soaked Washita and gotten an realistic reading. The entrapped oil weight would have to be figured in. New from the factory? Now we have a figure to look at.

This is certainly the case - SG readings on Washitas tend to go down as you degrease them.

Recently I was playing around to have a look at this. I took degreased and thoroughly dried Washitas and soaked them in water until they stopped gaining weight (about a week). As an example; a Washita that had an SG of 2.17 when dry, came in at 2.22 when saturated. Oil is a bit lighter than water so the difference would be slightly less pronounced.

(I do also actually have a 'factory condition' old LW. SG is a touch over 2.3 from memory.)

A black Ark will most likely show an elevated SG over a Trans just because of high SG impurities that cause it to be Black.

This is what I'd always assumed too. My old Norton Black comes in at 2.67, with Translucents around 2.63.

I don't see many Quartzite hones, my hunch? They are probably too soft and prone to glazing.

I have a couple of quite good stones that I believe are quartzites, but they're definitely not common.

And have also tried using a few bits of it I've found too; the main problem with that ime, is that they tend to be quite irregular in terms of both grain size and friability, so you get random sized particles released all over the place. So yeah as you said, a softness problem in a way.

FWIW though - Turkish / Cretan stones have a notable quartzite character to them I think. They seem to be a kinda quartzite-y, sandstone-y novaculite-y mashup.

 

[cotedupy]

There is that mention of Quartzite on Google and they mentioned a SG of 7.2.

I assume you meant 2.7 here...?

I actually didn't know that, interesting... these are the two whetstones I have that I believe to be quartzites, and they both have an SG of 2.70 on the nose:

Mystery hones... Let's see what you have

Here is a beautiful little ~2"x4" stone that I'm pretty sure is a streaky thuringian. This baby is slicker than snot! Like honing on a piece of warm dampened ice. Found in an old synthetic barber hone box with lots of hone wear.

www.badgerandblade.com

 

[Legion]

The best method to test, IMHO, is just measure LWH and weigh it.

Do me a favour. Get your ruler out and measure this one for me?

 

[H Brad Boonshaft]

Yup, have always questioned the reliability of the “test” and the condition of the stone.

Translucent can absorb oil, I assume Blacks can also. It takes months to get oil out of vintage translucent, but it comes out. Butterscotch is an oil soaked translucent.

For me old Ark’s work or not and SG is not something I deal with.

 

[Bowmaker]

I assume you meant 2.7 here...?

I actually didn't know that, interesting... these are the two whetstones I have that I believe to be quartzites, and they both have an SG of 2.70 on the nose:

Mystery hones... Let's see what you have

Here is a beautiful little ~2"x4" stone that I'm pretty sure is a streaky thuringian. This baby is slicker than snot! Like honing on a piece of warm dampened ice. Found in an old synthetic barber hone box with lots of hone wear.

www.badgerandblade.com

Is a spg of 2.7 in the same range as LI's?

 

[2bit_collie]

As @Legion points out, measuring dims and weight is accurate for some stones, not for others. Ditto for water displacement. I have a coffee pot with a narrow neck - If I can submerge a stone and get the water level near the narrowest part of the neck it gives me a very accurate reading. But that only works with stones up to 6”. For 8” stones I don’t have a good container to use (I end up using something with straight sides and a wide diameter, and in that case the water test is only accurate to about 0.05-0.08) and the dims and weight works better.

I measured one 6” stone that was cut very rectangular and without any chips or holes by both methods. The two volume measurements were within 0.5%. So that tells me that in the right circumstances, both methods can be quite accurate.

 

[Legion]

As @Legion points out, measuring dims and weight is accurate for some stones, not for others. Ditto for water displacement. I have a coffee pot with a narrow neck - If I can submerge a stone and get the water level near the narrowest part of the neck it gives me a very accurate reading. But that only works with stones up to 6”. For 8” stones I don’t have a good container to use (I end up using something with straight sides and a wide diameter, and in that case the water test is only accurate to about 0.05-0.08) and the dims and weight works better.

I measured one 6” stone that was cut very rectangular and without any chips or holes by both methods. The two volume measurements were within 0.5%. So that tells me that in the right circumstances, both methods can be quite accurate.

I’m not sure I understand your measuring technique with the coffee pot.

All I do is find a suitable size Tupperware container and fill it with water. Put that on the scales and tare it. Suspend the hone from a fishing line cradle in the water so it’s submerged but not touching the sides or bottom, and note the weight.

Divide the dry weight of the stone by the weight of displacement, and that is the SG. The magic of metric. And it works for hones of any weird shape.

 

[2bit_collie]

I’m not sure I understand your measuring technique with the coffee pot.

All I do is find a suitable size Tupperware container and fill it with water. Put that on the scales and tare it. Suspend the hone from a fishing line cradle in the water so it’s submerged but not touching the sides or bottom, and note the weight.

Divide the dry weight of the stone by the weight of displacement, and that is the SG. The magic of metric. And it works for hones of any weird shape.

So I think I’m doing it differently than you (not necessarily better!)
So here is my method…
1) Put the stone in a container
2) Fill the container above the level of the highest point of the stone (so it is completely submerged)
3) mark a line on the container at the surface of the water, Measure the weight = Weight 1
4) Pull the stone out and measure the weight = Weight 2
5) refill the container up to the line with water, note the weight = Weight 3
6) SG = (weight 1 - weight 2) / (weight 3 - weight 2)

Of course alternatively you measure the dry weight of the stone (weight 1 - weight 2) and then tare for weight 2 and then weight 3’ is weight 3 - weight 2 and you don’t have to do as much math. But since I’m using my calculator for the division anyway…

I may not be young (except by B&B standards) but I am young enough that despite being American I used metric throughout school and my Engineering career!) So we are on the same page in that regard.

The potential flaw in this method is how accurately you can refill the container to the line. That is why it helps to have a container that is narrow at the point where the line is - the narrower it is, the smaller the amount of water needed to raise the level by a noticeable degree.

If I am understanding your method correctly, you fill the Tupperware with as much water as it will hold and then you drop in the stone, actually displacing an equal volume of water from the Tupperware container, correct? Totally valid but I guess I would worry about making a mess on the kitchen counter. I assume you drop the stone in with the container in the sink and then move to the scale?

Anyway, if there is a better method, I’m all ears - I just got in a trans Ark with a claimed SG = 2.5 and I am eager to verify this claim.

 

[Legion]

So I think I’m doing it differently than you (not necessarily better!)
So here is my method…
1) Put the stone in a container
2) Fill the container above the level of the highest point of the stone (so it is completely submerged)
3) mark a line on the container at the surface of the water, Measure the weight = Weight 1
4) Pull the stone out and measure the weight = Weight 2
5) refill the container up to the line with water, note the weight = Weight 3
6) SG = (weight 1 - weight 2) / (weight 3 - weight 2)

Of course alternatively you measure the dry weight of the stone (weight 1 - weight 2) and then tare for weight 2 and then weight 3’ is weight 3 - weight 2 and you don’t have to do as much math. But since I’m using my calculator for the division anyway…

I may not be young (except by B&B standards) but I am young enough that despite being American I used metric throughout school and my Engineering career!) So we are on the same page in that regard.

The potential flaw in this method is how accurately you can refill the container to the line. That is why it helps to have a container that is narrow at the point where the line is - the narrower it is, the smaller the amount of water needed to raise the level by a noticeable degree.

If I am understanding your method correctly, you fill the Tupperware with as much water as it will hold and then you drop in the stone, actually displacing an equal volume of water from the Tupperware container, correct? Totally valid but I guess I would worry about making a mess on the kitchen counter. I assume you drop the stone in with the container in the sink and then move to the scale?

Anyway, if there is a better method, I’m all ears - I just got in a trans Ark with a claimed SG = 2.5 and I am eager to verify this claim.

No no. In my way you dont fill the container right up, just enough to cover the stone as it is suspended in the water without touching the sides or bottom. Because the stone is "floating", the weight of the container only rises by the weight of the water displaced, not the stone.

It's easy and very accurate.

Like this....

 

[Wid]

So I think I’m doing it differently than you (not necessarily better!)
So here is my method…
1) Put the stone in a container
2) Fill the container above the level of the highest point of the stone (so it is completely submerged)
3) mark a line on the container at the surface of the water, Measure the weight = Weight 1
4) Pull the stone out and measure the weight = Weight 2
5) refill the container up to the line with water, note the weight = Weight 3
6) SG = (weight 1 - weight 2) / (weight 3 - weight 2)

Of course alternatively you measure the dry weight of the stone (weight 1 - weight 2) and then tare for weight 2 and then weight 3’ is weight 3 - weight 2 and you don’t have to do as much math. But since I’m using my calculator for the division anyway…

I may not be young (except by B&B standards) but I am young enough that despite being American I used metric throughout school and my Engineering career!) So we are on the same page in that regard.

The potential flaw in this method is how accurately you can refill the container to the line. That is why it helps to have a container that is narrow at the point where the line is - the narrower it is, the smaller the amount of water needed to raise the level by a noticeable degree.

If I am understanding your method correctly, you fill the Tupperware with as much water as it will hold and then you drop in the stone, actually displacing an equal volume of water from the Tupperware container, correct? Totally valid but I guess I would worry about making a mess on the kitchen counter. I assume you drop the stone in with the container in the sink and then move to the scale?

Anyway, if there is a better method, I’m all ears - I just got in a trans Ark with a claimed SG = 2.5 and I am eager to verify this claim.

I do the same as David. When you got that Washita from me I do believe we both came up with the same SG. So it looks like both methods are legit.

 

[2bit_collie]

No no. In my way you dont fill the container right up, just enough to cover the stone as it is suspended in the water without touching the sides or bottom. Because the stone is "floating", the weight of the container only rises by the weight of the water displaced, not the stone.

It's easy and very accurate.

Like this....

Wow! I truly learned something today! Thank you @Legion!

I just ran on my reference stone (my 6x2x0.5 true hard) and my new stone
My true hard had measured 2.63 with LXWXD and 2.62 by my water displacement method. It hit 2.61 with your water displacement method and the only reason there was ANY variation is that my dry weight dropped 1g, the volume was an identical 100g = 100cm*3

So then I measured the new stone, with the claimed SG = 2.5. It came in at 2.55, so even a bit denser than I had expected.

For a final effort I remeasured the true hard. 100g when it was suspended in the water. Then I let the stone settle to the bottom of the container. The scale weight changed to 261, identical to the dry weight. So technically I could have gotten both measurement values within a few seconds simply by first suspending the stone in the water and then by letting it sink to the bottom of the container.

Will definitely use this in the future, much better than the way I had been doing it. I do have to sacrifice a few inches of string, but at least I don’t have to worry about scrubbing the Sharpie mark off the coffee pot!