Thread: What is the Sp Gr of Your Best Arkansas Stone

I have seen in several places that a specific gravity over 2.5 is considered True Hard which includes translucent and black. Hard Arkansas is between 2.30 and 2.45. From reading here and my experience all black and translucent Arkansas are not created equal. Do you have either a black or translucent that you use after a 12K stone and what is the sp gr? Please describe how you measured sp gr.

I measure volume by water displaced in a laboratory grade graduated cylinder then weigh the dry stone on a triple beam balance. I am not using distilled water and I did not correct for elevation. I have a 2x6 black with sp gr of 2.84 which I am experimenting with after my 12K Naniwa. Not sure if the edge is sharper but it definitely feels different. I also have a 2x4 black barber hone with sp gr of 3.23. I have not used this one enough to make any assessment.

Is there a different way to measure this without triple beam cylinders and laboratory grade graduated balances. I don't know what the specific gravity is and I haven't really cared because if it gets the job done it gets the job done...right? I do follow up a 12k with my Trans Black 4x2.

Originally Posted by cosperryan

Is there a different way to measure this without triple beam cylinders and laboratory grade graduated balances. I don't know what the specific gravity is and I haven't really cared because if it gets the job done it gets the job done...right? I do follow up a 12k with my Trans Black 4x2.

I am pretty sure there is - just not as accurate, though. Mind you, if you aren't correcting for elevation and buoyancy of the water, then the accuracy does not matter that much.

I haven't done it for a long time, but you measure the mass of the stone normally - in air. You then need to construct a sort of hammock affair that goes over a small balance - a digital one is ideal. You have to make allowance for the weight of the hammock you have constructed, so that gets weighed too.

The purpose of the hammock is to suspend the stone as freely as possible while a big bucket of water is placed under it - the bucket is elevated until the stone is fully submerged, not touching the sides, no air bubbles under it, etc. The bucket is not weighed and does not have any physical connection with the scales or the hammock.

The Sg is then the weight of the stone measured in air divided by the result of the difference of the weight of the stone in air minus the weight of the stone in water.

It is not brilliantly accurate. Things that make it more accurate are very thin wire (I used nylon fishing line) to create the hammock, and the hammock or tray or carrier has to be as skimpy as possible while still supporting the stone. Accuracy is further improved by making everything rigid, solidly made and well put together.

This sort of thing is routinely used to help identify mineral samples collected in the field, but you need some training in geology, etc, and a set of tables with the specific gravity of likely known minerals. You already know what mineral (novaculite) you are measuring on this occasion, so in effect one is relying on very fine differences to tell you how densely the abrasive grains are packed.

The machine may not be accurate enough for this, also the results taken as a whole would be meaningless unless everyone used the same machine at the same location under the same conditions.

Bit of a tall order, that...

Regards,
Neil

Yeesh...

I think I will just stick to the rating I've given it.... buttery smooth.

I'd recheck the measurement on the one that was 3.23.

Of all of my stones, all have fallen between 2.6 and about 2.85. A little more than 2.85 is probably possible, but I don't know if 3.23 is. Of course, I could be wrong and it might be possible, I figure that, generally, though, the particles in the hard arks have about the same density per volume and the air space between particles is probably what's accounting for the difference in density.

The stones that I've had in the 2.6 range are barely translucent before oil, and only a little better with it. Anything tending toward 2.5 IME has looked more like a finer stone than a soft ark, but still with the bite that a soft ark has and no translucence.

Fineness for me has been somewhat related to density. I haven't had a stone at 2.6 density that wowed on a razor, but the ones in the 2.7-2.85 range have better potential, but the one at 2.85 isn't necessarily better than the ones around 2.7.

If you want to do water displacement, you'll want a tall narrow vessel rather than a short wide one.

I've only ever measured stones that are new and that are tidy geometry and have relatively even thickness end to end and side to side.

I'm not sure what specific gravity has to do with hardness. Specific Gravity deals with density. Lead and Gold have high specific gravity however they are very soft materials.

Originally Posted by DaveW

I'd recheck the measurement on the one that was 3.23.

Of all of my stones, all have fallen between 2.6 and about 2.85. A little more than 2.85 is probably possible, but I don't know if 3.23 is. Of course, I could be wrong and it might be possible, I figure that, generally, though, the particles in the hard arks have about the same density per volume and the air space between particles is probably what's accounting for the difference in density.

The stones that I've had in the 2.6 range are barely translucent before oil, and only a little better with it. Anything tending toward 2.5 IME has looked more like a finer stone than a soft ark, but still with the bite that a soft ark has and no translucence.

Fineness for me has been somewhat related to density. I haven't had a stone at 2.6 density that wowed on a razor, but the ones in the 2.7-2.85 range have better potential, but the one at 2.85 isn't necessarily better than the ones around 2.7.

If you want to do water displacement, you'll want a tall narrow vessel rather than a short wide one.

I've only ever measured stones that are new and that are tidy geometry and have relatively even thickness end to end and side to side.

The 3.23 is one reason I asked this question. It is a black rock but I got it off ebay from a private individual. With the high sg I suspect it may not even be novaculite.

My understanding is that novaculite is made up of two minerals, one abrasive and one matrix. The abrasive is more dense so a higher sg indicates a higher percentage of abrasive and a smoother finish.

For a 2" wide stone a 1L graduated cylinder was the narrowest I could come up with. The one I used has 1ml graduations so accuracy should be +/- 0.1ml. This is really the only method I have available to me. Based on your description the 2.84 has potential to be a good finisher. I would love to see where a proven finisher comes in.

2.84 is definitely way into the density range for the true hard/black/translucents and has plenty of potential. hard arks are so variable that from that point, it really comes down to how you condition the stone and how you use it and maintain it.

Originally Posted by thebigspendur

I'm not sure what specific gravity has to do with hardness. Specific Gravity deals with density. Lead and Gold have high specific gravity however they are very soft materials.

You want to have a look at Dans Whetstone Site

The Specific Gravity range is set by the US Government, who assign soft and hard values.

They (and Dan, a trained geologist) ought to know what they are talking about...

Regards,
Neil

Specific gravity. Hmmmmmm. Why? If it WERE accurately determined, what does that tell you? Compared to what??