Hand Tools Archive

Re: How to measure density of ARK stones

David Weaver
There are charts at dan's, but the ratings in different systems are confusing. Here's why:
* some of the ratings consider anything over 2.4 or 2.5 specific gravity a hard stone
* we generally would start to regard a stone around 2.6 SG as a marginal finish stone (translucents start to not pass light very easily around there)
* Stones from dan's or good older translucent stones will be in the upper 2.6s for SG

Stones around 2.5 make a nice stone for prior to finish work.

All you do is measure the stones with something accurate (if they're new - if they're old and not flat, you're sol unless you can do water displacement), calculate volume and then compare volume and weight to the weight of water. probably easiest to do in CM and gram.

I have measured somewhere around 20 stones. It turns out that what we consider to be good stones are in a much narrower range than the charts would suggest.

I believe 2.65-2.7 range is the theoretical limit (siO2 itself is 2.67 on charts I've seen). This makes sense. If a stone starts having anything like 2% volume in air, it shows up as pores all over the stone.

The other issue with measurements is that the pike mine (dan called it perryn rock or something) stones can vary quite a bit. They are listed as a grade less dense than soft, but most of the folks who would use washitas and notice their qualities being different than other stones would say "Dense washita" or "low density washita".

I don't think cutting power of the washitas is quite as exactly related to density as the bright white clear low density washitas, to me, seem to settle in faster than the more mottled stones. I like the mottled type better.

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