I was thinking about this 'Butterscotch' thing the other day, and I came up with a theory that I didn't think had been mentioned before. But reading this thread back, it actually had been. Here's a post of Keith's from a few years back:
I'm pretty sure that light is the cause of 'Butterscotch' colouration in arks. UV light is a very strong and somewhat weird form of radiation that affects an awful lot of organic and inorganic substances in a variety of ways. And of all the whetstones in the world a Translucent Arkansas - just by it's its nature - is gonna be your most likely candidate.
But UV light isn't the same as visible light, a trans ark might block the former while allowing the latter to pass. We can probably get a reasonable idea though by comparing it to glass, which chemically is largely the same thing (silica) as a trans ark. Normal glass blocks almost all UVB and UVC light, but allows 60-70% of UVA through. I think we can also extrapolate from that, that UVA light does not affect silica, otherwise we'd all have butterscotch windows. But actually impurities in some old glass means it can discolour in light. Apparently it's a bit of a thing in glass collecting circles too:
So could UV light similarly affect something else in Arkansas novaculite and cause butterscotch variation? Well let's see... I put a 5p piece on a Washita yesterday and left it in the sun for 5 hours. And even after a short period in (British) sunshine, it has darkened the stone slightly.
Admittedly that still doesn't tell us though exactly what is being 'tanned', and it is certainly possible that impurities in absorbed oil could be the main culprit. But my hunch would be that's it's something within the stone itself. Trans arks absorb very little oil in comparison to any other novaculite, but they pass way more light, and it's in them that we see this colouration most often.
Though I do also have an old Pike LW that's extremely hard, fine, and high SG, that remains a obstinately light caramel colour despite having been degreased probably in total for about 6 months. But the stone, as you can imagine, is also really quite translucent, so again seems to confirm the theory.
Whatever's causing this may be found in certain stones but not others, or it may be possible in all examples. But either way - I'd be fairly confident that 'butterscotch' colouration in Arkansas stones comes from UVA alteration of chemical impurities.
I believe thete are a number of possible and actual factors, including oxidation of impurities, presence of oil, actual yellowiish stone due to a natural tint in impurities, sun-turning, and various different combinations of the above.
Because the stone is nearly pure micro/crypto crystalline silica, the effects of light diffraction are going to be seriously amplified - even a teensiest bit of impurity or oil or oxidation could have a major impact on the stone’s appearance. The crystalline structure can act like a signal repeater.
It’s an interesting phenomenon, the ‘antique’ coloring makes for a warm visusl aesthetic. Interesting and functional eye-candy can be fun to have around.
I'm pretty sure that light is the cause of 'Butterscotch' colouration in arks. UV light is a very strong and somewhat weird form of radiation that affects an awful lot of organic and inorganic substances in a variety of ways. And of all the whetstones in the world a Translucent Arkansas - just by it's its nature - is gonna be your most likely candidate.
But UV light isn't the same as visible light, a trans ark might block the former while allowing the latter to pass. We can probably get a reasonable idea though by comparing it to glass, which chemically is largely the same thing (silica) as a trans ark. Normal glass blocks almost all UVB and UVC light, but allows 60-70% of UVA through. I think we can also extrapolate from that, that UVA light does not affect silica, otherwise we'd all have butterscotch windows. But actually impurities in some old glass means it can discolour in light. Apparently it's a bit of a thing in glass collecting circles too:
"Sun-Changed" or Irradiated Glass information from the Glass Encyclopedia
Irradiated Glass: information, pictures and references on glass which has changed colour from radiation or sunlight; brought to you from the Glass Encyclopedia.
www.glassencyclopedia.com
So could UV light similarly affect something else in Arkansas novaculite and cause butterscotch variation? Well let's see... I put a 5p piece on a Washita yesterday and left it in the sun for 5 hours. And even after a short period in (British) sunshine, it has darkened the stone slightly.
Admittedly that still doesn't tell us though exactly what is being 'tanned', and it is certainly possible that impurities in absorbed oil could be the main culprit. But my hunch would be that's it's something within the stone itself. Trans arks absorb very little oil in comparison to any other novaculite, but they pass way more light, and it's in them that we see this colouration most often.
Though I do also have an old Pike LW that's extremely hard, fine, and high SG, that remains a obstinately light caramel colour despite having been degreased probably in total for about 6 months. But the stone, as you can imagine, is also really quite translucent, so again seems to confirm the theory.
Whatever's causing this may be found in certain stones but not others, or it may be possible in all examples. But either way - I'd be fairly confident that 'butterscotch' colouration in Arkansas stones comes from UVA alteration of chemical impurities.