Photosensitivity in Natural Stones

[cotedupy]

I was wondering if anyone knew of any other types of stone that show the effect I'm about to describe...

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This is an interesting coticule; it's fast and soft, self slurries with ease and the mud has a distictive soapy / lather-y feel, combined with a grittiness that reminds me of the King Deluxe stones.

Which are characteristics of a vein called 'La Grosse Blanche', and it has another interesting feature - photosensitivity. The exact reasons for this are not fully understood, but the stone tans or becomes darker with exposure to UV light.

Here's my coticule after sharpening that kiridashi, you can see the worn area on the right that I've been using is lighter than the rest of the stone. So I'm gonna leave it in a prime sunbathing spot on the roof all day and see what happens.

And... no more lighter area. If anything that part of the stone may now be darker than the rest.

I then decided to lap the stone, because it still has a low area, even if you can't see it any more. The picture below is halfway though lapping, and shows clearly the difference in colour; what was the worn lighter area is now the only low part of the stone that hasn't touched the atoma yet, thus the only part that's been exposed to the sun all day. And so it's now darker than the surrounding area which has been lapped.

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Fun eh!

Anything else do this...?

 

[cotedupy]

Here's the page from Grinding and Honing 3 describing this layer, and showing just how dark it can go...

 

[HumbleHomeCook]

I'm picturing you popping out of that window, happily placing your stone on the frame.

Your neighbors...

"What the hell is he doing?"

"I don't know. Probably playing his rocks again."



Pretty cool though. Don't know that I've heard of that with stone before.

 

[mengwong]

I found this on a random Internet web site, maybe relevant?

The most famous color change garnet, pyrope–spessartine garnet, was discovered in the 1990s. It changes color from blue-green to purple depending on the coolness or warmth of the lighting. Similar to the grey-green color-change sapphires, this is a result of relatively high amounts of the metallic element vanadium.

https://www.philophrosyne.com/blogs/jewelry-guide/color-changing-gemstones

 

[cotedupy]

I'm picturing you popping out of that window, happily placing your stone on the frame.

Your neighbors...

"What the hell is he doing?"

"I don't know. Probably playing his rocks again."



Pretty cool though. Don't know that I've heard of that with stone before.

Haha... I'm actually helping my parents move into a new place atm and it's got a very weird build into the side of a steep hill. The roof, of a four storey house, is at the same level as the first bit of the garden where I was standing to take that pic. Most of the garden is up some more stairs and well above the roof, all of which is gonna keep them fit if nothing else!

The coti is interesting eh. I tried putting it out again today, but British winter seems to have set in now, so probably won't see the sun again for 9 months.

 

[sansho]

lap it fresh again and put a piece of cardboard mask on it. maybe cut a star shape or something cool with some detail. get a pattern onto the stone.

 

[pavhav]

How about a depiction of Jesus, or Mary? Subsequently seal with UV protective lacquer, contact tabloids. Sell for outlandish price, and use proceeds to buy even more stones.

 

[mengwong]

Yes, and name the stone “Peter”

 

[Troopah_Knives]

Wait how do we know it is the light doing it as opposed to the atmosphere?

 

[MarcelNL]

Wait how do we know it is the light doing it as opposed to the atmosphere?

I thought about that too, and partially covering it loosely with something blocking light should prove photosensitivity (unless it's a UV-a/UV-B induced oxydation or something like that, but that could probably be excluded by checking if the discoloration fades away in the dark)

 

[cotedupy]

How about a depiction of Jesus, or Mary? Subsequently seal with UV protective lacquer, contact tabloids. Sell for outlandish price, and use proceeds to buy even more stones.

Now this is a good idea!

Especially considering its box that you can see in one of the pics there was actually whittled specially for the stone by Saint Domitian of Huy from a piece of the True Cross.

 

[cotedupy]

Wait how do we know it is the light doing it as opposed to the atmosphere?

I thought about that too, and partially covering it loosely with something blocking light should prove photosensitivity (unless it's a UV-a/UV-B induced oxydation or something like that, but that could probably be excluded by checking if the discoloration fades away in the dark)

I wasn't quite sure if that was a trick q. (or joke...?)

But yes - you could put the lid of the box on. Or leave it inside.

 

[Legion74]

Haha... I'm actually helping my parents move into a new place atm and it's got a very weird build into the side of a steep hill. The roof, of a four storey house, is at the same level as the first bit of the garden where I was standing to take that pic. Most of the garden is up some more stairs and well above the roof, all of which is gonna keep them fit if nothing else!

The coti is interesting eh. I tried putting it out again today, but British winter seems to have set in now, so probably won't see the sun again for 9 months.

They'll have to do the lawns with the mower tied to a rope.

 

[cotedupy]

They'll have to do the lawns with the mower tied to a rope.

You betray yourself as someone who's never abseiled with a scythe.

 

[Desert Rat]

A rare sighting of a cotedupy captured on video in NYC........

 

[Desert Rat]

Here's the page from Grinding and Honing 3 describing this layer, and showing just how dark it can go...

View attachment 202918
View attachment 202919

Kidding and jokes aside, it's the first I've herd of a positive test for a particular vein. They do mention the only one at the Ol'Preu quarry. So maybe there are others?
I think I will be giving some stones a fresh lap and cover parts of the stone with painters tape and see what happens.

 

[Grayswandir]

A rare sighting of a cotedupy captured on video in NYC........

Crazy. I could not do that. I don't mind heights, but the falling part is what gets me,

 

[MarcelNL]

Crazy. I could not do that. I don't mind heights, but the falling part is what gets me,

I do not see the immiment danger, he might fall off the weird structure he is standing on but will fall on the roof that is like 8-10 feet below at most (or I'm missing something).

 

[MarcelNL]

You betray yourself as someone who's never abseiled with a scythe.

I've never abseiled with one but have been known to mow grass with a scythe at an altitude of approx 2000 meters on a steep patch of ground...tricky stuff!

 

[Grayswandir]

I do not see the immiment danger, he might fall off the weird structure he is standing on but will fall on the roof that is like 8-10 feet below at most (or I'm missing something).

Fall the wrong way and he's hitting the street below, which is a good 20 stories down!

 

[Luftmensch]

I was wondering if anyone knew of any other types of stone that show the effect I'm about to describe...

I was told by a J-nat collector not to store stones in direct sunlight - that the sun could bleach some colours from the surface. I took the comment at face value. It seemed plausible. But I have neither put that hypothesis to test or really thought about how the physics/chemistry might work!

 

[Legion74]

I was told by a J-nat collector not to store stones in direct sunlight - that the sun could bleach some colours from the surface. I took the comment at face value. It seemed plausible. But I have neither put that hypothesis to test or really thought about how the physics/chemistry might work!

It might, but any change would only be skin deep. A quick lap and it would be back to normal. (Same with cotis).

 

[Vadoche]

Here's the page from Grinding and Honing 3 describing this layer, and showing just how dark it can go...

View attachment 202918
View attachment 202919

This is very interesting ! I'm curious about the actual mecanism behind the change of colours, it could be due to many things in the stone. Maybe the natural bonding of the stone gets affected by sunlight somehow...

 

[Luftmensch]

It might, but any change would only be skin deep. A quick lap and it would be back to normal. (Same with cotis).

Yeah! Agreed! UV wouldn't penetrate far.

 

[Logan09]

Nice thread, I may have to research my coticules more and see what they are. Boight several slurry stones and they seem to be a couple variants. Two turned dark brown like yours.

 

[cotedupy]

Nice thread, I may have to research my coticules more and see what they are. Boight several slurry stones and they seem to be a couple variants. Two turned dark brown like yours. View attachment 203541View attachment 203542

Cool! Yeah I'd say at least a couple of yours there appear to be La Grosse Blanche - 3, 4 and possibly 1 as well (?)

 

[cotedupy]

Here's a proper beauty I received today. Happily this one was largely flat already so I didn't need to lap through too much of the colour

 

[Grayswandir]

Very beautiful looking coti.

 

[KingShapton]

Beautiful color

 

[Legion74]

Here's a proper beauty I received today. Happily this one was largely flat already so I didn't need to lap through too much of the colour

View attachment 203960

View attachment 203959

If you ever need to recolour one of these feel free to post it to me, and I'll put it in the sun for you.