Glass? Obsidian? How can I tell? PHOTOS

I just found the greatest article EVER!!! Glass as radiation sensor: cdn.intechopen.com/pdfs/32110/InTech-Glass_as_radiation_sensor.pdf

And, yep, leaded glass containers for the specimens: www.schott.com/advanced_optics/english/our_products/materials/specialty_glass/radiation_shielding.html

The Uranium/vaseline glass come in a lot of different forms, some of which are still made today:). I love that stuff, even not glowing it's a beautiful color.

As for the kiln heating, I raised the temp very slowly. Your boy scout rock throwing wouldn't account for why it took 3 hours for them to crack while going from 70-1058 degrees F without cracking.

They were fine GETTING to 1058 degrees F, and didn't blow up til they'd been in there an hour or 2 at that temp. Since water boils at 212 degrees F, if it were ONLY the water content, it would have happened during the temperature raise.

Not to mention that flint is usually treated in turkey basters and fired up to 800 degrees for color enhancement (but apparently not 1058 degrees). 800 degrees is substantially hotter than the 212 degrees it takes for steam to rise.

I think what the problem was may be a low melt mineral in the rocks... ALTHOUGH... another possibility was my OPENING the kiln to look at the rocks gave them a sudden temp drop that they couldn't handle as well as glass can.

You can tell COE incompatibility from temperature drops faster than temperature rising. If say I put a blop of soda lime glass on borosilicate glass or vice versa, opening the kiln on them would crackle the entire piece (which is how I think Victoria/Imori glass is made).

John, that is actually really interesting!!! Granite is ALSO made of SiO2... 72% is the 'average' SiO2 content per wiki... wow: en.wikipedia.org/wiki/Granite

Soooo.... quartz is glass (SiO2) with a crystal pattern... and granite: "By definition, granite is an igneous rock with at least 20% quartz by volume."

And the wiki further says: "Granite is currently known only on Earth, where it forms a major part of continental crust. Granite often occurs as relatively small, less than 100 km² stock masses (stocks) and in batholiths that are often associated with orogenic mountain ranges. Small dikes of granitic composition called aplites are often associated with the margins of granitic intrusions. In some locations, very coarse-grained pegmatite masses occur with granite.
Granite has been intruded into the crust of the Earth during all geologic periods, although much of it is of Precambrian age. Granitic rock is widely distributed throughout the continental crust and is the most abundant basement rock that underlies the relatively thin sedimentary veneer of the continents."

Sooooooooo... most of the earth's crust is... glass?

Oh, I know that fast heating rocks leads to rocks exploding from water expansion. I was just saying that they didn't do that in the kiln, because I took the temp up 5 degrees a minute (which is unnecessarily slow for glass). I didn't pop the rocks cold into a preheated 1058 degree F kiln:).

The more I think about it, the more I think it was the fast cooling. I didn't mention that the 3 cracked/broken rocks that I pulled out of the kiln I threw in a thermal heat blanket (ceramic kiln blanket) so they didn't mess up the glass going into the kiln. Slowing down the cooling prevented further cracking on the 3, and again, the mozarkite held up best (that only cracked in 1/2).

I think those rocks at least contain minerals with different coefficiency of expansions, so are more subject to rapid cooling crackage than glass is. Or at least borosilicate glass... soda lime glass cracks while cooling if you look at it funny or breath in its general direction. I hate working with soda lime glass. Borosilicate glass is thermal shock resistant... so even plunging a 1700 degree molten piece into water may crack it, but not necessarily force it off a metal tool, it's that thermal resistant. That leads to my bad habit of opening the kiln whenever I want to look at something.

Here's an interesting... opals are SiO2 + H2O... water. It's the water trapped on a molecular level that gives opals their 'fire', but it's also why opals are so ridiculously heat sensitive that they can crack if you wash the dishes with an opal ring on, or crack going from a warm house outside to cold car.

A guy at our local rock show gave me an opal that he said could be heated... it was in fact heat treated and dark brown (not andamooka from Australia). He got it in Mexico. I stuck that in the kiln too... and it did NOT explode... yet maintained its 'fire'. What happened was it UNTREATED itself and bleached white... but kept the fire!!!

I've never seen anything like it. I figure I'd ruined the treatment anyway, so may as well go all the way, and ENCASED it in clear borosilicate...and while it fragmented a bit, and released some trapped air bubbles off the edges, it did not powderize like any other natural opal would have.

I'm still wondering how that's possible, it cannot have water or the water would expand. But how does it have opal fire with no water? It's chalk white, not the 'usual' opal translucent, so the fire doesn't have great contrast, but it's definitely still there. I have it here on my desk, encased in borosilicate glass, and it makes no sense.

Lots of great discussion and info!

Fulgurites are pretty cool, too!

Pat, it "looks" like slag to me, but, it is a pretty piece and color whatever it is called.

Here's some info on obsidian (and tektites, etc...) from one of my favorite websites:

stoneplus.cst.cmich.edu/obsidian.htm

Hi, Helen!! Hope you and your family are safe from Debby's rains and floods. I saw the storm was stewing over Jaxville and south Georgia (Valdosta) area earlier today.

I love the GemRocks website!!

I am going to post a pic of your gorgeous glass waves that you made for the opal fish on the forum. I keep forgetting to take a pic of it and posting it. :blush: I am still trying to maneuver my way around the forum and still haven't figured my way around yet. (Lost :blush: ) A lot to read.

Those two photos are definitely slag glass, not obsidian. I worked with slag glass from the formed glass industry for several years, and I know all the signs to look for. No question about it, IMHO.

Sorry about bumping this up. Couldn't resist.

Yup.

Yeah, Scott, can you imagine that? Hard to believe, lol.

Hey, donkeyrokman, nothing wrong with pulling up the old stuff, lots of old threads are worth revisiting. Are you going to introduce yourself? Welcome to the show! Jean

Don, do not poke sticks! That is one episode I would just as soon not repeat. Yeah, I know - neener-neener-neener! Feels good, huh?

Yeah, I thought of her as a friend at one time, too. But then her true colors came out after a while. Sorta like my ex-sister in law. Argumentative, opinionated and just plain crazy...