FINAL RESULT Mookaite heat treated at home

Neat experiment.

Jim, where is the link?

Was it Himalayan rock salt, Ed? I've seen pics of people cooking, seasoning and warming food on big pieces of it.

Oops! Oven goes only to 350 F. Will go to Goodwill tomorrow.
Sand is only $5, but it comes in 50 lb bags at the smallest size. Pet store has sand for $10 for three pounds, and it is colored. Don't want that.

Have a pan from the dollar store.

Is there anything I can use instead of sand? Kitty litter? Soil?

all those recipes say is several hundred degrees. a few as low as 300, most 400-500

some rock type are EXTREMELY generic (e.g., petrified woos)

there is also a reason KNAPERS did this. it does make some stones more brittle, and can affect how it polishes.

be sure to test a few slabs before doing a large amount.

I should have have said, "some rock types they list are not very precise" I am suggesting the list is not as informative as was represented)

for example, there are many types of petrified wood with all kids of impurities that might change with heat.

I think my caution was not for you Pat, but for anyone reading this thread or seeing that link and thinking that experiments were not necessary

good luck with your Mookite

slow roasted mook
with a side of pet gnoc
a rocking feast!
mostly toasty

As a knapper I heat treat chert, jasper and agate often. The main goal is the same... to make the stone easier to chip. The bonus for most materials is a higher gloss when chipped, color change, and more control over the conchoidal fracture. I wear an archaeologist hat during the day so I can tell you exactly what happens to the stone when it is heated too; the stone undergoes a sort of "recrystallization" process, the material surrounding the silica grains (called flux) will basically melt. The melting of the flux causes oxidation so, browns becomes reds and any air space will be gone leading to the higher gloss. Humans figured all this out about 20,000 years ago.

As a lapidary, I have worked lots of material that I heated for knapping. The only noticeable thing will be a reduction in the strength of the stone... it will grind away faster. This is true for heated stone knives too... the edges wears faster than raw rock.

Over heating stone is very possible. You can heat it too quickly and it will explode. The water molecules in the stone will get very excited and cause "pot lids" to come flying off the surface of the stone. If the stone gradually gets way too hot it will go beyond heat treated to what we call crazed. Crazed rock is gravel, it has lost all of its internal structure and if full of micro fractures.

Most siliceous rocks will heat in the 350 to 500 degree range. The best results will come from a gradual heating; I like to start them at 250 for an hour and then raise the temp 50 degrees per hour till I reach my max temp. Then I hold that temp for 4-5 hours. The stone will need to cool down gradually too. Let it sit in the roaster for at least 8 hours or till the sand is cool to touch. If they cool too quick you can get crazed and nobody wants that.

Keep on experimenting!
Michael

I can't say I've ever heated amethyst... it's not something that would have improved knapability seeing that it is just silica.

I'm certainly curious what would happen if it was heated. I recall from my geology classes that the amethyst color develops due to heat; if it gets really hot it turns to citrine (I think).

Michael