Ways to Heat Treat Rocks

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Heating stone changes them to a more
Glassy medium and eases the job of
Knapping and flaking tools and spear
points. It also Enhances the color on
some stones.
Heating Gem stones clarifies and changes
The color to make them more desirable by
The cave girls.
First a little
History………
Paleolithic Period (Stone Age) 2.5 million years ago – about 10,000 BC
Use of fire, Stone Tools.
Lower Paleolithic – Neanderthal
Middle Paleolithic - Cro-Magnon 40,000 years ago
Upper Paleolithic – 30,000 – 10,000 BC
Middle Paleolithic - Cro-Magnon 40,000 years ago
Upper Paleolithic – 30,000 – 10,000 BC
Cave Art from
The Cave of ChauvetPont-d’Arc France
Appx. 31,000 years ago
Upper Paleolithic 30,000 – 10,000 years ago
Heating of Cherts, Jaspers, & other stones for tool making
Early metal work, Art. Stone Working progressed to an art.
Neolithic (New Stone Age) 10,000
years ago to 2,700 BC
Beginning of Agricultural society,
Villages, Pottery making
And Ovens & Kilns.
Sphinx 2723 – 2563 BC
Current Era 2700 BC to Today
Pliny, (23 to 79 AD)
“To tell the truth, there is no fraud or deceit in the
world which yields greater gain and profit than
that of counterfeiting gems.” -History of the World
Tribal witch doctors, seers, and sages, and flourished during the Neolithic
era and were the precursors of Alchemists studying the transmutation of
stone and metals from one form unto another.
Today they are known as doctors and scientists.
Current Era 2700 BC to Today
From 2 Egyptian papyri
3rd or 4th century
False Ruby
“Take smoky crystal and make the ordinary stone from it: Take and heat it
gradually in the dark; and indeed until it appears to you to have the heat
within it. Heat it once more in gold-founder's waste. Take and dip the stone
in cedar oil mixed with natural sulfur and leave it in the dye, for the
purpose of absorption, until morning.
Corroding and Opening Up of Stones
“Grind alum and melt it carefully in vinegar. Put the stones therein, boil it up, and
leave them there over night. Rinse them off, however, on the following day and
color them as you wish by use of the recipes for coloring.
Current Era 2700 BC to Today
Teifaschi, in his treatise on gems from about
1240 AD, had this to say:
In Sarandib [Sri Lanka] and its environs, ruby is treated by fire. People
take pebbles from the earth and crush and compress them into a mass
with the aid of water. [This mixture] is daubed completely around a dry
stone. Then, the whole thing is placed on a rock with other rocks set
down around it. Dry firewood is thrown on top, lit and blown upon [with
bellows]. The blowing is applied, along with more wood, till any black
overtones on the ruby have disappeared.
The ruby is not heat treated a second time. After one treatment, its
color can neither improve nor diminish.
Current Era 2700 BC to Today
Natural Magic, John Baptist Porta, Naples
[ca. 1535–1615]
For example, we would have a Sapphire should be white on one side,
and blue on the other; or should be white on one side, and red on the
other: thus it may be done. Plaster up that side which you would have
red or blue, with chalk, and let it be dried; then commit it to the fire,
those ways we spoke of before, and the naked side will lose the color
and turn white, that it will seem a miracle of Nature, to those that know
not by how slight an art it may be done.
Current Era 2700 BC to Today
1850’s
Sri Lanka people coated Ruby rough with lime and Betel
Leaves, then cooked in hot fire until the blue tinge was gone.
Early 1900’s
Sri-Lank Ian's began heat treating Geuda (poor quality)
Sapphire to obtain the brilliant blue, and get rid of the ‘silk’ (rutile Strands).
1920’s – 1950’s
High Temperature ovens became in use & Professor
and Madame Bron of Company Grasset and Bron, rue Chantepoulet,
Geneva Developed the modern Method of heat treating Ruby and produced
the ‘Geneva Ruby’, a a forerunner of Verneuil synthetic Corundum.
Modern
By the latter part of the 70’s, large numbers of
heat-treated Sri Lankan sapphires were streaming out of Thai ovens.
In the early days of what amounted to the Great Geuda Rush, rough
could be had for a song and fortunes were amassed overnight.
The rest, as they say, is history…
The Mong Hsu ruby is almost always an ugly duckling. There are two
major problems. The first is dense silk/particle clouds and a strong
purplish color, making most stones look like low-grade, cloudy
rhodolite garnet.
This is mainly due to the crystal’s unusual blue cores. Ordinary
heat-treatment removes the blue, as well as removing silk,
making the final product a rich, clear red.
B
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Gemstones & Their Treatment Types
Most gemstones, with the notable exception of garnet, have a
particular treatment, or series of treatments that are commonly
used to increase the marketability of the stone.
Alexendrite: None
Amethyst: Heat Treatment
Aquamarine: Heat Treatment
Coral: Dyeing
Druzy Quartz: Dyeing, Heat Treatment, Irradiation, Vapor Deposition
Citrine: Heat Treatment
Diamond: Irradiation, Lasering
Emerald: Filling, Impregnation, Waxing/Oiling
Garnet: None
Lapis Lazuli: Dyeing
Onyx: Dyeing
Opal: Filling, Impregnation, Waxing/Oiling
Morganite: Irradiation
Pearls: Bleaching, Dyeing, Irradiation
Peridot: Impregnation, Waxing
Rubelite: Heat Treatment
Ruby: Heat Treatment, Flux healing, Fracture Filling
Sapphire: Heat Treatment, Flux healing, , Fracture Filling, Lattice Diffusion
Spinel: None
Tanzanite: Heat Treatment
Topaz: Heat Treatment, Irradiation
Tourmaline: Heat Treatment, Irradiation
Zircon: Heat Treatment
Ways to Heat Treat Rocks
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•
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Aboriginal or Fire Pit
Barbeque or Washtub
Kitchen Oven
Countertop Roasting Pan
Kiln
Special high degree heating Furnace for
gemstones
Ways to Heat Treat Rocks
Abo or Fire Pit
1.
2.
3.
4.
5.
6.
7.
8.
Dig Pit
Bed of Sand
Layer of rock
Sand/dirt on top *
Build fire
Tend for 2-3 days
Let cool
Remove rock
*Sand or dirt cover should be
at least 1”. Temp will decrease
About 50 Deg. Per additional ½ inch.
Heat is from the top.
Ways to Heat Treat Rocks
Barbecue or Washtub Method
Basically the same as the
Fire Pit only using a barbecue
Or a washtub and Charcoal
As the fire.
Use enough charcoal to cover
The sand completely and add
More as needed for a couple
Of days until done.
Heat is from the top
Abo or Fire Pit
Disadvantages
1.
2.
3.
4.
5.
6.
7.
8.
Dig pit
Enough wood/charcoal
Tend fire for long periods
May have to re-do a couple of times
Makes a hole in the yard
Hard to tell temperature
Hard to maintain temperature
Charcoal can be expensive
Advantages
1. Gives you the satisfaction of doing it the primitive way
Ways to Heat Treat Rocks
Oven Method
Use a Roasting pan that is at
Least 4” deep.
Put about 1” sand in bottom
Layer in Rock
Put about 1” sand on top
Cook for desired time
Let Cool
(can be smelly)
Oven Method
Disadvantages
1.
2.
3.
4.
Ties up the oven for a couple of days
Heats up the house in the Summer
Can get real smelly from oil evaporating from rocks
Electricity/gas to run oven can be expensive
Advantages
1.
2.
3.
4.
Even heat source
Maintain proper temperatures
Less tending
Less likely to have to re-do
Ways to Heat Treat Rocks
Roaster Oven
1.
2.
3.
4.
5.
6.
7.
8.
9.
Remove inner pan and rack
About 1” sand in bottom
Layer rock
½” Sand
More rock etc….
Top layer of sand about 1”
Cook for desired time
Let cool
Remove sand & rock
Roaster ovens can be purchased new for $45 and up depending on the
Brand and the temperature they go to. I bought mine used from Goodwill
For $15. The roaster oven needs to be able to go to 500 degrees.
Roaster Oven
Disadvantages
1. Heat is hotter in corners and on the bottom layer
2. Cost of oven if purchased new
3. Cost of electricity
Advantages
1.
2.
3.
4.
5.
6.
7.
Can be used in garage or carport and not smell or heat up the house
Fairly even heat
Can be stored when not using it
Can clean it and cook the Thanksgiving Turkey
Is fairly economical
Heat treat a good amount and variety of material at once
Less tending needed
Ways to Heat Treat Rocks
Kiln
1.
2.
3.
4.
5.
6.
7.
8.
About 1” sand in bottom
Layer rock
½” Sand
More rock etc….
Top layer of sand about 1”
Program cooking times
Let cool
Remove sand & rock
These are special made kilns for rock cooking. They are completely
Programmable and are made by a pottery kiln company. For more
Information http://www.thenaturalearth.com/kiln.htm. Cost is
$685.00 plus shipping.
Kiln
Disadvantages
1. Cost
2. Weight
3. Specialized equipment
Advantages
1.
2.
3.
4.
5.
Exact temperature control
Programmable ramp, hold, & cooling
Virtually no tending from start to finish
More predictable results
Will contain odors
How its Done !
•
Prepare rock so that you have slabs or in the case of flints & cherts
Make your bifaced blanks.
•
Thickness matters, try to use pieces of the same thickness
•
Over ½” pieces take too long to heat
•
Place thicker pieces on bottom or if you are doing various types
of rock, put the ones requiring the most heat closest to the source.
•
Some people use Vermiculite instead of Sand
•
Abo/pit style the heat is at the top
•
Oven/kiln heat is surrounded
•
Roaster heat is mostly on bottom
Heating Phases
Phase 1. Drying the rock
Bring temperature up to 150 – 250 degrees and cook for 4 – 8 hours
Phase 2. Ramp up temperature
Begin raising temperature by 50 or 100 degrees per hour to the desired
Temperature. (depends on the type of rock)
Phase 3. Hold temperature
Hold the temperature at the desired maximum for anywhere from
0 minutes to 30 hours depending on the type of rock.
Phase 4. Reduce temperature
Reduce temperature by 50-100 degrees per hour to about 125 degrees
Then you can shut off your oven-roaster-kiln and let finish cooling
Rock Recipes
The ramp times, temperatures, & holding times vary depending
On what you read. But generally they are as below. (From several
Knapping Sites, Holding times may need to be longer for Lapidary)
Montana Agate
450 - 500
gets glassy like obsidian
Brazilian Agate
450 – 600
Hold for 1- 2 hours
Rhode Island Jasper & Chalcedony
300 - 350
Most Flints & cherts
400 – 550
Mook Jasper
425 – 550
India Agate (Bloodstone)
450 – 500
More Transp. less heat
Mexican Agate
500
Most Jaspers
350 – 500
Red Jasper
450 – 500
Picture Jasper
500 – 600
Sunset Jasper (Richardsons Ranch) 400 – 500
Gets brittle over 500
Turns yellow at 400, orange at 425, dark red at 450 – 475
Petrified Wood
300 – 480
Moss Agate
450 – 500
More transp. Less heat
Coral
450 - 600
Things to Think About
Ask 3 different people about Temperatures, ramp times, or hold times
You will get 10 different answers. Nothing is ‘set in stone’ !
Reasons for Fracture or Pot-lidding during Heating
1. Dissimilarity of material within a piece of rock
2. Not drying out the rock enough
3. Ramping up temperature too fast
4. Cooling too fast
5. Too high a holding temperature
6. Some stones will just crumble if too high a temp
7. Air or liquid pockets in the rock will go BANG !
8. Most of the temperatures and holding times are from
Knapping sites. So for lapidary I would increase the holding
Times to get more color depth.
9. Longer holding time = deeper color
Do Not handle HOT ROCKS
Let them cool or use tongs
What to Expect
Most flints and cherts will benefit from heating.
Glossiness, transparency, and workability will improve.
Agate & Jasper benefits
1.
2.
3.
4.
5.
Finished Cabs will be glossier.
Slabs will feel smoother and have a waxy feeling.
Slabs will work into cabs in less than ½ the time
Colors will change or intensify
Color change and intensity can be different in
sequentially cut slabs.
6. Color depth may vary depending on thickness,
temperature, and holding time.
What I have for Display
(Display Pieces on following Slides)
1.
2.
3.
4.
5.
Rough or Slab Unheated
Slab heated
Unheated Cab
Cab made AFTER Heating Slab
Cab made Before Heating Slab
Some of the display pieces I did not have enough material
To provide all 5 display pieces. However, there are some
Additional pieces that show the effects of heat treatment.
Note the color differences between the different
Pieces of the same stone
Also, while it is difficult to see here, on the flaked pieces
You will see a waxy/glossy surface to the flaked portions
Some stones may have a “healed” fracture, they may or
May not break during heat treatment
The following display items were glued on butcher paper
So that it was a ‘hands on’ experience for those attending
The meeting presentation. I have included some extra notes
That were brought out verbally at the presentation.
I use an old roaster oven, purchased at Goodwill for $15.00
And sand.
I started with a drying temp of 200 for about 4 hours, then ramp
Up to 475 at about 100 deg/per hour.
Hold time was about 6 hours then ramp down. As the sand holds
The temperature up pretty well in my roaster, I usually just
Turn it down to 250 then when it starts to kick in to heat up again,
I turn it off.
Depending on how full it is, (3 Layers of sand and rock) I will
Just turn it on to 500 deg. It will take about 2 days to reach 475.
Then I just turn it off. (A newer unit may work better) And another
Day or 2 to cool down.
Do not be afraid to experiment ! Above all else, HAVE FUN !!
Plume Carnelian
Unheated Slab
Un Heated Cab
Heated Slab
Banded Carnelian
Unheated
Un Heated Cab
Heated Slab
Heated then
Cabbed
Heated then
Cabbed
Cabbed then
Heated
Cabbed then
Heated – Fracture
popped off during
cabbing
Banded Agate
Unheated Slab
Sunset Jasper /
Polka Dot
Unheated
Un Heated Cab
Un Heated Cab
Heated Slab
Heated then
Cabbed
Cabbed then
Heated
Heated Slab
Heated then
Cabbed/Flaked
Cabbed then
Heated
Blue Brazil
Brown Brazil
Unheated Slab
Unheated Slab
Un Heated Cab
Heated Slab
Un Heated Cab
Heated Slab
Heated then
Cabbed
Heated then
Cabbed – Healed
fracture, broke
when cabbing
Cabbed then
Heated
Cabbed then
Heated – healed
fracture did not
break
Montana ??
Red Carnelian
Unheated Slab
Unheated
Un Heated Cab
Un Heated Cab
Heated Slab
Heated then
Cabbed
Cabbed then
Heated
Heated Slab
Heated then
Cabbed
Cabbed then
Heated
Brazil - Fractured
& Pot Lidding
Ramp up too fast,
& cooled too fast
Brazil, Tumbled
then Heat Treated
Notice the color
depth on the
flaked edge
Brazil, heated too
fast not long
enough
Notice the lack of
color depth on
ground and flaked
areas