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Brittle Carnelian


#1

I have heard that most carnelian is heat treated, does anyone know
precisely how this is done? I’m wondering whether it’s done to
maximize the color without regard for the integrity of the stone.

About a year ago, I bought some big rough carnelian cabs of a lovely
reddish orange. Last weekend I began to cut them, and noticed that
some had developed meandering hairline cracks. Well, I had intended
to recut them anyway.

The first one cracked when I let it wander onto a dry part of the
carbide stone for about a second. I cut another one very slowly,
using finer wheels and it turned out quite lovely.

I lost it to the trolls for a while, and they left it underneath a
closed-bottom filing cabinet that had not been moved for months. I
cannot explain that part.


#2
I have heard that most carnelian is heat treated, does anyone know
precisely how this is done? I'm wondering whether it's done to
maximize the color without regard for the integrity of the stone. 

Carnelian is routinely heated to attempt to change the color from a
yellowish-brown to a deeper rust color. This can be accomplished at
fairly low temps, like 400-450 F. This causes a conversion of the
limonite impurity in the chalcedony to convert to hematite, which is
a rusty red color. It’s a change in hydration state, basically, no
re-ordering of atoms or color centers is involved, hence the low
temps. The integrity of the stone is rarely affected, although
trapped water, heated too quickly will turn to steam and cause an
impressive explosion in your kitchen stove.

The first one cracked when I let it wander onto a dry part of the
carbide stone for about a second. I cut another one very slowly,
using finer wheels and it turned out quite lovely. 

The carbide wheel generated too much heat, which will immediately
cause chalcedony to crack. Lower speeds, more lube, less pressure,
finer grit, all can help. Also, the coarse grits ALWAYS cause
sub-surface damage which is hidden until we get to the polishing
stage. The cure is to use the proper succession of grits and
pre-polish well. In essence, you must grind away the surface damge
caused by the coarse grits, and it can go surprisingly deep in
chalcedony…sometimes.

BTW, if you happen to live near Tenino, WA, you live an area once
considered the source for the beat carnelian available. It still
washes out of the banks of many of the creeks in fist-sized chunks
and is easily collectible if you don’t mind getting wet and muddy.
Don’t forget to ask th landowner’s permission, lots of dairy farms
there. Courtesy and packing out what you pack in goes along way in
that part of the world.

Wayne Emery
The Gemcutter


#3

Harry

Depending on the size of the stone I will go as much as 5 hours at
around 550F and then turn off the kiln and let it cool with the door
closed. I do it to maximize color. The stone will be brittle after
heating that is true. Most of the stuff I get is such and ugly gray
until heated, I could not imagine messing with the stuff were it not
heated.

I never grind dry, I don’t even begin grinding until the wheel is
evenly wet and the same with my sanding. I have had no problems with
cracking though, how I noticed that it was more brittle was in the
course grinding, it would chip more than a non heated stone.

I don’t know what you mean ‘without regard to the integrity of the
stone’. Under 350F the color does not come out, or it goes brown,
550F seems to give the best color change for me, so if you want the
color, that is the temperature you use, and the stone will be come
brittle. Higher temperatures, and there is enough moisture in the
stone they will break apart. If they have pockets they will break
anyhow, but the crystals do something really unique in the way of
coloration. As far as the integrity of the stone, to me it is like
any other stone you work with, what you make from it has to be in
balance with its characteristics. It would not do well to make a very
thin disk of turquoise and then not back it, it would break or
crumble very easy. If you know the stone is brittle, or has a very
strong cleavage, your make it thicker to allow for it, or you
mechanically protect it. Simply cutting the stone alters it integrity
from what it was. I have a green stone I really like to work with,
but don’t cut it in advance, the slices “dry” and will split, if you
cut it and then immediately work it to shape and finish, it does fine
and I have had no pieces crack even after several years, it is just
the characteristic of that stone.

Another thing is most stones will contain water, and they will dry
and crack, it is not only Opal that is prone to this. Usually
finishing them slows the evaporation or seals the surface that I have
only seen one stone that cracked after completion. Wait till you get
a chance to work on really fine grained dense Jade, that one you have
to make shellac wheels to work with. It was the most expensive 12
hours I have ever had in my life short of surgery.

Don’t know if that answered your question, but it is what I have
observed in the cutting I have done. Also, you might want to start a
notebook on the stones you work, what you notice and how they react
to your efforts, it will help in the long run and keeps down the
learning curve. There is a world of differences in the same stone but
from different locations on the planet.

Terry


#4
Depending on the size of the stone I will go as much as 5 hours at
around 550F and then turn off the kiln and let it cool with the
door closed. I do it to maximize color. The stone will be brittle
after heating that is true. Most of the stuff I get is such and
ugly gray until heated, I could not imagine messing with the stuff
were it not heated. I never grind dry, I don't even begin grinding
until the wheel is evenly wet and the same with my sanding. I have
had no problems with cracking though, how I noticed that it was
more brittle was in the course grinding, it would chip more than a
non heated stone. 

It sounds like I will just have to cut slow if I want the pretty
color. OK.

I don't know what you mean 'without regard to the integrity of the
stone'. 

At shows I’ve always seen lots of carnelians that were a set of
cracks held together by a bright color. It made me wonder if they
were over- heated, cooled too quickly or otherwise improperly
treated.

Under 350F the color does not come out, or it goes brown, 550F
seems to give the best color change for me, so if you want the
color, that is the temperature you use, and the stone will be come
brittle. Higher temperatures, and there is enough moisture in the
stone they will break apart. If they have pockets they will break
anyhow, but the crystals do something really unique in the way of
coloration. 

Does this produce usable drusy? I don’t live near any source of
carnelian, but I guess I could purchase some untreated material and
try this.

If you know the stone is brittle, or has a very strong cleavage,
your make it thicker to allow for it, or you mechanically protect
it. Simply cutting the stone alters it integrity from what it was. 

Oh well, at least I figured this out the first time.

Also, you might want to start a notebook on the stones you work,
what you notice and how they react to your efforts, it will help in
the long run and keeps down the learning curve. There is a world of
differences in the same stone but from different locations on the
planet. 

Excellent idea.
Thanks to all who replied.


#5

Harry

It sounds like I will just have to cut slow if I want the pretty
color. OK. 

Don’t know if you mean cut as in grind or cut as in slice. Grinding
is slower and there was a post to your subject that I totally
neglected to mention, and that is the course grind does damage the
stone. There is a formula for this and I forget what it is, but the
rule of thumb that I distilled it down to from what I remember is,
leave 3 times the depth of the grit to be removed for the next
process. This only applies as I have seen, to below about 150, 400
and up does not leave appreciable damage, but it must be accounted
for or you will over grind to get a good finish. If you are making
the stone, and then the mount to fit, no problem, if you are making a
stone to fit a piece, it then you have to follow this rule or you
will be always under, or poorly finished. In cutting, sometimes
slower is good, sometimes it is bad for the blade. I vary the
pressure on the blade to minimize glazing of the blade surface. I
have read 5lbs of weight is about right, but for me 1.5 to 3.5 lbs
seem to get the best blade life and cut quality. I have had where I
was getting a lot of glazing, and increased my weight by 1/2 lb and
the glazing stopped and my cut speed went up.

At shows I've always seen lots of carnelians that were a set of
cracks held together by a bright color. It made me wonder if they
were over- heated, cooled too quickly or otherwise improperly
treated. 

Both will do it, I start from a cold oven add the stone, and cool
down is slow, it will be a day after heating before the stone is
removed. I wait until I can touch it by hand. I have not had that
type of cracking in any of my stones.

Does this produce usable drusy? I don't live near any source of
carnelian, but I guess I could purchase some untreated material and
try this. 

Yes, and it will have a very unique look. I have material that is
red on the base, very like the color of ruby and gradually changes to
clear at the tips, I also have some that is lavender at the base and
changes to orange at the tips unfortunately I had not started my
notebook at the time and do not have a record of conditions or of the
stone before heating. A good inexpensive stone to try this with would
be Brazilian Agate. It will have a red to orange rind that is very
shallow in depth, and ugly in the middle. Heating this stone does
very nicely, and I would recommend it to learn heating with. To try
for the drusy, get the larger stones, they seem more likely to have
usable pockets, large orange size to grapefruit. As they will pop
when you find one, I put a grate or some kind of restraint over it, I
have not lost a heating element, but I felt it was only a matter of
time before I did.

Good luck.
Terry


#6

Nobody else has mentioned this, and I don’t know if it applies here
or not. We useta could tell when turquoise was blasted (as in
dynamite) because it would just break into itty-bitty pieces on the
wheel, no matter what we did. Everything was Devcon backed, it wasn’t
that, it was just fragile and fractured to pieces…

http://www.donivanandmaggiora.com


#7

John

Nobody else has mentioned this, and I don't know if it applies
here or not. We useta could tell when turquoise was blasted (as in
dynamite) because it would just break into itty-bitty pieces on
the wheel, 

Ran into some of that type of thing myself. In my case it was
Sugilite, very good stuff by the pictures, but could not be cut
without crumbling. Definitely from the shock zone.

Terry


#8

I found some interestingly shaped but too thick carnelian beads and
cut them down. When you look at the cut side of the stone, you see a
carnelian red colored region around the perimeter and then a lighter
browny-yellowish region in the center. Can anyone tell me whether
this is characteristic of heat treatment or dying?

Thanks
Ivy


#9

Ivy,

Those beads would be dyed.

Wayne Emery
The Gemcutter


#10

Ivy

When you look at the cut side of the stone, you see a carnelian
red colored region around the perimeter and then a lighter
browny-yellowish region in the center. Can anyone tell me whether
this is characteristic of heat treatment or dying? 

It is died, heating will go all the way through. When died, the
color is very shallow about 1 or 2 mm, heating goes all the way
through when done properly. Some of my early attempts were improper
and only ran about 1/2 inch and then gradually tapered to the
original color. Carnelian and many of the chalcedony materials dye
readily, but it is thin.

Terry