guys, we are going on a little excursion into the world of
titration. What is "titration"? Well, its a method
of finding out something that you want to know by setting up an
experiment in graded steps so that we can know when a certain
event takes place. For example, If you are near deaf, like me, and
go for a hearing test one the things that they test for is the
frequency range that your ears respond to. A person with normal
hearing can detect frequencies of about 20 to 20,000 cycles/sec.
As we get older we usually start to lose our high frequency
acuity. When I got out of the Navy, after being around aircraft at
high-power turn-up, for 4 yrs, my hearing acuity was about 20-
14,000 cps. Now, I suspect its about 500-10,000 cps.
guys, how the hell did we get to this point? We started out to
just explain some facts about mercury. Its your fault. Well,
maybe just a little bit mine. Fact is, now that I have created a
whole new group of basement chemists out there (I take credit for
that). But, you keep sending requests for more and more info so it
sort of means that a lot of you are hooked on basement chemistry
and might never be the same again.
get on with it. First, what do we mean by the word "assay"?
It means determination of what is present in whatever sample you
are working on. Doesnt have to be gold. We can assay for
silver, rhodium, sulfur, water, bubble gum, anything. Now you must
understand that there are two basic classes of assays. Qualitative
tells you what. If someone brings in a sample of quartz rock and
says, "I want to know what is in it". We take it to the
basement and run it through our procedure and issue a report that
says "sir, you have quartz, sulfides, arsenic, silver,
copper, and gold". This is a qualitative assay.
course, our client who is a prospector immediately asks "how
much gold"? Our reply has to be "who knows".
$63.00, Will that be cash or credit card? "But I have to know
how much gold is in it. Great, now we can run a quantitative assay
for only $89.99. Gold only. Quantitative tells you how much.
not telling you that everyone should run out and start buying
equipment to install an assay lab in the basement. I just thought
that you might like to know what is involved when you take a
sample to an assay lab and why it costs what it does.
going to put in also some Q&D methods that you can do in the
field to help you determine what spots have more fine gold than
others do. You are going to hate me because Im not going to
tell you where this gold mine of information is located on the
page. Hence, you will have to read the whole thing in order to
obtain this very necessary information. You wouldnt want to
miss it, would you? That might allow your partner to know more
than you. Wouldnt want that. Naaah, you are a basement
chemist and are hooked on information.
page is going to be very difficult to write and its going to
take time but you guys seem to be worth it, so--- if you will hang
with me we are going for a MAGIC CARPET RIDE!!
|Types of Assays
are a few different ways of assaying for precious metals. Some of
these are relatively new such as "ion probe analysis".
This is where you take a sample (it has to be in one piece) like a
little BB of metal to a lab and they put it in the Scanning
Electron Microscope and push the button. ZAP!! It spits out a
chart with a plot of all the precious metals that are in the BB
and the amount of each. Another is a method whereby you take
ground sample and put it in a little instrument called a
Fluorescence Spectrometer. This will give you an instant readout
of the amount of gold etc in your sample. We are not going into
the details of these hi-tech methods. We are going to concentrate
on the tried, true and more available methods such as "Aqua
Regia Assay" and the more familiar "Fire Assay". We
will also look at an old "sourdough" method that can be
method a weighed amount of the ground sample is covered with about
2-3 volumes of Aqua Regia and heated for a specified time. The
cooled sample is then filtered and the remaining solids washed 3
times with dilute hydrochloric acid. All the liquid is pooled and
evaporated by gentle heating until only a very small amount is
left. Then, hydrochloric acid is added and the evaporation
repeated. This Hydrochloric reduction is repeated 3 times to be
sure that all of the nitric is gone. Now you have the values in a
solution of concentrated hydrochloric acid. Now add about two
volumes of water. If there was silver in the ore it will
precipitate as a white cloud of silver chloride. The silver is
point there are several options as to how the next step is
accomplished, but the usual is to extract the gold chloride into
an organic solvent, make it up to an exact volume and put it into
an instrument called an atomic absorption spectrometer that
measures the gold content.
type of assay ore is ground and a weighed amount, usually 30 or 40
grams is mixed with an assay flux. We will go into fluxes later.
The ore and flux is placed in a clay crucible and heated at about
1100 C or 1850 F for about an hour or until it is a thick,
homogenous liquid. This melt is then poured into a cone shaped
mold. This cone will consist of a bottom piece of lead with all
the values dissolved as an amalgam. This cone of lead is called a
"prill". The top or wide end of the cone will consist of
a lump of glass containing all sorts of impurities.
lead is separated from the glass by simply smashing it with a
hammer. The lead is further pounded into a cube. This cube is
weighed. The lead cube containing the values is now put into a
sort of a cup with a very thick bottom, made from bone ash or
cement. This cup is called a cupel. That is Q-ple not cup-l. If
you drop this info at a cocktail party you wouldnt want to
cupel with the lead is placed back into the furnace and heated at
1000 degrees or so. The door to the furnace is usually left open a
crack so that air can enter. At this temperature the lead rapidly
combines with the oxygen in the air and is converted back into
lead oxide. You see, another redox reaction. First it was reduced
from lead oxide to metallic lead and now it is being oxidized back
to lead oxide. Ill explain this better when we get to
molten lead oxide now soaks into the cupel. Dont ask why, its
just the nature of the beast. Bone ash or cement will absorb large
quantities of lead oxide. When all of the lead oxide has soaked in
you will be left with a very small sphere of metal. This sphere
contains all of the precious metals except for Osmium. In the
trade they say, "if you can cupel it, you can sell it".
Anything that remains after cupeling is precious metal.
this tiny lump is weighed. Next it is flattened by smashing it
with a hammer. This is to increase the surface area. Remember
about surface area? The flat metal piece is put into a small
porcelain dish and treated with 50% nitric acid until there is no
more reaction. The remaining piece of metal is dried and weighed.
reason for weighing before and after the nitric acid treatment is
that you probably want to know how much silver you had in the
sample. Since the nitric acid removes the silver the difference
between the first weighing and the second is the amount of silver
that was in the sample. The metals remaining after this acid
treatment are gold and the platinum metals.
|How they do that
thing that makes a fire assay work is the flux. What is a flux
anyhow? Why do we need it? A flux is (usually) a mixture of common
chemicals that react to produce a desired result on the metal of
interest. It can reduce it or it can oxidize it. We must use
fluxes because it is imperative to provide the right environment
for the precious metals to be recovered for whatever purpose we
example, If we are assaying a sample of ore for gold we must first
divide all the ore as well as possible by grinding it. Usually, 30
grams of this ore is mixed with the flux. The flux is composed of
a calculated amount of silica (white sand), lead oxide (red lead
or litharge), sodium carbonate (washing soda), and flour,
cornmeal, or cornstarch. That is not a mistake. All the
ingredients including the sample are mixed thoroughly and put into
a clay crucible. The crucible is put into a muffle furnace and
heated at 1100 C or 1850 F until everything is melted and is a
order to understand what is happening it is necessary to review in
detail the things that are taking place in this fearsome mixture.
Guys, we are going to get inside a melted, boiling mixture that
will reduce anything. As the dry mixture melts, some very
interesting things start to happen. First let me say that the
carbonate is in the flux simply to keep the pH on the alkaline
(basic) side so that you cant lose your gold as a vapor of
gold chloride. The sand (quartz) is there to make the melt liquid
so that it can be poured into a mold and to form a glass with
impurities that can be removed after pouring.
still with me?? It aint particle physics, but it is pretty
good chemistry. Apart from the sand and carbonate, that are sort
of inert ingredients, what do we have left? Well, we have the
precious metals, the flour, and a large quantity of lead oxide.
What is happening to those?
course you remember the other pages where we talked about redox
(oxidation/reduction). You also remember that I told you that if
you were to understand anything about metallurgy/chemistry you had
to have a concept of redox. If you forgot, go back and read it
as our mixture starts to get hot the flour starts to try to burn
but it cant. Why not? Well, there just aint no oxygen
available. At the melting temperatures of these materials there is
no air (oxygen) in the furnace. At these temperatures, the flour
is crying to burn. So what does it do? Its desperate. It
finds the only oxygen available to it. That is, the oxygen that is
contained in the lead oxide (PbO). The flour becomes a reducing
agent. In other words it strips the oxygen from the lead oxide and
burns to carbon dioxide. Well, that leaves the lead with no
oxygen. The lead has been reduced to metallic lead instead of the
salt, lead oxide.
a rather traumatic change for the lead. One minute it was a happy
contented salt of lead, a paint pigment. Now it is a heavy metal.
As a metal it is now heavier than any of the quartz or carbonate
so it has no option, it heads for the bottom of the crucible. Now
the lead was reduced in very tiny droplets that now sort of "rain"
down through the melt. As they pass through the thick, viscous
melt they amalgamate with any gold, silver, or platinum metals
that they encounter.
lead/precious metals are now in the bottom of the crucible with
the sand/carbonate/impurities "glass" floating on top.
This mixture is poured into a mold and allowed to cool. You now
have a lead "prill" containing all the precious metals.
The prill is cupeled as described above to determine the precious
got here, is anyone still with me? If not, take a break, have a
beer, read it again, thats what Im gonna do. After
all, If you are going to be a Basement Chemist you gotta be
willing to suffer a little bit.
in my life I have encountered statements which left me speechless.
Many years ago while inspecting a new car then called an "economy
car"; I told the salesman that I thought the price was a bit
steep for the rather shoddy workmanship. With a deadpan face he
replied "sir", if you want an economy car, you must to
be willing to pay for it". What do you say to a naked lady?
Its like this site, if you want to know what is going on,
you gotta be willing to suffer just a little. Spread it out, it
wont hurt so much.
are certainly more fluxes than there are people using them. Why?
Well, you have to have more than one flux. The composition of the
ore sample determines the composition of the flux. That is, the
flux must be "tailored" to the sample to be run. If the
sample contains lots of oxidizing agents then your flux must
contain more reducing agent (flour) than usual. Remember that the
oxidation/reduction system must balance out. In contrast, if your
sample contains lots of reducing minerals you will need less
reducing agent. Well how the hell does anyone ever know what kind
of flux to use? You will soon know. You realize that from this one
website you will have enough information that you will not be able
to "drop" all of it at cocktail parties for the rest of
flux is calculated to contain enough lead Oxide (litharge) so that
if it is completely reduced, it will produce a prill weighing
(usually) 25-28 grams. How do I know how much that is? Im
going to tell you. Aint gonna be no more igerunt prospectors
out there. Litharge is composed of one atom of lead and one atom
of oxygen. We represent it as PbO. Pb being lead and O being
oxygen. Lead has a molecular weight of 207 and oxygen 16. Since
they are combined the molecular weight of PbO is 223 (see link for
Periodic chart of elements). In other words pbO Is 98.2% lead. If
we reduce it to metallic lead then for every 100 gr of pbO we will
be left with 98.2 gr of pure lead. Does that help? Well, it helps
me because I know something that you dont. I know that we
want a prill of around 25-28 gr. So I am going to put 26 gr of PbO
in my flux. When this PbO is all reduced by the flour/ore it will
produce a lead prill of 25.79 gr. That is plenty close enough for
government work. If we get a prill that only weighs 18 grams, we
know that some of the flour must have been used up in reducing,
not the litharge, but the sample. There wasnt enough flour
to reduce both the sample and the PbO. So, what can be done to fix
this problem? Simple, we run the sample again and this time we put
in a little more flour. How much more? Im going to get to
that but right now my ESP tells me that some brains out there are
beginning to overheat and some folks are thinking bad thoughts
about me and why they ever started reading this to begin with. I
think there are two things that we must accomplish. We must cool
down the brain cells and we must adjust the attitude of the
reader. I feel this can best be accomplished by a single action.
Take a trip to the fridge and recover another (or two) of those
cool, soothing, foamy, unguents that I know you have squirreled
away. Hey guys, we aint playing now. This is "Jet
that things are back to normal (do I hear muttering out there?), Im
going to give you a recipe for a flux that will, with minor
adjustments, work most of the time. I know you arent going
to set up an assay lab but this is what happens when you send in a
sample to be assayed.
a good starting flux for quartz or "neutral" ore
Ground sample ---------30 gr
Litharge (PbO) --------- 30 gr
Soda (Na2CO3) -------- 30 gr
Flour ------------------------ 2 gr
Silica Sand -------------- 10 gr
amounts used are not correct for all ores. If your prill is too
small, add more flour. I happen to know that 1 gr of flour will
produce about 12 gr of lead from the litharge. So, if your prill
is 10 gr too light you should add another gram of flour to the
next assay. Itll be close enough. Another thing that can go
wrong is that sometimes the ore produces a melt that is too thick
and viscous to pour properly. Could probably add more sand or,
more effective, would be to add a couple of grams of Borax. Borax
produces a thinner, more liquid melt. If borax is used you should
be aware that it attacks the clay crucible so dont use so
much that you get a hole in it.
should also mention that molten Litharge will dissolve the
crucible and the firebricks that line the furnace. That is another
reason for the quartz sand in the flux. It protects the crucible.
reality, the business of Fire Assay is some science, some art,
some patience and a little luck.
intended to make assay chemists of you. This page is intended to,
hopefully, give you a little insight as to what goes on in those
mysterious backrooms that you never get to see at the assay
a lot of folks get confused between assaying and smelting. In
fact, smelting is sort of the opposite of assaying. They both are
carried out at high temperatures. I think, perhaps this is where
the confusion begins. In an assay melt, as I have explained, the
environment that is created in the melt is reducing. In fact, it
will reduce almost anything known to science.
smelting melt we accomplish just the opposite. We want to oxidize
everything except the precious metals. As all the base metal salts
are attacked or oxidized by the smelting flux they dissolve in the
flux where they remain until the melt is poured into a mold. These
base metals will end up in the slag or glass that floats on top of
the gold. As in assaying, the flux depends to a large degree on
the composition of the material to be smelted.
term "smelt" seems to mean different things to different
people. To many the word brings up visions of smoking, boiling,
caldrons of metal. To others it refers to any purification which
results in the separation of the gold from most of the
contaminating materials. The gold that most of us will be working
with is in pretty good shape to begin with. Sure, it probably
contains some black sand and maybe a few other minor contaminants
but not 50% black sand etc.
smelting is usually carried out using a crucible made of graphite
(a crystal form of carbon). Of course I dont have to tell a
basement chemist why we prefer to use graphite, but for any casual
visitor I should explain that at high temperatures the graphite
(carbon) becomes a reducing agent that helps keep gold and
platinum metals in their reduced or metallic forms. Some silica
sand or ground glass is usually added so that there will be a
glass matrix that floats on top of the metal. Some sodium nitrate
(Chilean nitrate or Saltpeter) is added. This nitrate is a rather
strong oxidizing agent. When hot it will oxidize almost any metal,
except for gold and the platinum metals, to its nitrate salt.
These salts combine with the molten sand. Usually some borax is
added to thin the viscous, molten glass.
mixture is heated until the melt becomes "quiet" with no
bubbles, foam, or lumps in it. The precious metals are now poured
into a mold and the glass or other "gangue" is removed.
The black sand that was in our melt was oxidized to iron nitrate
by the sodium nitrate and is now dissolved in the glass that we
discard. Your gold, at his point will not be 100% pure. It
probably contains small amounts of copper, silver, tellurium, etc.
The good news is that it looks like gold and should be of fairly
high quality. Plenty good enough to sell.
|A field assay that you can do
prospectors might have known a few things that we dont
usually give them credit for. They knew about fine gold and they
knew that it was worth just as much as coarse or nugget gold. They
also had a very simple method of determining about how much of
this fine gold that was in the ore that they were currently
interested in. I have no idea where they learned how to do this
assay, who told them about it, or where the procedure originated.
I never hear of anyone using this method anymore. Dont know
why. If you read my page on halide leaching you know that
solutions of Iodine will dissolve fine gold very rapidly. Well,
the old sour-doughs knew this too. And, somehow they devised
the following method for using it as an assay tool.
need to do this assay is get some "Lugols Iodine".
This is simply Iodine dissolved in a solution of Sodium Iodide.
You can buy this from any pharmacy. Just be sure that it is in
water, not "tincture of Iodine", that is in alcohol
solution. You can make it yourself (see page on Halides). You will
also need a little dilute nitric acid, 2 normal or so.
just take some material that you think might contain a significant
amount of fine gold and grind it, pound it with a rock or a
hammer, or whatever until it is as fine as you can get it. Take a
measured amount of the sample and put it in some convenient vessel
such as a large test tube, a small baby food jar, or other. You
can use whatever measure is convenient for you. Five level
tablespoons, one coffee scoop, etc. You do want to use the same
amount each time so that you will have some idea of the amount of
gold in the ore. At least you will be able to say that this ore
has more than that one.
your ore is in the jar. Pour in the Iodine solution so that the
ore is well covered. Shake it occasionally for some time, maybe ½
to 1 hour. Now you must remove the ore. You can let it settle and
pour off the Iodine containing the gold or you can filter it with
a small funnel and some coffee filter paper. The idea is to get
the solution as clear as possible. Now you add a small glob of
mercury and shake. At some point the solution will lose it's
red-brown Iodine color and become a (usually) clear yellowish
liquid with sediment of heavy floured mercury in the bottom of the
jar/test tube. Allow this to settle for a few minutes and then
carefully pour off the liquid. Add some water to the mercury,
shake, allow it to settle, and pour off. Dont throw the
liquids away. Now you have your precious metals amalgamated in the
floured mercury. Now you simply add some nitric acid (not more
than 50/50 with water) and dissolve the mercury. When the mercury
is all dissolved you will be left with a black or brown material
that you cannot dissolve. This is your precious metals. Dont
throw the nitric solution away. Now take a good look at the black
sediment. Try to get a feel for how much there is so that you will
have a comparative idea of how much you have. You can buy, from
chemical supply companies, a graduated, conical test tube. If you
run the assay in this you can simply read off the quantity of
precious metals on the scale engraved on the tube. Its a
nicety that the old timers didnt have but it is convenient.
reason I told you to save all the liquids is simple, we are going
to recover all your Iodine and mercury. The solution that
contained Iodine is treated with a few drops of clorox. The Iodine
will settle to the bottom. The liquid is poured off and then you
add a little lye water until the solution becomes clear and
colorless. Your Iodine is in solution ready to use again (see the
page on Halides). The nitric acid solution of mercury is treated
by your favorite method to recover the mercury (see the mercury
see, now that you have become a basement chemist, you dont
have to stay in the basement, You can take it right out into the
field with you.
now we know a few things that the old-timers knew 150 years ago.
The question is how many of you knew about or have used this
method? Give it a try, it works.
|Aint Science Wunnerful???