Assaying and Smelting
Dr. A.K. Williams, Ph.D
San Pedro, Costa Rica


O.K. guys, we are going on a little excursion into the world of titration. What is "titration"? Well, itís 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 itís about 500-10,000 cps.

Well guys, how the hell did we get to this point? We started out to just explain some facts about mercury. Itís 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.

Letís 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. Doesnít 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 and quantitative.

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.

Of 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.

Now Iím 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.

Iím 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 Iím 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 wouldnít want to miss it, would you? That might allow your partner to know more than you. Wouldnít want that. Naaah, you are a basement chemist and are hooked on information.

This page is going to be very difficult to write and itís 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

There 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 very useful.

Aqua Regia

In this 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 removed.

At this 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.

Fire Assay

In this 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.

The 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 wouldnít want to mis-pronounce it.

The 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. Iíll explain this better when we get to fluxes.

The molten lead oxide now soaks into the cupel. Donít ask why, itís 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.

Now 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.

The 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

The 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 desire.

For 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 thick liquid.

Now, in 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 canít 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.

Are you still with me?? It ainít 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?

Of 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 again.

O.K., as our mixture starts to get hot the flour starts to try to burn but it canít. Why not? Well, there just ainít 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? Itís 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.

This is 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.

The 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 metal content.

WOW! I got here, is anyone still with me? If not, take a break, have a beer, read it again, thatís what Iím gonna do. After all, If you are going to be a Basement Chemist you gotta be willing to suffer a little bit.

Twice 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? Itís 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 wonít hurt so much.

Assay Fluxes

There 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 your life.

The 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? Iím going to tell you. Ainít 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 donít. 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 wasnít 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? Iím 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 ainít playing now. This is "Jet Airline" stuff.

Now that things are back to normal (do I hear muttering out there?), Iím going to give you a recipe for a flux that will, with minor adjustments, work most of the time. I know you arenít going to set up an assay lab but this is what happens when you send in a sample to be assayed.

This is a good starting flux for quartz or "neutral" ore samples.

Ground sample -----------30 gr
Litharge (PbO) ---------- 30 gr
Soda (Na2CO3) -------- 30 gr
Flour --------------------- 2 gr
Silica Sand -------------- 10 gr

The 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. Itíll 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 donít use so much that you get a hole in it.

I 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.

So, in reality, the business of Fire Assay is some science, some art, some patience and a little luck.

I never 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 back rooms that you never get to see at the assay laboratory.


I think 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.

In a 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.

The 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..

Furnace smelting is usually carried out using a crucible made of graphite (a crystal form of carbon). Of course I donít 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.

This 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!

The old prospectors might have known a few things that we donít 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. Donít 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-doughís knew this too. And, somehow they devised the following method for using it as an assay tool.

All you need to do this assay is get some "Lugolís 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.

Now, 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.

Now, 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 Ĺ 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. Donít 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. Donít 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. Itís a nicety that the old timers didnít have but it is convenient.

The 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 page).

You see, now that you have become a basement chemist, you donít have to stay in the basement, You can take it right out into the field with you.

O.K. 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.

Ainít Science Wunnerful???
Assaying and Smelting