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The Art of Assaying

Last Updated: 6th Apr 2019

By Dave Crosby

What is assaying and how do you go about doing it?

The art of assaying is a branch of analytical chemistry in connection with mining and metallurgy. It's object is to obtain the value of a stated quantity (usually in avoirdupois ton) by determining the value of a small representative sample.
- J. Reginald Smith "Assaying For Everyone" 1902
(Printed on Demand)
Modern Assaying

Modern Assaying: A Concise Treatise, Describing Latest Methods and Appliances (Classic Reprint) (Paperback)
- by J. Reginald Smith (Author) Paperback $10.57

So assaying is an attempt to determine the amount and value of a metal in a field, stream bed, mine, or vein.

I think of the Assay Process as functioning like a pinhole camera where:

1. the subject is the mine,
2. the pinhole is the sample, and
3. the image is the emerging estimated amount and value.
Art of Assaying

As with most things, you can make it as difficult as you want.

A. A rough assay can be done by crushing ore samples and panning the results. With some experience and a careful examination of the amount of gold present, it is possible to make decent visual estimates of the amount of free gold in the ore.
If you have watched Gold Rush on the Discovery Channel, they often test a pan of the supposed gold rich layer.
Finding less than 5 colors (flakes) in the pan determines the layer as not worth the expense of mining.
Panning for Gold

B. Assaying by the blow-pipe furnace is another inexpensive path.

Egyptian Blowpipe

Assaying with a blowpipe probably began in ancient Egypt.

Portable Assay kits were first developed in the 1830s at the Freiberg Mining Academy in Germany allowing someone to set up a table and do the assay right at the mine site instead of taking samples back to the lab.
Blowpipe kit
Blowpipe Kit Contents

Bramble, 1898 - www.gutenberg.org/files/45287/45287-h/45287-h.htm
The ABC's of Mining p-12
The prospector of to-day is often a very different man from his predecessor of a generation ago. The old gold hunter used to sally forth armed with a pick, shovel and pan, and usually a very little grub. In his stead men are now taking the field who have had the benefits of a thorough education, both practical and theoretical, and provided with all the equipment necessary for their work.

Some of these men carry an outfit somewhat as follows: An iron mortar holding half a gallon, together with a pestle a rough scale for pulp, a more delicate one showing troy grains and pennyweights, a 40-mesh sieve, a burro furnace and muffle, one cupel mould, a couple of dozen scorifiers, tongs to handle the cupel and scorifiers, two annealing cups, a spirit lamp, a dozen test tubes, a pouring mould, five or six pounds of borax and about as much carbonate of soda, five pounds of bone ash, ditto of granulated lead, a pint of nitric acid, ditto of hydrochloric acid, ditto sulphuric acid, ditto of ammonia, twice as much alcohol and two pounds or so of granulated zinc.

As a blow pipe outfit he will take a blow pipe, spirit lamp, nitrate of cobalt in solution, cyanide of potash, yellow prussiate of potash, red prussiate of potash, a sheet or two of filtering paper and a couple of three-inch glass filters. With this outfit he can determine any mineral he may come across.

By patience and observation the man who starts out to take up prospecting as a road to fortune may easily master the rudiments of his business. It will not take him long to become familiar with the commoner rocks, and the more valuable ores. His own rough tests in the field must be confirmed by competent assayers upon his return to civilization, and in this matter he should be very guarded. The most reliable assays are made either at the different government assay offices or by some of the large metallurgical works whose reputation is world wide.

Prospecting is hard work, but the life is healthy and full of excitement, only the explorer should have courage, hope, and good temper, for each and every one will be as necessary in his chosen vocation as his pan and pick.

Here's the Blowpipe Assay Process:
1. Extract a small pulverized most carefully prepared true sample of the layer/vein/ore being assayed as possible and weigh it with the provided balance scale.

2. Combine the ore with some tiny lead pellets in a scorification dish and heat the whole thing with the alcohol lamp using the blowpipe. There's a trick to blowing through puffed out cheeks while simultaneously inhaling more air through your nose to keep the flow going. By exhaling steadily through the blowpipe you add oxygen to the flame that can increase the temperature above 2,000 degrees Celsius (3,632 degrees Fahrenheit).

The lead will oxidize and react with any silicates in the ore to produce a glass-like substance called slag, and the rest of
the lead would form an alloy with any gold or silver in the sample.

3. You end up with a small chunk of glass and a small bead of metal alloy in the scorification dish.

4. you pick out the metal alloy and put it in a bone ash dish and heat it up enough to oxidize all of the lead. The lead melts and the surface oxidizes. The oxide is absorbed into the cupel, exposing more lead which oxidizes.

As gold and silver do not oxidize until well above 1000°C, they continue to concentrate until the last of the lead is absorbed; leaving only a "dore bead" (containing only gold and silver) in the bottom of the cupel.

After cleaning, the beads' weight is recorded to get the total precious metal content of the sample.
The bead is then "parted" by dissolving the silver from it with nitric acid which dissolves silver but does not affect gold.
After parting, the weight of gold is recorded.
Subtracting the gold weight from the dore weight provides the silver weight.

5. By weighing the final bead and dividing by the weight of the original ore sample, you can calculate the percentage of precious metal in the ore.

6. Multiply the percentage of precious metal by its part of a ton and the going price and you have the value of the mine per ton of ore.

These kits were widely used in the late 1800s, but their use started to wane around the end of the century because of more trust in the two MAIN methods of modern assaying: the "Dry Assay" and the "Wet Assay."

The larger the assay sample, the greater possible accuracy, but as Agricola stated in his book "De Re Metallica" (1556):

they can assay the ore at a very small expense, and smelt it only at a great expense.

Where is the balance between too little a sample and too large?

Of great value in making calculations is the Assay Ton: An Assay Ton (A.T.) = 29.166 grams.
Definition of Assay Ton: A weights system created by combining aspects of Troy, Avoirdupois and Gram weights into a simple labor saving calculation unit. It was derived by reckoning in the following manner:

1 lb. av. = 7,000 troy grains.
2,000 lbs. = 1 Ton
2,000 X 7,000 = 14,000,000 Troy grains in 1 ton Av.
480 Troy grains = 1 oz Troy.
14,000,000/480 = 29,166 Troy oz. in 2,000 lbs. Av.
There are 29,166 milligrams in 1 assay ton (A.T.);

Therefore 2,000 lb. is to 1 A.T. as 1 oz. Troy is to 1 milligram.

So now using this system, if 1 A.T. of ore assays 1 milligram of gold or silver, the ton contains 1 ounce Troy.

Old Assay Building
C. Dry - AKA Fire Assay (assaying proper)- using heat, reducing fluxes. Includes cupellation and scorification.
Dry assaying is much like blowpipe assaying, only on a larger scale and is well covered in Reginald Smith's book

D. Wet - AKA Chemical Assay (analysis)- includes volumetric, gravimetric, calorimetric, and electrolic.

Volumetric is the quickest and simplest as less manipulation is required. The sample may often be titrated without lengthy chemical separations.
The weight of ore taken for wet assay is much smaller than is required for Fire Assaying, 1/4 - 2 grams generally being sufficient. It is essential that the sample be finely crushed, especially if it is to be dissolved in acids, as particles of the mineral may be encased in the insoluble gangue. Minerals which are not decomposed by acids are fused with a flux which will render them soluble in water or acids.

The assay of gold and silver may be done by cupellation or by wet chemical volumetric methods. The cupellation method is subject to more errors than the wet assay. If accurate values are needed the chemical method must be applied.

WET ASSAYING SILVER ORE: Is the same process usually used to Refine Silver.
How to Refine Silver
By Laura Hageman, eHow Contributor

When refining silver you want to make sure you do this outside where there is plenty of ventilation. Give yourself plenty of room in order to have a quicker dissolving reaction from the silver. Depending on how much silver you have to refine will depend on how many 5 gallon buckets you will need. You can have larger buckets as well just as long as there is enough room for a reaction of bubbling and fuming you will get during this process. You can refine silver using acid only or acid and baking soda.

Things You'll Need

Rubber apron
Rubber gloves

Put 150 ml of nitric acid for every ounce of silver you have in each bucket. Do not overload the bucket since it could overflow. You will need about two to three times more room than the amount of silver in the bucket. You will see the reaction of bubbling and fuming which can take at least 30 minutes. Once this stops you can move onto the next step.

Take the acid and the dissolved silver and pour it into another bucket. Don't allow solid pieces to be poured with the silver and acid since it will contaminate the silver. From here you will add an ounce of silver precipitant crystals or SAC to every 40 ounces of dissolved silver. This will develop silver that will sink to the bottom of the acid. Give 30 minutes or more. Then pour out the acid. Neutralize and dispose of the acid by filtering it making sure that no particles of silver are lost.

Wash the silver thoroughly. You want to rinse off any acid remaining on the silver. Then add about three or four drops of aqua ammonia which will appear blue if there's traces of acid left. If so, then continue rinsing and reapply the ammonia. Do not add too much ammonia since it could damage the silver. Then dry.

Use baking soda along with karo pancake syrup, salt, water and Red Devil drain cleaner in order to use a different process of refining silver. It is basically the same procedures with the nitric acid but this way may be easier to not lose any silver particles. You will begin by dissolving the silver with acid in the plastic bucket. Add salt to acid until white clumps stop appearing in the acid. Then pour off the acid into another bucket.

Add baking soda to the acid. Add about two tablespoons of baking soda which will neutralize it. Then filter the white precipitants that the salt had formed which is pure silver chloride. Rinse the silver with water. Then add the Red Devil drain cleaner to the silver chloride until it turns black. Rinse with water again. Then add karo syrup until the particles turn silver. This will result in refined silver. Be sure to rinse and make it clean of any extra chemicals left on it.

From 1859 to 1891 the Comstock lode in Nevada produced $325,000,000. This lode is a belt of quartz in a contact vein between diorite and diabase.It is 10,000 feet long and several hundred feet wide.
The Anaconda mine in Butte was one of the largest producers of silver in the country. In 1896 its output was 5,000,000 ounces. The Anaconda was also the heaviest copper producer in the United States, its yield of copper being 125,350,693 pounds.

All silver ores are heavy, and many of them are sectile - may be cut with a knife.
Most silver ores are sulfides as silver easily enters into chemical combinations with sulfur.

In America galena is the principal source of silver. Next are the chlorides and oxides, and finally the silver that is parted from gold when it reaches the mint, as gold always contains some of that metal.
Silver is found in many different locations, and is associated with all sorts of minerals. It is never found in placer deposits, as it easily breaks up under the influence of water, air, etc. It is also found in igneous rocks in association with augite, hornblende and mica.

The commercial ores of silver are:
Argentite Ag2S - - - - - - .- 87.1 per cent silver
Cerargerite AgCl - - - - -. - 75.3 per cent silver
Stephanite 5Ag2SSb2S3 -- 68.5 per cent silver
Proustite 3Ag2SAs2S3 - - - 65.5 per cent silver
Pyrargyrite 3Ag2SSb2S3 - - 59.9 per cent silver

Silver miners have tested ore for silver by heating the ore and dipping it into water.
If silver is present some metal will come to the surface as a greasy scum.

Heated above 600°C silver tends to join with silica to form yellow crystals of silver silicate - Ag2O3Si. For this and other problems the cupellation assay of silver has been largely replaced by volumetric methods. Chief among these is the "Gay Lussac Assay", which was introduced in Paris in 1830 for testing silver bullion.
An exact weight of the bullion is dissolved in nitric acid, and very nearly all the silver is precipitated at once by the addition of a known volume of a standard solution of salt. When the precipitate has settled, the remaining silver is precipitated by the further addition of a small quantity of a more dilute solution of salt, the precipitate forming a white cloud in the supernatant liquid. The quantity of this silver is judged by the appearance of the white cloud.

Wet Assay of Silver Ore:

Begins as all assays - Collecting an honest sample, crushing, quartering, pulping, selecting final samples of 1/4 to 2 grams.

Now place the sample in a tube and dissolve the powdered ore in nitric acid.
The sulfur in the silver is replaced with Chlorine and turns from black to white.
Add a solution of common salt or hydrochloric acid to precipitate chloride of silver.

If chloride of lead and mercurous chloride are absent, then decant or filter the solution.
Dry and weigh the chloride of silver. Three fourths of the weight is nearly pure silver.
Collect and fuse the silver chloride to release the silver.
Weigh the metallic silver collected.

Once you have the weight of the silver extracted from the pulp, the following math will expand the numbers to the weight per ton of silver in the ore:

Weight of the sample in grams / 908000 grams per ton = weight of the sample in tons.

Or Weight of the sample in carats / 155.5 carats per troy ounce = wt. of the sample in ounces.

Next, to determine the ounces per ton, calculate "the Factor":

1/weight of the sample in tons = the Factor

Then the weight of the silver in troy ounces times the Factor = ounces of silver per ton of rock.

Finally, ounces times the spot price = the value.

BTW, How large is a "TON" of sandstone?
The average weight of a sandstone boulder is approximately 150 pounds per cubic foot. Limestone boulders and granite boulders in most cases weigh more. They average about 175 pounds per cubic foot.
To estimate the quantity, you need to convert the area into cubic feet by multiplying length x width x height. Once you have this number, multiply it by the boulder's weight per cubic foot, then divide by 2000 to convert this number into tons.

13.333 cu. Ft of sandstone = 2.37124' X 3 X 150 Lbs = 2,000 lbs/2000 = 1 Ton.

A cubic foot of iron would weigh 491 lb.
A cubic foot of copper would weigh 559 lb.
Silver is even denser than copper, at 655 lb for a cubic foot.
Gold is really heavy at 1206 lb for a cubic foot.

1 ounce (troy) (oz t) of silver = 0.18 cubic inches in silver. ~ 3/16"
One cubic inch (cu in - in3) of silver = 5.53 troy ounces.
One cubic foot of silver = 9,552.0833 Troy Ounces = 655 pounds.

One Ton

The silver would make it heavier, so ~ less than a two foot cube.

Then even newer techniques like X-ray spectroscopy began around the 1920s. By bombarding a sample with X-rays and measuring the wavelengths of the X-rays it emitted back, a chemist could determine what elements were in a sample, and in what percentages.

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