Fire assaying is a method of determining the proportion of precious metals like gold and silver in ores. It involves melting samples with fluxes at high temperatures. The metals dissolve in molten lead and are separated from slag. Lead is removed by oxidation, leaving a gold-silver alloy called dore. Acid treatment separates silver from gold. Fire assaying provides an economical way to analyze large samples and is still widely used today due to its accuracy and simplicity despite the development of modern analytical techniques.

1. Fire Assaying

2. A. Definition and Scope
Assaying is the process of determining the proportion of metals in ores and
metallurgical products. There has been a tendency to restrict the term to fire
methods for the determination of metals so that wet chemical analytical
procedures and spectrographic methods of analysis are usually termed “analysis”
instead of “assaying”.

3. Fire Assaying is a branch of quantitative chemical analysis where metals are
determined in ores and metallurgical products with the aid of heat and dry
reagents.

4. Fire assaying is carried out today as much as they were then. This ancient method
is still taught in university courses for metallurgist because it offers a convenient
way of studying the reaction of igneous chemistry.
It is still used in the industries, mining and refineries, because modern science has
been unable to develop better methods of determining precious-metal content of
ores. This is because, large samples can be economically and accurately assayed
by fire method but spectrographic method of analysis are not adopted to operating
on large sample portions, while wet chemical method, involving digestion of large
samples would be very expensive.

5. The fire assay method is generally applied to gold, silver and some metals in the
platinum group. The fire assay of gold and silver depends mainly upon:
a. High solubility of these metals in molten metallic lead and their almost complete
insolubility in slag.
b. The difference in specific gravity between the two liquids, lead and slag, which
permits the complete separation of precious metal-bearing lead from the slag.
c. Complete removal of lead (Pb) from the previous metals by controlled oxidizing
fusion involving differential filtration, in a porous vessel known as cupel.
d. The solubility of silver and complete insolubility of gold in dilute nitric acid.

6. B. Objectives of Assaying
1. Valuation of mining property.
2. Basis of buying and selling ores and metallurgical products.
3. Guide to prospecting and development of ore bodies.
4. Delimitation of boundaries of marginal ores.

7. 5. Check against dilution in mining operation.
6. Control of average grade of ore mined and milled.
7. Investigation and control of metallurgical process.
8. Accounting of all metals in process.

8. C. Degree of accuracy required in assaying depends
upon which of the above purposes is involved.
Highest degree of accuracy on individual sample is necessary in buying and
selling ores and metallurgical products, so average of 2 to 3 determinations is a
must. If parties do not agree a duplicate pulp is submitted to umpire who reports
average of four or more determinations.
Samples for mine valuation will need rigid precautions to avoid contamination or
accidental salting.

9. Mill tailings and smelter slags require special care since small error will seriously
affect the metallurgical balance.
Other purposes of assaying as outlined above do not generally require a high
degree of precision, single determination with ordinary precaution against errors
are usually adequate.

10. D. Unit in Fire Assaying
In the USA, Canada and South Africa, the assay of precious metals is expressed
as troy ounces per short ton (2000 avoirdupois pound) of ore. A system of assay
ton weighs is used in weighing the assay portion of ore, which is taken for assay
process. The assay-ton contains the same number of milligrams (29,166), as there
are troy ounces in a short ton. Hence, the number of milligrams of precious metal
found in an assay-ton of ore indicates the assay in troy ounces per short ton.

11. In the Philippines and other countries including the countries mentioned above
who used or shifted to metric system. The proportion of precious metals is
reported in grams per metric ton of ore. The metric ton contains 1,000,000 grams;
consequently one (1) milligram of gold from a 10-grams assay sample or ore
indicates 100 grams per metric ton of ore.

12. E. Requirements in Fire Assaying

13. 1. Apparatus
1. Muffle Furnace - oil fired or electric furnace
2. Balance and Weights - should be sensitive to
0.005 mg at least. It need not have a capacity of
more than 1,000 g but should be accurate with that
load.
3. Cupels - a flat, shallow crucible of bone ash,
used in cupellation.
4. Fusion Crucible - 20 g capacity
5. Iron Mould
6. Annealing Crucible – 10 g Coors porcelain
crucible.
7. Hot plate

14. 2. Reagents
A flux is a substance which when heated in contact with some difficulty fusible
compounds either combines in it or takes it into solution, in each case producing a
compound or mixture which is easily fusible at ordinary furnace temperatures. The
principle fluxes and other reagents used in fire assaying are the following:

15. 1)Borax and Borax Glass – these are active and readily fusible acid fluxes. These
lower the temperature of slag formation and are considered excellent solvent for
metallic oxides. These are used as cover to prevent loss of ore which results from
the escape of gas at a temperature below that incipient fusion.
2)Silica – acid reagent which combines with metal oxides to form the slag, protects
crucible from corrosion.
3)Litharge – basic reagent and aids in the fusion of acid substance. When
reduced, it supplies the lead for the button.

16. 4)Soda Ash – basic flux and when molten is very fluid and holds in suspension
finely ground infusible materials. To some extent, it reacts with the metallic sulfides
to form alkaline sulfide and sulfates.
5)Flour – powerful reducing agent and reduces litharge to lead for the collection of
gold and silver.
6)Niter – this is a powerful oxidizing agent. It oxidizes sulfides, arsenides,
antimonides. It is used to control the size of the button when the reducing agent
effect of the charge is high.

17. F. Process Terminology Involved
1. Sampling – grinding sample too fine a mesh should be avoided, 80-120 mesh is
probably the best range.
2. Fusion – melting together of the fluxes and the sample at 1,000°C. Time
ranges from 40-50 minutes. A long-continued fusion at low temperature means a
small lead button (ideal weight is 23-30 g), and an imperfect collection of gold and
silver. Pouring melt in molds separates lead buttons which are hammered in rough
cubes.
3. Cupellation – the separation of gold and silver in a cupel, wherein the
impurities are volatilized or absorbed by the cupel, and a button of noble metal
known as dore (an alloy of gold and silver left after cupellation) is left. Pre-heat the
cupel in the furnace for at least 20 minutes. Starting temperature is ideal at 900°C.

18. 4. Parting – the separation of gold from silver by dissolution with dilute nitric acid.
5. Annealing – tempering by heating, then cooling, to render the gold less
brittle.
6. Weighing – values may sometimes be nil (not visible) or trace (visible but
negligible in weight).
7. Inquartation – addition of inquartz with known Ag content.

19. G. Procedure
1. After drying the sample, stage-crush to minus 1.82 mm and reduce by a series of crushing, mixing and cutting until the
desired amount of approximately 300 grams is obtained for pulverizing to minus 100 mesh.
2. Mix the sample thoroughly by rolling in a rubber cloth and scoop sample at random for weighing. Normally 10, 15,
20 or 30 grams is used depending on the grade of the precious metal.
3. Add the necessary fluxes such as soda ash, litharge and silica. Add the reducing or oxidizing agent. These must
be calculated to suit the nature of the ore for assaying.
4. Mix the sample and reagents thoroughly in the clay crucible and cover the charge with a mixture of borax and
soda ash at 2:1 ratio.
5. Charge the crucible in the furnace, pre-heated at 1,000°C. When the charge is completely melted, remove the
crucible from the furnace and pour the contents into an iron mould and cool.
6. Break the core of the lead from the slag by hammering and form it into a cube. Place two inquartz into the lead
button, also by hammering to press the inquartz into the cube.
7. Put the button into the cupel previously heated inside the furnace. The lead is melted and subsequently oxidized
and absorbed by the cupel. An alloy of gold and silver which is called dore remains in the cupel.
8. The dore is cooled, flattened and weighed.
9. The weighed dore is treated with hot nitric acid to dissolve the silver, first with 1:4 then 1:1 HNO
3
to water. It is
then washed with distilled water.

20. H. Guide in Calculating Fluxes, Reducing and
Oxidizing Agents
1. Determine the nature of the sample and classify them as siliceous, low sulfide,
high sulfide, oxide or basic ore. Each type of ore will have a different combination
of fluxes.
2. After the nature of the ore is known, use the following method in calculating
the respective reagents.

21. a) Siliceous Ore
1. Weight of soda ash – weight of ore
2. Calculate for the bisilicate silica equivalent of the soda ash. If there are
other basic compounds in the charge, calculate for their bisilicate silica equivalent.
3. Deduct the total silica in step 2 from the silica content of the ore.
4. Add litharge equivalent to the remaining silica.
5. Add litharge for the lead button (the equivalent of 25 g Pb).
6. Add reducing agent

22. b) Low Sulfide Ore
Low sulfide ores are treated as siliceous ores and the slag is aimed to be
bisilicate. However, the reducing effect of the ore should be considered.

23. c) Sulfide Ore
1. Wt. of Na
2
CO
3
= Wt. of ore
2. Wt. od PbO = 2 times the Wt. of ore
3. Calculate bases in the ore
4. Calculate the monosilicate silica equivalent of the basis in steps 1,2, &3.
5. Add silica as flux.
6. Add PbO for button.
7. Find the reducing effect of the ore. Subtract the desired button weight form the reducing effect and divide
the result by the oxidizing power of niter. This gives the amount of niter.
8. Add soda ash for the sulfate layer equivalent to ¼ of the niter.
9. Add borax as cover.

24. d) Basic Ores with Alumina with Bisilicate Slag
1. Add Na
2
CO
3
equal to the weight of the ore.
2. Add litharge for the slag equal to the weight of the ore if low in Al
2
O
3
. If the ore is high in Al
2
O
3
, double
the quantity of litharge.
3. Calculate the bases in the ore other than alumina.
4. Calculate the bisilicate silica equivalent of the bases in steps 1,2&3. Deduct the silica in the ore and replace
1/3 of the remaining silica with borax glass.
5. Add more litharge for the button.
6. Add reducer for the button after calculating the oxidizing effects of the bases in the ore.
7. If the ore is high in alumina, estimate the alumina content and provide an equal weight of lime.

25. I. Calculation of charges for Copper Concentrate
1. Estimate the approximate percent Cu and calculate the weight of the sample
such that the Cu content of the charge is not over 7.5 grams. Add Na2CO3 equal
to the weight of the ore.
2. Add litharge for the slag equal to 30 times the weight of the copper.
3. Calculate the bases in the ore.
4. Calculate the subsilicate silica equivalent of the bases in steps 1,2 &3.
Deduct the silica in the ore.
5. Add more litharge for the button.
6. Calculate the niter by considering the reducing effect of the ore.