The beginning history of fire assaying can be traced to the finds in Troy II (about 2600 B.C.) and in the Cappadocian Tablets (2250-1950 B.C.). These finds prove that very pure silver was made in the twenty-fifth century B.C. From this evidence we must conclude that the cupellation process, and therefore fire assaying, was invented in Asia Minor in the first half of the third millennium B.C. shortly after the discovery of the manufacture of lead from galena (Forbes, 1950, p. 213). Theodore A. Wertime (1973, p. 883) wrote that the first convincing evidence of the production of silver from lead ores is the cupel buttons found at Mahmatlar in the late third millenium B.C. and that are now in the Hittite Museum in Ankara, Turkey.

Biblical references to the use of fire-assay techniques can be traced to the prophet Moses (about 1300 B.C.). Numbers 31: 22, 23 states: “Only the gold, and the silver, the brass, the iron, the tin, and the lead, every thing that may abide the fire, ye shall make it go through the fire, and it shall be clean:.” From David (about 1000 B.C.), Psalms 12: 7 (Dahood, 1966 p. 72) states:

“The promises of Yahweh (God)
are promises unalloyed,
silver purged in a crucible,
of clay refined seven times.”

From Solomon (10th century B.C.) Proverbs 17: 3:

“The fining pot [cupel] is for silver
and the furnace for gold:
but the Lord trieth the hearts.”

References to the dross obtained in cupellation are contained in Proverbs 26: 23:

“Burning lips and a wicked heart
are like a potsherd covered with
silver dross.”

From Jeremiah (born after 650, died 570 B.C.) 6: 29 (Bright, 1965 p. 47):

“The bellows blow fiercely,
But the lead comes whole from the fire.
It’s useless to go on refining,
The wicked are not removed.”

Zechariah (sixth century B.C.) 13: 9 states: “And I will bring the third part through the fire, and will refine them as silver is refined, and will try them as gold is tried:.” More reference is made to the “fining pot” (cupel) in Malachi (fifth century B.C.) 3: 3: “And he shall sit as a refiner and purifier of silver: and he shall purify the sons of Levi, and purge them as gold and silver, that they may offer unto the Lord an offering in righteousness.”

These references are but a few that demonstrate the extent to which fire-assay techniques permeated Asia Minor in ancient times.

The Romans were able to desilver lead down to 0.01 percent or even to 0.002 percent in some cases and marked their cupelled desilvered lead “EX ARG.” Roman lead pipe was found to be desilvered almost completely. In addition to the employment of fire-assay techniques for refining lead and silver, the Romans were the first to make extensive use of the amalgamation process. This process consisted of mixing the finely ground ore with salt, copper sulfate, and mercury and exposing the mixture to the air in heaps which were constantly worked. The silver-amalgam (silver-mercury compound) was then distilled, the mercury recovered as distillate, and the silver as residue (Forbes, 1950, p. 208). Another version of the amalgamation process used by the Romans in treating rich gold ores consisted of crushing the ore and mixing it with mercury. The gangue was separated by forcing the mercury through leather, and then the amalgam (solution of the gold and silver in the mercury) was heated to recover the noble metals (Forbes, 1964, p. 173). The amalgamation processes were made possible because the Romans produced mercury on a large scale. The ore worked was cinnabar, crushed in sandstone mortars with stone pestles, and heated in furnaces where the mercury distilled off to collect in the fore-hearth.

With the fall of the Roman Empire by the invasions and conquests of the barbarian hordes, intellectual life stagnated for about 500 years. It is doubtful that any significant advancement in the art of fire assaying took place during that period. With the revival of learning in the latter centuries of the Middle Ages, the art of fire assaying was renewed vigorously. One man stands out above all in this period — Agricola (1494 —1555). In about 1529 he began to write “De Re Metallica” (1556), which he completed in 1550 but did not send to the publishers until 1553. Preparation of the woodcuts delayed publication until a year after the death of Agricola. In this book, Agricola, who was the first author to attempt to cover the subject of fire assaying in its entirety, arranged his material systematically. He described the tools and equipment to be used in assaying, stressed the quantitative aspects of each step, and for the first time gave instructions for assaying. “De Re Metallica” was definitely the first and foremost textbook on fire assaying, and it served as a guide to miners and metallurgists for the following 180 years.

Although some publications on fire assaying and metallurgy appeared in the sixteenth and seventeenth centuries following Agricola’s “De Re Metallica,” it was not until the first half of the eighteenth century that creditable works (for instance, C. A. Schluter’s “Hutte-Werken, Braunshweig,” published in 1738, cited by Hoover and Hoover in their translation of Agricola, 1556) began to supersede Agricola’s book. Numerous publications on fire assaying, mining, and metallurgy followed in the 19th and 20th centuries. With continuing worldwide use and interest in the noble metals, one can confidently predict that fire-assay techniques will still be used in the future for the determination of the noble metals.

History of Fire Assaying in the US Geological Survey

In 1879, Clarence King, the first Director of the newly-formed U.S. Geological Survey, instructed Samuel F. Emmons, geologist-in-charge, to undertake a detailed study of the geology and mining industry of Leadville, Colo. (Emmons, 1886, p. VII -XI). In 1880 Emmons was fortunate in securing the services of Antony Guyard, a former pupil of the Ecole Des Mines, and, for 12 years, chemist at the well-known metallurgical works of Johnson & Matthey, London. He was assigned to make chemical investigations of the processes of lead smelting used by numerous smelters at Leadville and vicinity.

Thus, in Denver, in 1880 (Emmons, 1886, p. 621) Guyard was the first in the U.S. Geological Survey to use fire assay as an analytical technique. W. F. Hillebrand was also employed in 1880 and, with Guyard, started a chemical laboratory in Denver. Guyard died in 1884. As a result of a general reorganization, Hillebrand was transferred to Washington, D.C., in 1885 (Clarke and Hillebrand, 1897). By 1888 the Survey’s Denver laboratory was closed.

Although Hillebrand’s major interests lay elsewhere, he had become sufficiently familiar with fire assaying and had the equipment necessary to investigate, with E. T. Allen, the special steps needed for fire assaying of gold telluride ores (Hillebrand and Allen, 1905). The results of these investigations remain the basic study for gold telluride analysis. Their experiments, though extensive, suggested the need for additional work. However, the Washington quarters occupied by the Survey had become so crowded that the assay furnaces were dismantled, and assaying was abandoned altogether.

In 1948 there was a need to determine the gold and silver content of samples collected for uranium and thorium investigations, particularly in the Front Range of Colorado. Both the volume of this work and the security requirements of that time made it imperative for the Survey to do its own fire assaying. D. L. Skinner, who had joined the staff of the chemical laboratory in Denver, had had extensive fire-assaying experience at the U.S. Mint in Denver. He supervised the purchase and installation of equipment and in 1951, after an interval of nearly 50 years, revived the use of fire assaying in the U.S. Geological Survey. The furnace was christened by baking a pan of biscuits in it before it was contaminated by lead.

Again, in 1966, investigations for the heavy-metals programs — particularly those concerned with gold and the other noble metals — encouraged the expansion of existing analytical techniques and the search for new ones. Some of the results are given in this manual. One interesting and important result is the combining of fire assaying with such techniques as atomic absorption, optical emission spectrography, and neutron-activation analysis. Thus, in one sense, the very oldest analytical method has been updated by combining it with the most modern techniques.