The status of precious,metals in today’s society becomes more significant, due to various technological, advances and the world’s monetary problems. The price structure and supply of these metals have consequently been drastically change in recent years. We find ourselves mining lower grade ores, and reclaiming scrap with a lower noble metal content. This has led to a critical assessment of the methods used to sample and analyze these materials. One from many method to analyse the precious metals in the rock, mineral processing final product slurry or any form of sample is fire assay.
The origins of fire assay
The origins of fire assay are invisible in antiquity. Some writers/researchers say fire assay start in Egypt in the 5th Millennium BC. Although the written records only date back to the 3rd Millennium BC. They found good descriptions of the process were recorded on Cappadocian Cuneiform tablets. Then, in excavations at Troy II, mid 3rd Millennium BC, cupels & cupel buttons were found. Even the Old Testament of the Bible has numerous references to assay. Numbers 31:22,23; Psalms, 12:7; Proverbs 17:3 and 26:23; Jeremiah, Zechariah, Malachi – all use aspects of fire assay in allegory to connote honesty and purity (Haffty, et. Al, 1977).
In the middle 1st Millennium BC, King Croesus ruled the Lydian Empire and recent excavations in the ruins of Sardis, Croesus’ capital city, have turned up evidence of the first “mint” that controlled the gold and silver content of coins regardless of size (thickness or diameter) – this requires sophisticated assay and smelting/alloy techniques! (Ramage, et al, 2000). Later, in Roman times, lead pipes notice by “Ex Arg” (no silver or without silver) so, again, fire assay comes into play.
Fire assay and alchemy
In the 8th or 9th Century AD philosophy and mysticism entered the picture in the form of Alchemy. Fire assay led to Alchemy which had as its guiding principles the search for a method to transmute base metals into precious metals, the search for the “Philosophers Stone” and the search for an “Aqua Vitae”. Most of the knowledge behind the alchemist’s research was claimed to have derived from “Ancient Manuscripts” written by historically unidentifiable “Ancient Philosophers”. The true departure from a scientific method and a move into the mystical realm took place in Europe where alchemy really took hold using the supposed 14th Century manuscripts of the legendary and probably mythical philosopher Hermes Trismegistos.
Fire assay in old literature
The first verifiable texts script by the 10th century Arabian physician Avicenna. Coupled with his medical writings, he produced manuscripts on astronomy, chemistry, geology, philosophy, physics, theology, among other subjects. Around 250 of his manuscripts still exist.
The literature states that cupellation already take place in 12th century England. In the 13th century, the parting start to use in France. Furthermore during the 16th century, the basic fire assay procedure was essentially the same as that in use today. Although the use of the fire assay is now largely use to the analysis of precious metals, it is also use in previous times for the assay of base metals, such as lead, bismuth, tin, and copper.
The first complete work on metallurgy, assay and mining was written by the 15th century German naturalist/ scientist Georgius Agricola. Many of the tools, equipment and techniques detailed in his “De Re Metallica” illustrate that little has changed in assay in 5,000 years. We have past US President Herbert C. Hoover and his wife, Lou H. Hoover to thank for the only translation into English of Agricola’s work. Furthermore, the work is profusely elaborate and I recognize my current tools in the woodcuts – cupel tongs, crucible and scorifier tongs, crucible shapes, cupel shapes, etc.
Fire assay as an Art
Fire assaying has always well known more of a backroom art than a science. The reason for this is partially due to the high degree of practical knowledge and manipulative skills needed to complete a successful fire assay. However, the theoretical chemistry pertaining to the fire assay has never been completely investigated. In fact this has left us with a process based upon some fundamental principles. The fundamental principles depends upon experience and observation alone, without due regard for theory. The fire assay remain as a prolific subject for basic research.
In his classical text De Re Metallica, written in 1556. Agricola offered this ageless advice on the fire assay:
“It is necessary that the assayer who is testing ore or metals should be prepared and instructed in all things necessary in assaying, and that he should close the doors of the room in which the assay furnace stands, lest anyone coming at an inopportune moment might disturb his thoughts when they are intent on work.”
Fire assay nowadays
Even though a carefully performed Fire Assay remains the most economical, accurate and precise method of determining the quantity of precious metals in both ores and alloys. The resultant bead from a cupellation remains the standard for calibration and control of instrumental methods such as Atomic Absorbtion (AA), ICP‐AES, Elemental Spectroscopy, X‐Ray Fluorescence, Li Metaborate Fusions with ICP‐AES finish. With proper care and control, fire assay will find gold at 1 part per 2.5 million, silver at 1 part per million and platinum group metals at 1 part per 0.5 million at a fraction of the cost of instrumental method.
source and further reading:
Fire Assay – What is it? by Don Saathoff
THE CLASSICAL FIRE ASSAY by M. A. McGuire, Newmont Exploration Limited Danbury, Connecticut