Ohinemuri Regional History Journal 14, October 1970
By LES MORGAN
One of the professions practised in Waihi that is forgotten, now that mining has ceased, is that of the Assayer, the man who tested the ore for the value of gold and silver it contained, and also the refuse from the various stages of the treatment, so that a check was kept on the extraction.
The method employed at the mines was known as the fire assaying method. After the samples to be assayed, the methods of obtaining which I will explain later, reached the assay office they were all treated in the same manner.
It was necessary to have the material to be assayed in a very fine condition, something like the consistency of flour, and for it to be thoroughly dry. The sample was then thoroughly mixed and quartered and one quarter further mixed and thus reduced to a very small quantity, from which about 10 grammes was weighed. This was then mixed with certain reagents, Soda, Borax and oxide of lead or lithage [litharge –E]. The mixture was then placed in an earthen crucible and this placed in a very hot furnace. Early furnaces were heated by coke and the crucibles packed into a bed of coke. Later compressed air and kerosene blast was used, and I understand that electric furnaces have also been used.
The intense heat melted the quartz and the action of the reagents freed the minerals. The lead was freed by the heat from the litharge, and as it sank to the bottom of the mixture it also collected all the minerals. When the sample was cooked, a condition the assayer was able to determine by its appearance, it was poured into an iron mould and the lead and minerals again collected together at the bottom. The piece of lead, known as the button, was then detached and cleaned by tapping with a. hammer and was ready for the next step which was cupelling.
This was done in another furnace which had an oven heated to a high degree in it. Here again the heating agents were coke, air and oil, or electricity. The cupel was about the size and shape of an egg cup, and was made from a sand like substance called Morganite. This substance had the property of absorbing all minerals with the exception of gold and silver. The lead button was placed in the cupel and then placed in the oven. After a period the lead and base metals were absorbed by the cupel and the small bead containing the gold and silver from the sample left on top.
Different metals stained the cupel different colours, and when there was a combination of copper, iron, nickel and lead some striking effects were seen.
The small gold and silver (bullion) bead was then removed and weighed. The scales used in the weighing were very sensitive and were kept in a glass case. As absolute dryness was necessary a jar of moisture absorbing crystals was always kept in the case. Alteration to the hair like weights which were used was done by means of an arm which protruded through and was worked from the outside of the case. The scales were so sensitive that a speck of dust on one pan would put them out, and it is said that a postage stamp was weighed and then marked with a pencil and weighed again, and the weight of the pencil mark thus obtained.
The bullion bead is weighed on these scales and then it is beaten flat and boiled in nitric acid. This acid will dissolve all the silver and leave the gold. The gold is then weighed. The difference in that weight and the bullion bead gives the silver content, and from these two weights the amount of gold and silver and the value per ton is calculated. Extreme care had to be exercised throughout this process, for it must be realised that some hundreds of tons of quartz were treated each week.
Samples of quartz were obtained in various ways. If a drive was following a reef, at intervals pieces of quartz were taken from all over the face and all crushed together to get a fair sample. In the stope, sometimes samples were taken from the roof, but there were also boxes at the hoppers and samples from the trucks which came from below were put into them. I understand that in later years a diamond drill which took out cores from the reefs was used to sample them. These samples enabled a record of the value of reefs at any locality to be kept.
There was also a continuous sample taken of the quartz as it entered the treatment plant or battery, and with this a check is kept of the value of the battery heads, as they are known.
At various stages during the treatment waste material was washed away and samples of this were continuously taken and assayed, so as to make sure that there were no mistakes in the various stages of the treatment. Waste solutions were sampled by a tube attached to a wheel set in the launder through which they ran, a. measured quantity of this solution was then dried out in a shallow dish, usually on a litharge base, and the residue treated as a dry sample. As some of those waste samples returned a value as low as 9 pence per ton, it will be seen how exacting this work was.
When the gold and. silver was finally refined and melted into bars, ready for shipment, it then had to he assayed to prove its purity. To do this a small hole was bored right through the bar and the borings were then assayed for their purity.
It will be realised that a great deal depended on the accuracy of the result obtained by the assayer in this work, as it was on the value shown by his work that decisions were made as to whether or not a reef was payable and the extraction rate in the battery was satisfactory. It was remarkable how close the actual return of gold and silver at the end of each four weekly refinery period was to that shown by the assay value of the number of tons treated through the battery.
It was necessary for an assayer to take a course at a School of Mines in order to qualify, and in addition to the ordinary Certificate for Fire Assaying there was also another known as a Bullion Assayer's Certificate, which was issued by the Customs Department.
I realise that the method I have endeavoured to describe, and which I used at the Waihi Grand Junction, may be replaced by some more modern technique, but from the slides shown at the Waihi Historical Society evening a short time ago it does not appear so. The sight of the crucible being lifted from the furnace and the cupel oven brought back to me rather vivid memories.