After some practice, assays of copper ores cau be made by this method in a little over an hour, with great accuracy. It is well to have two assays of the same sample going on at the same time, and to take the mean of the results as to the correct one. Until the operator has had considerablo practice, he will find that hie results are generally too low, owing to a slight loss occurriog in the washing. In using the above method, the assayer will do well to remember the following rules:— To be careful, in attacking the ore, to use enough nitric acid to free the whole of the sulphur ; if it contain only a small quantity, very little will be needed, but in the case of copper or iron pyrites, which contain a large percentage of sulphur, considerably more must be employed. The contents to be completely dried in the flask, for the purpose of rendering the silica insoluble and of driving off every trace of nitric acid. In washing the filter, not to use more water than is necessary, as the copper is more easily precipitated from a concentrated solution than from a dilute one. The washing should cease as soon ns the ammonia test shows that all the copper has been washed down. To use only the best wire for precipitation, and to heat it to about the same heat as the solution before introducing it. As soon as all the copper is precipitated, to remove the wire, or an excess of iron salts may be formed which is very difficult to get rid of. It is unnecessary to remind the operator to avoid any loss of the copper in the washing processes ; the washings should be examined care fully before being rejected.
This method has beau proved by long-contiuued use to be reliable and accurate, and at the same time expeditious enough for all ordinary purposes ; it ie, therefore, deemed unnecessary to describe other and more elaborate processes.
Assay of Gold.—Gold is usually found in the native state enclosed in other minerals. A rough assay may be performed by pounding and washing the ore, and collecting and weighing the separated gold ; but such an assay would be very inaccurate, becanse a large proportion of the fine gold would 2 be washed away. The best method, in all cases, is to assay the ore by the process of smelting. As gold ores arc usually poor, it is useless to smelt a small quantity ; five ounces are little enough in any case for the production of a reliable assay. The ore is finely powdered and sifted, after which, at least an equal weight of litharge is added to it. In most cases, as when the ore is very siliceous or sulphurous, twice or three times its weight is required. The litharge must be pure, and is best obtained by drying acetate of lead at a strong heat, constantly stirring it, meanwhile, to prevent fusion. To the mixture of ore and litharge add a little black flux. Half an ounce of the latter will reduce metallic lead from the litharge in sufficient quantity to absorb all the gold contained in the five ounces of ore taken, which is the object of the process. If sulphides be present in the ore, a little nitre must be added ; but if there be none, carbonate of soda or borax may be used instead. The assay is performed in a crucible made of pure clay, or, better still,
in an iron pot. When the latter is used, the contents must be poured out into a mould while still hot, or if left till cold they will adhere firmly to the pot. They should be covered with a layer of common salt to facilitate smelting, and the separation of the gold. The melted lead thus obtained contains all the gold and silver derivable from the ore. The gold is obtained in the pure state by destroying the lead and any other metals which may be present by means of a process called cupellation, now to be described. The process is founded upon the feeble affinity which gold and silver have for oxygen. The oxides of lead, &c., formed during the process, melt and sink into the pores of the cupel, leaving the precious metals behind perfectly pure and nnoxidized. A cupel containing the lead to be refined is placed at the mouth of a muffle furnace, so as to receive the heat very slowly. When all moisture is expelled, the cupel is pushed farther into the muffle, and a stronger heat applied. The heat is now gradually increased, but if the lead becomes too lively, and particularly if there is a considerable evaporation, it must be reduced, or the vapours of the lead will carry away particles of the gold and silver. The best beat is that which just keeps the lead in the liquid state and allows the oxide formed to sink into the cupel and not form a cold, black ring around the hot lead. When nearly the whole of the oxide has been absorbed, the heat is increased to bright whiteness in order to melt the gold and silver into a round globule. When all the lead has disappeared, and the cupel contains only a bright, shining, metallic globule, it is removed from the muffle and cooled. When cool, the globule is detached, cleansed from dirt, and weighed ; the yield from an assay of this description is always a little too small. Gold and silver are almost invariably fouud alloyed together in all assays made immediately from the ore, and the separation of these two from one another, after cupellation, is an operation of much importance. To effect this in the dry way, the alloy should be mixed and melted with three times its weight of crude sulphide of antimony. The melted mass is run into a mould and cooled, and the button obtained is melted in a fresh crucible and treated as before. If, after the pot has been cooled and broken, the button is found to be brittle and hard, it should be subjected to the same process again, with the addition of a little saltpetre and common salt. If the button of gold still remains impure, more antimony must be melted with it, and the refining process repeated until a perfectly pure metal is obtained. When this is the case, the button represents the amount of gold contained in the five ounces of ore taken, and the difference between this weight and the weight of metal received from the cupel represents the amount of silver.