Table of Contents
For years past I have traveled in quest of promising mining- properties, over almost impassable mountain-trails to remote places in the mining-regions, usually many miles from an assay-office.
If, upon examination, the formation and the ore-deposit appeared favorable, a quick determination of the value of my samples was necessary, in order that I might take intelligent action with reference to the acquisition of the property. A journey with my samples to civilized regions often meant weeks of delay, and perhaps loss of opportunity. How many, under similar circumstances, have returned with their assay-results from such a journey, only to find that some audacious speculator had “optioned” the property! True, a first payment could have been made and the chances taken on the results; but experience has taught that rich-looking ore is not of necessity valuable. Personally, I would not hazard a ten- dollar bill on any property without first determining by assay the value of the ore.
To carry the usual complete assay-paraphernalia over such mountain-trails was impracticable by reason of the loss of time involved, the great expense of transportation (charged according to weight), and the fact that a few such trips would almost certainly wreck the outfit.
For these reasons I began a process of elimination and substitution which finally enabled me to carry, in an ordinary 26-in. valise, sufficient apparatus and supplies for, say, 100 assays of adequate commercial accuracy. My final outfit was as follows:
Equipment
Small hammer; steel mortar (2.5 in. wide, 3 in. high, with stand sawed off flat, so that it can be used as an anvil and for slag-molds); small steel pestle; blow-pipe and blow-pipe cupel-holder; small spatula; large spatula; button-pliers; light 1-in. cupel-mold; two 3.5- by 8-in. muffles; 18 “F” crucibles and 6 crucible-covers; short crucible-tongs; 25-cc. burette; six 0.5-oz. parting-bottles; six 0.5-oz. Berlin annealing-cups ; two or three beakers; hand-balance, such as is used for weighing gold-dust (folding in tin box), and set of gram-weights; a 6-in., 60-mesh screen; 6 small glass stirring-rods; teaspoon and tablespoon of usual camp-style; and a micrometer for measuring the diameters of buttons. (This may be a short, fixed microscope-stand, in which has been placed a scale of 0.1 millimeters—two or three extra object-glasses being carried for safety. If one is willing to be burdened with the extra weight and bulk, a small portable button-scale and weights may be substituted. The micrometer, however, is sufficiently accurate, and possesses the advantage that if one’s mule should roll down the mountain it will not be injured; whereas a scale may be hopelessly ruined for further immediate field-use.)
Chemical Reagents, and Supplies
(To be carried in bottles enclosed in metal cases): 0.5 lb. of potassium cyanide; 1 lb. of C. P. nitric acid; 1 lb. of C. P. hydrochloric acid, and 2 lb. of aqua ammonia.
A few ounces of sheet-copper; 1 oz. of C. P. sheet-silver; 0.5 lb. of sheet-lead; 2 lb. of ten-penny wire nails; 5 lb. of bone-ash; 5 lb. of sodium bicarbonate; 5 lb. of litharge; 3 lb. of red lead; a few pieces of prepared charcoal for blow-pipe use; half a dozen miners’ candles and half a dozen old newspapers.
Practicable Substitutes in the Field
Sodium bicarbonate can be obtained at most country stores. If the supply of litharge be exhausted, red lead (used in the proportion of about 3 assay-tons) makes a fair substitute. For a long trip I increase my supply of bone-ash and litharge. Under extreme conditions I have been compelled to get bone-ash by calcining and pulverizing old bones, and to substitute powdered beer-bottles for borax, filings from coins (of known fineness) for silver, and flattened bullets for sheet-lead. If, as is not unlikely, the supply of beakers should have been destroyed en route, beer-bottles with the tops broken off constitute a tolerable substitute—and, I need hardly say, can be obtained in abundance at all mining-camps!
The outfit above described can be packed in an ordinary valise, say, 26 in. long. To illustrate its use in the field, I present the statement which follows.
Gold and Silver Assay
The ore-sample is broken to very small size, quartered, and passed through the screen. The final assay-sample (0.5 A-T., unless the ore be very poor, but up to 2 A-T., if necessary) is weighed in the gold hand-balance. To a sample of 0.5 A-T., add one even teaspoonful of litharge. If the ore is very heavy with sulphides, 5 even teaspoonfuls of red lead and 10 nails may be substituted. To this should be further added a “ heaping ” tablespoonful of sodium bicarbonate, and another of borax, for which, if the ore is very basic, ordinary powdered glass may be substituted, or also added. The final addition of 0.5 g. of wheat-flour or powdered charcoal will give, after melting, a lead-button of from 7 to 8 g. weight, unless the ore contains a large proportion of heavy sulphides. The sample should be thoroughly mixed, wrapped in newspaper, and properly numbered for future identification. If the ore be believed not to carry sufficient silver for successful subsequent parting, a small measured portion of C. P. silver may be added.
For crucible-melting, a blacksmith’s forge is available in every mining-camp. In using such an apparatus for this purpose, a small inclosure of brick or loose stone should be placed around the tuyere of the forge, built up nearly even with the tops of the crucibles, and a layer of coal or coke should be put directly over the tuyere on which the crucibles stand. The spaces between the crucibles and the walls should be filled with fuel. The crucible and covers (or, in case of muffle-roasting, the muffle) should be thoroughly dried and warmed beforehand, to prevent cracking. The charge having been put into the crucible, with a layer of borax on top, the crucible- cover put on, and the whole covered with charcoal, it is only necessary to take care not to make too hot a fire (thereby melting off the bottom of the crucible) and to turn the crucible from time to time, in order to secure an evenly-distributed heating. During the roasting-process, the crucible-cover should be removed.
If the fluxing was correct, the melt will pour well, and the crucible can be returned to its place in the fire, and the next charge, paper and all, put into the hot crucible. There need be no fear that the crucible will “ salt ” the subsequent assays.
With care one can make from 6 to 18 assays in each crucible. Fuel must always be kept between the bottom of the crucible and the tuyere-iron. Unless the samples are very base, from 15 to 30 min. will be sufficient, after the first melting, to melt each lot of four.
When all the samples are melted and ready for cupellation, remove the stone or brick around the tuyere, and place on each side of it a brick (or a flat stone of about the same size), the two being about 6 in. apart. On these place the muffle, forming a bridge over the tuyere, with the mouth of the muffle facing the front of the forge. In using stones for this purpose care must be taken to select those that will not spall under the heat and break the muffle. Set the muffle level and build a wall around it, excepting a front space about 3 in. away from the muffle, and to about the height of its top. Cover with charcoal, and keep plenty of fuel always under the muffle on the tuyere. Put in the empty cupels and close the mouth of the muffle with a piece of charcoal or coke whittled to fit it. After the muffle has become hot put in the buttons.
In the absence of charcoal or other suitable fuel, I have made charcoal in an open fire or fire-place by a liberal use of water to cool it or by covering the charred fuel with dry dirt.
The above method of fire-assay requires less fuel than any charcoal-furnace I have ever used; and I have often made, in this crude way, 40 assays in a day. The buttons come out clean and bright; and the cupellation requires but a few moments longer than in a regular cupeling-furnace.
Sometimes the muffle breaks, and must be set up on pieces of iron running lengthwise, and plastered with clay to hold it together. When a muffle is entirely gone beyond such patching, and no other is to be had, a new one may be drilled out of a brick. But I have often made 20 runs with one muffle.
Having cupeled all the buttons, one should find some place out of the wind and, if one has the scale, part and weigh the gold in the usual manner. If using the micrometer, measure the diameter of the button and determine its weight by a previously-prepared scale based on specific gravities, multiplied by solid contents. This method of determining weights was published by Luther Wagoner, of San Francisco, some ten years ago. After parting, the gold is dried, wrapped in a small piece of sheet-lead and cupeled with blow-pipe and miners’ candle on a little bone-ash pressed into a cupel-holder. The button being perfectly round, the weight is determined by measuring the diameter, as described. The cupel-holder is made from a piece of volcanic tufa cut down to a cylinder in form, about 1.5 in. in diameter, and 2 in. long. The top is hollowed out like a bowl about 0.5 in. deep; and a small hole, 0.5 in. long and about 0.25 in. in diameter, is cut in the bottom. Into this hole fits a piece of wood, which the assayer can hold in his left hand, so as to turn the cupel-holder with his thumb and forefinger, and make a perfect button. The tufa having been heated in a fire to a red, nearly white heat, and then dropped into water, comes out as hard as a rock.
A few years ago, I forgot, on one trip, to take with me the pulp-scale and weights; nor did I discover the omission till I was ready to weigh the pulp, in a locality more than 100 miles away from the nearest town. I melted the tops from two “ Carnation ” cream-cans, and punched three equi-distant holes through the rim of each top and put in strings of equal length, thus making pans for a scale. Then I put a common dressing-pin through the middle of a thin strip of pine, and another through each end, at equal distances from the first; balanced the stick; hung my pans to it; and having balanced these, was ready to weigh the pulp. I had a worn half-dollar silver piece, which, with a new ten-cent piece, would exactly balance a new half dollar, weighing 205 grains. Then, by adding a 5-cent silver coin, I had 230.625 grains, near enough for my purpose to 0.5 assay-ton.
If one doubts the existence of any commercial value in the ore, one may carefully weigh out 0.5 g., run it down on a piece of charcoal, and measure the button with the micrometer, even though it be so small that the natural eye cannot see it. In this way, the trouble of making a regular assay may often be avoided.
Copper and Lead Assays
For volumetric copper-assays, it is necessary to standardize a solution at each camp; since it is impossible to transport the solution without standardizing it when next used. Therefore it is best to throw away the old solution when packing, and make a new one when required. Often I make rough fire- assays of copper, to determine whether it is worthwhile to investigate the copper-value more thoroughly. This can be done in connection with melts for gold and silver.
Lead can be determined in the same manner. An excess of carbon should always be used in the charge for lead. It is advisable to keep a duplicate sample of each assay, and have it checked on the return to civilized regions. After one has become familiar with the above plan of assaying, however, the testing by duplicate assays of such preliminary determinations of probable value will soon be discontinued.
