CHAPTER III.

THE ASSAY LABORATORY: ITS FITTINGS, APPARATUS.

THE ASSAY LABORATORY.-For fire-work a room should be set aside, and for convenience it should be situated near the balance room and the

chemical laboratory. Gas and water must be laid on if possible, and provision must be made for lighting by day and night (if necessary).

The modern assay laboratory frequently errs on the side of over-equipment. The assayer many times in his after-career has to work without gas and many other luxuries found in large laboratories. It would be well, then, for the student to remember that a coke-' or 'coal '-fired muffle, besides being suitable for cupellation, can be readily adapted to the needs of the chemist. Incinerations, silicate fusions, etc. can with care be accurately performed in the muffle.

Again, a small blast kerosene stove, costing from ten to twenty shillings, can be surmounted by an iron plate or sand bath suitable for boiling, evaporation, etc.

The student may, without unduly interfering with his work, spend some time in exercising his wits on the construction of apparatus. Some of the finest chemical research has been carried through with home-made apparatus ; and he should remember that here, as in many other cases, more depends on the skill of the operator than on an elaborate equipment.

No attempt will be made to furnish detailed plans, but a few general points may be noted:

Floor; preferably of concrete. Roof; slate or iron (painted), and ventilated by a lantern.' Walls; preferably of brick. Benches and Lockers; of wood, and placed round the walls. Furnaces; in the centre of the laboratory. Fume cupboards, gas, water, grinding appliances, rolls, etc. etc., may be fitted where most convenient.

Besides the laboratory proper, a special room must be set aside for the finer balances. The tables on which these are placed must be of solid construction, and designed to reduce vibration to a minimum. Provision must also be made against dust and fumes from the assay and chemical laboratories.

FITTINGS, FURNACES.

Two types of furnaces are required for the efficient performance of the work of the fire-assayer; these are, melting or wind furnaces and muffle furMany varieties of design are on the market, much depending on the nature of the fuel used, which may be either gaseous, liquid, or solid.

naces.

Furnaces for Gaseous Fuel.-These furnaces are designed to consume ordinary illuminating gas, and are simply large and modified bunsen burners. They give good results if carefully handled and regularly cleaned, but can only be recommended where gas is cheap; and as the assayer can but rarely obtain gas at a low figure, the student should, at least at the outset, confine

his attention to some other form of furnace. Figures 73 and 74 represent two common forms of Gas Melting and Muffle Furnaces respectively.

Furnaces for Liquid Fuels.-During the past ten years this type of furnace has been brought to great perfection, and portable outfits for melting and cupellation can now be had at a reasonable figure. The outfit consists of three parts, the oil-reservoir and pump, the burner, and the furnace proper. In out of the way places, where coal or coke is unobtainable or very expen

FIG. 75.

sive, this outfit is very serviceable, as the 'oil' or 'gasoline' can easily be transported, the volume of oil required being much smaller than that of coal or coke for equal heating effect. Very many varieties of these furnaces are obtainable, some of them displaying much ingenuity. Figure 75 shows a combined Melting and Muffle Furnace.

If gasoline be used as fuel, special care must be taken in following out the directions given regarding the storage and use of this dangerous liquid.

The makers generally supply full instructions regarding the setting up and operation of the furnace.

Furnaces for Solid Fuels.-The fuel used may be coal, coke, or char coal; generally coke or coal. For the wind' furnace coke is the most suitable fuel, though for the 'muffle' furnace, coal has certain advantages over coke; the muffle may be 'underfed,' the fuel being fed underneath and not on the top, thus prolonging the life of the muffle; any freeburning coal may be used, one cwt. of coal running an 8" x 14" muffle for eight hours. With ordinary prices a saving in fuel as compared with coke results. The fire is quickly started, and the temperature is readily controlled.

The melting furnace may be constructed of red brick, with a lining of firebrick. It should be provided with movable fire bars, a cast iron top, and a damper to regulate the temperature. Figure 76 shows a convenient design.

The muffle furnace will vary in design according to the nature of the fuel used.

In

some of the later designs of

FIG. 76.

coal-fired muffle furnaces, two muffles are employed, the one above the other,

the upper being used for cupellation and the lower for scorification. Figure 77 shows a furnace designed for the use of coke, and figure 78 a design for a free-burning coal (preferably bituminous).

Fire-brick may be used in the construction of these furnaces, though red brick answers well if supported by angle iron and straps. A damper to regulate the temperature will be found very convenient, though not absolutely necessary.

Where proper furnaces were not available, assays have been conducted successfully in an ordinary kitchen range, or on a blacksmith's For such impromptu

forge, with the aid of a small flower-pot for a muffle.

appliances the wit and resources of the assayer must be depended on, and in the course of his work the student will have the opportunity of noting where he may replace, partially at least, the appliances of the laboratory by those of everyday use.

GRINDING APPLIANCES.

The mortar and pestle may be used for reducing an ore from the coarse state to a fine powder, but in a laboratory where any considerable amount of work is to be attempted, a division of labour must be adopted by providing appliances for coarse and appliances for fine reduction; by coarse' is meant particles of inch to inch in diameter; by 'fine' is meant particles passing through the meshes of a sieve with, say, 20 to 120 holes to the linear inch.

Appliances for Coarse Reduction.-For pieces of ore 9 to 12 inches. or more in diameter an iron plate or slab about 36′′ × 36′′ × 4′′ (cast iron); a 'spalling' ring about 2 feet in diameter and 4 inches deep and a heavy sledge or knapping hammer will be convenient. By this means the lump. may be reduced to pieces 2 to 3 inches in diameter. For further reduction some form of fine pulveriser must be employed.

If 'power' be available, a small rock-breaker driven from shafting by a belt and pulleys can be employed to advantage. Here, as in other reducing machinery, a point of vital importance is that the machine be so designed that it may be quickly and thoroughly cleaned. Many otherwise efficient machines are deficient in this respect, and cannot therefore be recommended. If 'power' be not available, the same type of machine can be obtained adapted for hand-power. These machines can readily be adjusted to prepare the ore for fine pulverisation on the bucking-plate, or by other means. Fig. 79 shows a convenient form of pestle and mortar.

Where expensive machinery is unobtainable the prospector's 'dolly' can, with a little ingenuity, be adapted to the needs of the assay office.

Apparatus for Fine Reduction-(a) Grinding Appliances; (b) Sieves.

For coarse reduction sieves may generally be dispensed with, but for fine reduction they are indispensable to the student. The experienced assayer can with care reduce a sample on the bucking-plate without the aid of a sieve, but to the beginner this practice cannot be recommended.

(a) Grinding Appliances.-The mortar and pestle have already been mentioned. The form shown in fig. 79 is convenient where much work has to be done. The short pestle, though not to be recommended, is frequently used. It may be safely stated that this grinding appliance is exceedingly laborious, and wherever possible should be replaced by the bucking-plate, which is a cleaner and much more efficient 'fine-grinder.'

The simplest form of bucking-plate consists of an iron casting 18′′ × 24′′ × 1′′, the upper face of which is planed smooth. On this face is worked the grinder of cast iron, about 4" x 6" on the upper face and 14" thick at the middle, from which, on both sides, it falls off in a curve of wide radius, the grinding face being planed true longitudinally. To the upper surface of the grinder is attached a handle, preferably curved like that of a hatchet.

No description of the operation of the grinder will be given; a few hours' practice will teach more than pages of writing. Fig. 80 shows the plate and grinder.

The form shown is simple, and if placed overlapping the edge of the table

or other support by about 2 inches the powdered ore is easily brushed off into a scoop or on a sheet of paper. The two longer edges of the plate are often provided with raised flanges to confine the ore. Some makers pivot the plate at the two sides so that it may be tilted for cleaning, but this seems to be an unnecessary refinement. A radical modification of the plate and grinder has recently been placed on the market by an American firm, and combines the grinder and sieve. The pressure between the grinder and plate is adjusted by simple spiral springs, whilst an ingenious design of the grinder provides a regular feed of ore. The principle of the machine seems goodthe feed being regular, the ore being discharged as soon as fine enough, and the pressure permitting of easy adjustment.

(b) Sieves. These are generally designated as 20, 30, 40 ... 100, etc., according to the number of meshes per linear inch, a 20 sieve having 20 holes per linear inch, that is 400 holes per square inch, an 80 sieve having 80 holes per linear inch, or 6400 holes per square inch, and so on. A convenient diameter of sieve is nine inches. It is important when selecting a sieve to choose one that can be readily inspected and cleaned. The most suitable material for the wires is brass; for the body, wood or iron. For general purposes the following sizes will be useful:20, 40, 60, 80, 100.

These sieves may be had in box sets provided with a lid and receiver, thus retaining the dust. The student will find that, when dealing with an ore which clogs the meshes, the operation of sifting is much hastened by placing in the sieve along with the ore two or three small copper coins; if copper be harmful, discs of some other metal may be employed. The student is reminded finally that he must be absolutely certain of the cleanliness of all reducing and sifting apparatus before proceeding to crush a sample of ore.

GENERAL FITTINGS, APPARATUS, ETC.

The following list of headings, though by no means complete, includes the most important of the smaller pieces of apparatus necessary in the assay laboratory.

Crucibles.-These are made of various materials, and are known as "Clay," Graphite," ," "French Clay," "Hessian," etc.

"Clay" Crucibles.-These are best suited for the assays of gold, silver, lead, and tin ores. In the selection of a crucible for a particular assay, the nature of the charge must be taken into consideration; for a basic charge a basic crucible should be chosen, and vice versa. The student will note that when fusing certain charges in these crucibles it is necessary that some