Without being loaded with detail, it should contain sufficient to characterise each sample.
Key to following example page of Assay book: DR = DATE REPORTED.
Not Det. = Not detected
EXAMPLE OF PAGE OF ASSAY BOOK.
-------------------------------------------+----+-------+---------------+---- DESCRIPTION OF SAMPLE. Water Assay on ------+--------------------+--------------- Lost at the Dry Date. Material. Weight. No. 100 C. Material. DR ------+--------------------+---+---+---+---+----+-------+---------------+---- 1889 ton cwt qrs lbs Feb. 1 Tough cake copper 482 Arsenic, 0.52% 7 " Tough cake copper 2082 Arsenic, 0.63% 7 " Tough cake copper 491 Arsenic, 0.68% 7 Feb. 2 Nickel disc for C.R. X Copper, 73.75 7 Nickel, 24.34 Iron, 2.18 ----- 100.27 ------ " Silver precipitate, 24 1 0 73 Not 4 casks det. Silver, 4.851 10 Gold, 0.0215 Lead, 19.37 Zinc, 2.00 Silver, 1584.7 ozs. per ton Gold, 7.0 ozs. per ton " Purple ore 200 494 Not Copper, 0.13% 11 det. Sulphur 0.15% ------+--------------------+---+---+---+---+----+-------+---------------+----
When the number of samples is small, the Sample Book may be omitted, and the entries made in the Assay Book as the samples arrive.
_Report-forms._ These should entail as little writing as possible in making out the report. For general purposes the form given on p. 12 is useful.
~The quantity of substance~ to be taken for any particular assay depends largely upon the method of assay adopted. There are, however, some general considerations which should be remembered, and some devices for simplifying the calculations which should be discussed.
The smaller the percentage of the substance to be determined, the larger should be the amount of the ore taken. The following table will give a general idea as to this:--
Percentage of the substance Amount of ore, &c. to to be determined. be weighed.
100-10 1 gram.
10-5 2 grams.
5-1 5 "
1-0.1 10 "
0.1-0.01 20 "
[Illustration: ASSAY NOTE]
The rougher the method of assay adopted, the larger should be the quantity of ore taken. If the degree of accuracy attainable with the methods and instruments at the assayer's service is known, it is easy to calculate what quantity should be taken for any particular case. If the results are good within 0.001 gram, then, taking 1 gram of ore we can report within 0.1 per cent., or if they are good within 0.0002 gram, taking 20 grams of ore, we can report within 1 part per 100,000, or very closely within 6-1/2 dwt. to the ton. If it is wished to be yet more particular in reporting, larger quantities must be taken. The difficulty of manipulating very small or very large precipitates, &c., must be borne in mind. So, too, must the fact that the greater the weight of the final product of an assay, the less, as a rule, is the percentage error.
The distinction between absolute and percentage error, often overlooked, is important. If 0.5 gram of silver be cupelled with 20 grams of lead, there may be obtained a button of 0.495 gram; the absolute loss is 0.005 gram, and this equals 1 per cent. of the silver present. Similarly, cupelling 0.1 gram, the resulting button may be 0.098; the absolute loss is only 0.002 gram, but this equals 2 per cent. of the silver present.
In the same way the student should see that the two results, 91.5 per cent. and 92.0 per cent., are really more concordant than the results 9.1 per cent. and 9.2 per cent.
A device often adopted in practice where a large number of assays of one kind are made, and the report is given as so many ounces or pounds to the ton, is that known as the _assay ton_. The assay ton may be any arbitrary and convenient weight, but its subdivisions must bear to it the same relations as pounds and ounces bear to the actual ton. On the other hand, in a laboratory where many kinds of work are performed, different sets of weights of this kind would only tend to confusion, even if they were not unnecessary. With a set of gram weights and its subdivisions anything may be done. If it is desired to report as pounds to the ton, then, since there are 2240 lbs. to the ton, a weight of 2.240 grams may be taken as the assay ton, and each 0.001 gram yielded will equal 1 lb., or 22.4 grams may represent the ton, and each 0.01 gram a pound. Similarly, since there are 32,666.6 ozs. troy to the ton; if we take 32.6667 grams as the assay ton, each 0.001 gram will equal 1 oz. to the ton. In some cases it may be convenient to have, in addition to the usual gram weights, one or other of the "assay tons" mentioned above, but generally it is better to work on a purely decimal system, and convert when required into ounces per ton, &c., either by actual calculation or by reference to a set of tables.
PRACTICAL EXERCISES.
The student should practise such calculations as the following:--
1. Calculate the percentages in the following cases:-- (a) Ore taken, 2 grams; copper found, 0.2155.
(b) " 1.5 gram; iron found, 0.8340.
(c) " 30 grams; lead found, 23.2.
2. Calculate the parts per thousand in the following:-- (a) Bullion taken, 1.1 gram; silver found, 1.017.
(b) " 1.14 gram; silver found, 1.026.
(c) " 0.6 gram; gold found, 0.5500.
3. Calculate parts per 100,000 in the following:-- (a) Ore taken, 20 grams; silver found, 0.0075.
(b) " 50 grams; gold found, 0.0026.
(c) Water taken, 500 c.c.; solids found, 0.1205.
4. Calculate cwts. to the ton in the following:-- (a) Ore taken, 5 grams; tin found, 2.816.
(b) " 5 grams; tin found, 3.128.
(c) An ore with 68.2 per cent. of tin.
5. Calculate lbs. to the ton in the following:-- (a) An ore with 3.28 per cent. oxide of tin.
(b) Ore taken, 20 grams; oxide of tin found, 1.67.
6. Calculate ozs. (troy) to the ton in the following:-- (a) Ore taken, 50 grams; gold found, 0.0035.
(b) " 20 grams; silver found, 0.0287.
(c) " 25 grains; silver found, 0.0164.
7. Calculate in grains per gallon:-- (a) 0.51 gram per litre.
(b) 24.6 parts per 100,000.
(c) Solution taken, 100 c.c.; copper found, 0.0045 gram.
(c) " 50 c.c.; iron found, 0.165 gram.
8. Convert into ozs. (troy) per ton:-- (a) 7 loths per centner.
(b) 30 grams per quintal.
(c) 15 parts per 100,000.
FOOTNOTES:
[1] Ether or carbon bisulphide.
[2] Such substances are best dried by pressing between folds of dry filter-paper.
CHAPTER II.
METHODS OF ASSAYING.--DRY GRAVIMETRIC METHODS.
The methods of assaying are best classed under two heads, Gravimetric and Volumetric, in the former of which the final results are weighed, whilst in the latter they are measured. A commoner and older division is expressed in the terms much used in practice--wet assays and dry assays.
Wet assays include all those in which solvents, &c. (liquid at the ordinary temperature), are mainly used; and dry assays, those in which solid re-agents are almost exclusively employed. Dry assays form a branch of gravimetric work, and we shall include under this head all those assays requiring the help of a wind furnace. Wet assays, as generally understood, would include not only those which we class as wet gravimetric assays, but also all the volumetric processes.
~Gravimetric Methods~ aim at the separation of the substance from the other matters present in the ore, so that it may be weighed; and, therefore, they must yield the _whole_ of the substance in a pure state.
It is not necessary that a metal should be weighed as metal; it may be weighed in the form of a compound of definite and well known composition. For example, one part by weight of silver chloride contains (and, if pure, always contains) 0.7527 part of silver; and a quantity of this metal can be as exactly determined by weighing it as chloride as by weighing it in the metallic state. But in either case the metal or its chloride must be pure.
Exact purity and complete separation are not easily obtained; and methods are used which are defective in one or both of these respects.
It is well to note that an impure product increases the result, whilst a loss of the substance decreases it; so that if both defects exist in a process they tend to neutralise each other. Of dry methods generally, it may be said that they neither give the whole of the substance nor give it pure; so that they are only calculated to show the amount of metal that can be extracted on a manufacturing scale, and not the actual quantity of it present. Their determinations are generally rough and always low. The gold and silver determinations, however, will compare very favourably with any of the other processes for the estimation of these metals in their ores.
The calculation of the results of a gravimetric assay has already been referred to. If the result is to be stated as percentage, it may always be done by the following rule:--_Multiply the weight of the substance got by the percentage of metal it contains, and divide by the weight of ore taken._
Gravimetric methods are divided into three groups: (1) mechanical separations; (2) dry methods; and (3) wet methods.
~Mechanical Separations.~--Under this head are classed the method of assaying tin ores, known as vanning, and the amalgamation assay for gold. A set of sieves to determine the relative proportion of powders of different degrees of fineness is sometimes useful. A set with 10, 20, 40 and 80 meshes to the inch is convenient.
~Dry Assays.~--An important distinction between wet and dry methods of assaying is, that in the former the substance is got into the liquid state by solution, whilst in the latter fusion is taken advantage of.