A Textbook of Assaying | Page 8

report-forms. 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