Mineral Assay & Testing Laboratory Services

Mineral Assay & Testing Laboratory Services

After the completion, of the Hammond Mineral Assay & Testing Laboratory of the Sheffield Scientific School, Yale University, it became necessary to secure and assay a large assortment of ore-samples, and to arrange the results in such a manner as to admit of use with the least amount of work on the part of the instructor.

At first, pulp-samples, together with their respective assays, were secured from the mines, smelters, and public assayers, but this practice did not prove satisfactory. It was not only difficult to secure a sufficiently large supply of material, but it was also impossible to secure the proper assortment required for instructing students. It involved considerable work for the donors to select the samples and furnish a record of the assays, and also for the instructors to examine each pulp and determine its character. Some of the samples did not contain a sufficient quantity of pulp for the student to check his work in case his first assay did not correspond to the record.

In order to increase the number of samples and to vary the character of the material, pulp-samples were later mixed with barren rock and with sulphides, but this method also was unsatisfactory and has been discontinued. mineral_assay_and_testingAt present, small lots of ore are secured by gift or purchase from engineers, public assayers, and the managers of mines, mills, and smelters. These lots weigh from 10 to 100 lb. each, and are rarely finer than 0.25 in. in size. The larger samples permit of a careful study by the instructor in order to determine the best method of assaying to be followed.

In universities in which assaying is taught the record of the samples should fulfill the following requirements:

  1. Ease of record, avoiding all unnecessary duplication of work, and, so far as practicable, the possibility of error from duplicating numbers or in recording the assay.
  2. Piling of assay-records properly, especially if the card containing the record has been taken from the file for comparison.
  3. Facility in securing the character of sample desired.
  4. No duplicating of numbers for the same sample.
  5. The number of the last sample recorded should, be easy of reference.
  6. The number on the sample-sack should not in any way indicate to the student the character of the pulp contained.
  7. Finding promptly the assay-record of a sample by having only the number on the sack.
  8. The total number of different samples of any character should always be known, so as to replenish the stock when necessary.

The Dewey decimal system of record was adopted so as to indicate:

  1. whether the sample is an ore or a furnace-product;
  2. the character of the gangue;
  3. for what metals the sample is to be assayed; and
  4. the method of assaying to be used by the student.

This system has been in use at the Hammond Mineral Assay & Testing Laboratory for four years, and with a few slight changes has proved entirely satisfactory.

The records of the assays are entered on cards, 5 by 8 in. in size, indexed as shown in Fig. 1. Numbers in the left-hand column on the key are placed to the left of the decimal point on the assay-cards and indicate whether the sample is an ore or a furnace-product, and also for what metals the sample is to be assayed. Thus, 23 indicates gold and silver; 13, lead and silver; 83, a scale-ore of silver, such as ores from Cobalt; 4123, a lead-bullion containing gold and silver; and 913, a special furnace-product containing lead and silver.

The second set of numbers are placed to the right of the decimal point and indicate the character of the gangue and the method to be employed in assaying; thus, .13 indicates the gangue to be a basic oxide and may be either limestone or an oxide of one of the base metals; .14, the ore has a siliceous gangue which requires a basic charge with the addition of reducing flux; .5, the ore is a sulphide, 1 g. of which will reduce more than 2.5 g. of lead, and should be run by the crucible-method with the addition of nails; .6, the ore is a sulphide, 1 g. of which will reduce between 1.8 and 2.5 g. of lead, and can be run by the crucible-method with the addition of niter; .7, the scorification-assay will give higher results, otherwise it is an impure ore giving a matte or speiss by the crucible-method. The decimals .57 and .67 indicate that the gold is to be determined by a crucible charge, using the nail- or niter-method respectively, and the silver by scorification.key to classification of assays

The index- and assay-cards are contained in ordinary filing- cases. One file contains the records of the common ores and another the records of the furnace-products, and ores and products requiring special treatment. The first index-card to the left, containing only the number to the left of the decimal, is of one color, and the remainder of the index-cards, indicating both the content and the character of the sample, are of another color. Cards showing the same character of sample and method of assaying are placed in the same relative position for each set of ores, an arrangement which greatly assists in the finding of a desired sample.mineral_assay_&_testing_laboratory__sample_preparation_&_tracking

The assay-card, shown in Fig. 2, contains a complete record of the sample, entered from the instructor’s assay-certificate, shown in Fig. 3. The series number in the upper left-hand corner of the assay-card, 23.67, indicates that the ore contains gold and silver, the gold to be determined by the crucible- method with the addition of niter, and the silver by the scorification-method.

record of assay

The number in the lower right-hand corner indicates that the card is the sixth in position under the index- number, 23.67. At the top of the card are entered also the results of the fire assay, the estimated percentage of base metals determined by vanning, any note of special interest under “ character,” and the names of the assayers.

The cards are ruled to contain 8 samples, but in case there are 16 duplicate samples these are usually recorded on the same card. The first samples recorded on each line are assayed by different instructors. The names of the students are entered under the number of the sample assigned to them, and when the samples are returned the series-number and students’ assays are entered on the pulp-sack and later checked on the assay-card as having been returned. If the sack returned contains too little pulp to be of value, it is destroyed and the number on the assay-card crossed out. If the returned sack contains sufficient pulp to be of value the number is checked and the sample filed. When all of the samples recorded on one card have been given out and are also returned, the pulps are re-mixed and divided into new samples, and new numbers are given to each one, the record being entered on a new card. One sample of each lot of mixed pulps is assayed by an instructor so as to avoid the possibility of error arising from a student placing the pulp in a wrong sack.

Duplicate samples are given out under different numbers, and rarely does any one class of students receive more than two samples from the same card. The samples, as mixed, are numbered consecutively, regardless of the contents. Card 6, series 23.67 (Fig. 2), records pulps 1492 to 1507; the following card, 7, of the same series, may record pulps 6001 to 6016. Pulp 1507 is in series 23.67 and the following number, 1508, may be in series 1.5. By this method the number on the sack does not indicate to the student the character of the sample or the method to be employed in assaying.

In case it is desirable to find the assay of any special number, the series of which is not recorded on the sack, reference is made to the location-card, Fig. 4, which contains the numbers of the samples and the series to which they belong.

For instruction-work it is desirable to have as large an assortment of samples as possible with different assays. At present there are recorded and ready for use in the Hammond laboratory about 7,000 samples. The annual consumption amounts to about 2,000 samples, and the new ones added to the list each year amount to about 3,000, which gives a gain of about 1,000 samples per year. It is hoped in the near future to have on record 25,000 samples, which should afford a sufficiently large assortment of assays requiring different methods of treatment. In order to obtain this variety, products are used from the testing of ores, from screen-tests of 25-lb. lots, from mixing 5-lb. lots of comparatively coarse gold-ore with different quantities of pyrite, and silver-ore with different amounts of galena. The samples thus obtained are pulverized in jar-mills and passed through an 80-mesh screen. For 16 gold-pulp samples, weighing about 125 g. or 4 A. T. each, 2,000 g. of pulp is taken, thoroughly mixed on a rubber sheet, and riffled into two lots, marked A and B. These lots are separately mixed and riffled into eight samples each, mixing thoroughly between each riffling, and placed in numbered pulp-sample bags.


The numbers of lot A are entered on the top line and the numbers of lot B on the lower line of the assay-record cards. The cards are then filed under No. 11, to be assayed, and the samples placed in stock. The first recorded samples of each lot assayed by different instructors must check within a commercial limit before the record is accepted. In case the assays do not check, which is rarely the case, the samples are re-mixed and re-assayed.

The weight of the pulp in the sacks varies slightly, depending on the character of ore. Ores requiring charges of 0.5 A. T. weigh about 120 g.; low-grade ores requiring charges of 1 A. T. about 220 g.; and scorification-ores about 100 g. Each sack contains sufficient ore for preliminary vanning and testing, and for two sets of assays if necessary. The student reports his results, and in case his results do not check, he does not consume unnecessary time in re-assaying without personal instruction.

Assay Certificate

On the assay-certificate, Fig. 3, used by the instructor in this work, the number of the sample is entered on the lower left- hand corner, followed by the date of the assay and the name of the assayer. The “ character of the gangue ” is either checked before pulverizing or determined by vanning-tests, preferably the latter, as the student determines the character of the sample by this method. The preliminary assay is made, if necessary, and the oxidizing- or reducing-power entered on the certificate. The weight of the charge used and the notes taken on the furnace-work are next entered under the heading “Assay and Method.”

The above information gives the series number, which is next entered, thus completing the certificate for file and entry on the assay-card. These certificates are kept on file for future reference in case an assay is disputed.

In laying out a term’s work for a class it is necessary to select a large number of different samples having the same general character and method of assaying. The entire assortment of samples of gold-ore requiring a basic charge will be found under 2.14, 23.14, and 123.14, the number .14 remaining in the same relative position in the file. In like manner gold-ores to be run by the niter method will be found under 2.6, 23.6, 23.67, 123.6, and 123.67.

Although at first sight the above system may appear to be complicated and to call for a large amount of work, in practice it has worked out very satisfactorily. The system has been evolved during the past four years, until at present there is but one entry from the assay-certificate to be made on the record-card. Two duplicate assays, as a rule, suffice for recording the assays of 16 samples.

Assay Certificate
Assay Certificate