Table of Contents
Analytical Balances
Buy a good button balance, for should the final weighing of the gold or silver recovered from an assay charge be inaccurate, all the previous work will have been wasted.
A good button balance can be purchased for $100.00 (Fig. 42); higher grades that are more sensitive and delicate of construction will cost proportionally more. It is desirable to have all the bearings of the button balance of agate or sapphire. By the bearings are meant the knife edges and all the parts called planes that make a contact with the knife edges.
Pulp scales sufficiently delicate may be bought at prices ranging from $5.00 to $50.00. It is advisable, however, for this purpose to have a balance sensitive to at least 1 milligram (Fig. 43).
Analytical balances (Fig. 44) range in price from $50.00 upward, and are intended to be accurately sensitive to a weight of from 0.1 to 0.05 milligram.
Any first-class pulp balance will be sufficiently accurate for all the purposes of the ordinary assayer, such as weighing the pulp used in making volumetric
assays of copper, etc., or any ordinary gravimetric determinations.
Keep the button balance in a clean dry place, and do not expose it to acid fumes. Set the button balance upon a support that does not vibrate, and always level the balance before using.
Do not handle the small weights except with the forceps (Fig. 45) supplied for that purpose. Remove and replace the scale pans with the forceps, and brush off any dust that may gather in the pans with a camel’s-hair brush (Fig. 46).
Weights
The French or metric decimal system of weights and measurements is eminently the best, and is in general use for the laboratory (see tables, page 128).
Assay Ton
The assay ton consisting of 29.166 grams is the unit of pulp measurement for fire assay (see table, page 128). The milligram, which we may term the unit weight of the assay balance, bears the same relation to the ounce troy that the assay ton (29.166) bears to the ton avoirdupois.
The assay ton weight was created so that when 1 assay ton of ore was taken, each milligram of pure gold as shown by the button balance would represent 1 ounce troy in gold to the ton avoirdupois of the sample assayed. This system simplifies the figuring of valuations, as each milligram of gold recovered from an assay ton of pulp represents 1 ounce troy per ton avoirdupois, or a value at present of $20.67 per ton.
Weighing
The assay is weighed by putting the bead into one pan of the button balance and counterpoising with the requisite number of milligram weights placed in the other pan. The bead should never be handled except with the forceps. It is placed upon the left pan of the balance, and the weights are then put in the right pan also with the forceps. The balance must always be arrested before putting anything in or removing anything from the pans; by arresting is meant that the pans must be rested upon their supports. Adjust the balance every time before using, as a small deviation will materially affect the result. Estimate the probable weight the bead will have and put the highest estimated weight in first, then the next lower and so on until the exact scale weight is obtained. This is the only way to preserve the balance from bumping, and time will be saved by being systematic. The weighing operation is then completed by sliding the rider along the beam until the pan is exactly counterpoised.
The weight is written decimally in milligrams and fractions thereof; the number of milligrams being shown in the weight pan, and the fractions of a milligram being shown by the rider.
If the beam has fifty graduations on each side of the central bearing of the beam, each graduation or division (provided a 1 milligram rider is used) represents the 1 /50 part of a milligram or decimally 0.02 of a milligram.