Weighing:

Weighing of the resulting precious metal beads produced from fire assaying is a very critical step. .

We are focusing on a one ton assay charge (29.167 grams of ore) From this amount of ore we should have after cupelling, a small bead containing the precious metals. These are the metals that were collected in the lead button. After the lead has been absorbed by the cupel cup, there may or may not be a small bead containing the precious metals. Base metals such as copper and zinc have joined the lead absorbed into the cupel cup. Only a small percentage of fire assays produce any measurable results. If the bead weighs less than one tenth milligram, it might take a magnifying glass to discover it lodged in the bottom of the cupel cup. In a small mine such results would probably be uneconomical to mine.

The use of "money" created in very ancient time instead of bartering, consisted of coins made of valuable metals, bronze, silver, and gold. Kings and rulers not trusting the purity of such coins employed assayers to protect the coffers. The science of fire assaying was developed as early as 1300 BC.

Being able to measure the small beads presented by a fire assay, was a challenge to the assayer with the result, many mechanical methods were invented to weigh these small beads. One of the more important methods was the use of a pivotal balance with two equal size pans hanging on either side of a beam. The beam was then balanced at a single point on a stand which allowed the beam to freely move in the direction of the heaviest weight in a pan balanced against a known weight in the other pan.

It wasn't until late in the 1900's that modern electronics allowed the creating of electronic balances capable of repeatable accuracy. The mechanical balances used up to that time, with a capable assayer, could weigh to an accuracy of one hundredths of a milligram. Only the more expensive electronic balances of today can duplicate this accuracy. Most small assay laboratories use an electronic balance accurate only to one tenth of a milligram, a speck so small it is hard to see without magnification. Values lower than this lend themselves to large companies that can process thousands of tons of low grade ore each day.

The electronic age gave rise to many other methods of assaying via chemical, atomic, and spectrometry, all of which cost thousands of dollars to purchase, and without the accuracy of the common fire assay. The advantage is a complete analyses of all metals and the short time it takes to do the assay. Fire assays take a couple of hours, and to save on costs, many are done at one time.