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Assay Balances

Assay balances were used for one of two purposes: Either the determination of the content of a precious metal (usually gold) in an alloy, thus establishing the fineness of the latter (usually in carats or per cent). Or the determination of metal content (usually in per cent) in an ore. Both these processes come under the name of assaying, and the balances were mostly found in assay offices, national banks, metal processors and mining laboratories. They normally have a very small maximum load (commonly 5 grams) and high resolution (commonly 5 ... 1 micrograms). For this reason the more modern assay balances are closely related to microbalances. In terms of balance design they are fairly basic, often lacking damping mechanisms and optical projection, achieving their performance purely by careful design and very fine manufacture and adjustment. The shape of their pans and suspension wires is typical for this application and not seen outside this field.

Model MC9 microbalance. Further development led to the MC9. Performance and resolution as per MC8 but without the use of a rider. This was the culmination of mechanical microbalance design, in the early 1960's. At that time new analytical methods did no longer require balances of such high capacity, and in the following decade electronic balances arrived on the scene.

The beam of the MC9.

A model HP (high-precision) balance with long-range optical projection, note prism on top of case. Not strictly a microbalance, but achieving the highest possible repeatability (micrograms or better) at a capacity of a few grams. Used for weight calibrations, i.e. a mass comparator. Has many features in common with microbalances, hence included here.

The inner workings of the HP model.

Model HP41 high-precision balance. Much the same as the HP above, but a somewhat later design.

The inner workings of the HP41.

Model SA1 assay balance, 5 g capacity, 0.005 mg resolution, weight loaded to 0.1 g, additional rider. Stanton assay balances are relatively rare. They are based on the MC series, using the same beam and case.

A much older long-beam assay balance (late 19th century) is shown in the Oertling slideshow

The beam, pans and suspension of the SA1.