Process of Coining

Panel 1

From Mines, to Mills, to the Carson Mint

The United States Branch Mint established in Carson City, Nevada produced its first coin early in 1870. The mint was located here because the precious metals needed to make coins were close at hand. Miners on the Comstock Lode, and from other mining districts of the state, extracted the ore. Mills processed the ore, producing silver bullion bars and doré bullion, a mixture of silver, gold, and trace metals. Shipments were delivered by the Virginia & Truckee Railroad.

The Superintendent’s General Department

The Mint Superintendent oversaw all activities at the mint, but more closely managed the General Department which included Deposit Weighing, the Treasurer’s Office, and Deposit Melting. This department dealt with the initial receipt of metals, record keeping, and preparation of samples for testing. At the end of the process, the Treasurer’s Office took charge of the finished coins.

1. Deposit Weighing received the bullion, gold deposits, and retorted amalgam (the last is the same as doré bullion). These items were recorded and weighed on large bullion scales. (See scales in this room).

2. The Deposit Melting Room took materials brought from depositors (individuals or companies) and melted them. A small portion of this melt was ladled out and poured slowly into a vat of water to form granulations. The sudden chill produced by putting the hot liquid into cold water formed granules of equal fineness. The rest of the melt was poured into three-part molds to produce shoe-bars.

Shoe-molds at the Carson mint had two semipartitions which rose to a little more than half the thickness of the metal cast. This produced a long, thin block that could easily be broken into three pieces that would melt quickly in the Refinery.

Samples for the Assayer were either tiny chips removed from bars, granulations, or both.

The Assay Department

3. The Assayer tested ores, minerals, and bullion samples to determine their value and composition. Fire and humid assay methods were used. The analysis also determined the fineness (based on percentages of gold and silver contained in the bullion).

The depositor’s pay was based on the total weight of gold and silver contained in the shipment.

Panel 2

Melter & Refiner’s Department

4. After boiling the bullion in sulphuric acid for 16 hours in the Refinery, the gold separated from the mix and, being heavier, sank to the bottom of the vats. The acid, with dissolved silver, was siphoned off, and the remaining gold was collected. The gold was boiled again to further refine it.

Silver was removed from the acid by adding water and heating the liquid again, this time with copper slabs placed around the sides and bottom of a special vat. The silver oxidized and precipitated (separated from the liquid as a solid) onto the copper. Silver was then scraped from the copper.

5. In the Ingot Melting Room, a small amount of copper was added to give more strength to the future coins. These nearly pure metals were then cast into ingots of various sizes depending upon the type of coin to be made.

Coining Department Machinery

This work room in the coining department of a mint shows part of the extensive system of leather belts, metal shafts, and pulleys required to propel the various machines operated by a single steam engine.

6. Ingots were passed through two sets of rollers. The first set, which reduced the ingots roughly, was called the breaking down rolls. The second, further reduced the thickness.

7. In the Annealing Room, several ingot strips were fit into a copper tube, the cap was sealed with clay, and the tube was put into an annealing oven. The strips were heated for about 40 minutes and then gradually cooled in warm water in annealing vats. This process prevented the metal from being brittle.

8. The Cutting Room at the Carson Mint had one draw-bench and two cutting machines. The draw-bench was adjustable; it reduced the strips to the standard thickness required for different coins. One cutting machine could cut 280 blanks per minute.

Panel 3

9. Each coin blank was individually weighed in the Adjusting Room. Women generally handled this task at the Carson Mint, utilizing scales so sensitive that a slight breeze might throw off the result. Blanks that were too heavy would be filed and sent on to the coining room. Underweight blanks were remelted.

10. In the Whitening Room, the coin blanks were cleaned in a hot alkaline bath and rinsed with hot water. Then they were heated, immersed in a light acid bath (whitening), rinsed with cold water, and partially dried in basswood sawdust. Final drying was done in enclosed pans heated by steam.

The Coining Room

11. The blanks were loaded into cartridges on the milling machines. The Carson Mint had three machines for different sized denominations.
When the edge was thickened on the milling machine to form a lip, and the diameter was adjusted to fit the collar on the coin presses, the blank became known as a planchett.

Note: Women were often employed by the various mints to operate the milling machines and coin presses. In 1880, women were operating milling machines and presses in Carson City.

12. Coin press operators made sure the hoppers or tubes that fed the coin press were kept full of planchetts. There were three presses at the Carson Mint. One machine could produce around 100 silver dollars per minute.

Three dies created the images and texture on a coin: one for the face or head, called the obverse side, one for the tail called the reverse side, and a collar die. The collar die created the parallel lines called reeding on the edge of the coin.

Finished Coins

13. Counting boards were used by the Coiner and his assistants to rapidly and accurately count coins.

14. Finished coins were passed on to the Treasurer’s Office where they were bagged for shipment or storage in a vault.

Panel 4

Locations of Activities

“IN THE ATTIC are five water tanks, having an aggregate capacity of 2,500 gallons. One of these tanks contains hot water, three of them cold, and one is kept filled with ‘sweet water,’ or water purified from all salts by distillation. The attic is also used as a store room for crucibles, etc., and contains a number of hot-house plants of luxurious growth . . .”

– Nevada State Journal, December 1870

“IN THE BASEMENT the hydraulic press is located, and near it is a large cage for hoisting or lowering heavy material. In another room is a small force pump, run by steam, used in hoisting water to the tanks in the attic. Further on is a small rotary crusher to reduce bone to powder of which cupels are made, and near this is a large one used in pulverizing old melting pots, bricks, ashes, sweeps, etc., so as to recover every particle of the precious metals which may have adhered.”

– Nevada State Journal, December 1870

Notes on the west addition: A larger rear, two-story wing off the main building was completed in 1881. Refinery activities were extended into the 2nd story section at that time. The engine and boiler rooms continued to occupy the lower floor areas. This stone addition was deemed unsafe in 1971 and was removed.

From Mint, to Assay Office, to Museum

The Mint ceased operations in 1893. Coining Department machinery was removed in 1899 and sent to other mints. The building, however, continued as a United States Treasury Department assay office until 1933.

When it reopened as a museum in 1941, exhibits filled many of the rooms. As a result, the original six ton coin press, reacquired in 1958, was first placed in the former Deposit Weighing Room. It is now located in the former Treasurer’s Office.

The Steam Engine

The Steam Engine Room was located on the basement level and was also open to the first floor. This accommodated the engine’s 12-foot diameter fly wheel, as well as numerous counter-shafts leading from the room to operate machinery in both the basement and on the first floor.

The engine’s speed was 52-54 strokes per minute. It ranged from 45-75 horse-power, depending on the number of machines being driven.

Hospital

During World War II, the basement was set up as a 24-bed emergency hospital. The mine exhibit, which currently occupies much of the basement, was completed in 1950.