Assay Ton :

In the United States, assay offices commonly use for the unit of measure, Troy ounces of precious metals per ton of sample.

There are 2000 pounds avoirdupois per ton. This type of ton is called a short ton. There are 12 ounces in a troy pound and 31.1034768 grams in one troy ounce

If the amount of ore used for a fire assay is a one assay ton sample, then the weight of the sample in grams is 29.1666667. This is rounded in some offices to 30 ounces and others to a more exacting 29.17 grams per crucible charge. Accuracy beyond 1/100 of a gram is not needed.

The resulting precious metal bead after fluxing, and cupelling to remove the lead, copper and other impurities, is weighed on an analytical balance with a accuracy to tenths or hundreds of milligrams. Each milligram being equal to one ounce troy of precious metal per ton of sample. Scales capable of weighing to hundredths of milligrams are quite expensive and many offices turn to other methods to determine low amounts of precious metals below .1 milligram (.0001=1/10,000) gram.

Common readings on low grade ore are in hundredths of a milligram i.e.; .015 milligrams which equals .015 troy ounce of precious value per ton of ore to be milled. With gold at 470.00 per ounce, this amounts to $23.50 worth of precious metal per ton of ore processed. This normally would not begin to recover the cost of a mining operation.

The cutoff point for smaller underground mines happens when the cost of mining and milling exceeds the value of the precious metals recovered. For instance the supervisor of the now closed Goldroad mine told me that their cutoff point was .16 ounces per ton, and that with a mine producing dore bars that consisted of 60% gold and 40% silver. Some of the tunnels at Goldroad were sixteen feet which is the reason there are huge dumps of waste rock with very little value.

An average lode mine with a production output of 50 to 100 tons per day, may have a cutoff point of .25 troy ounces per ton while an open pit mining operation producing thousands of tons of ore per day may be as low as .032151 troy ounces of precious metal or even less (1 gram). This given that the precious values are primarily gold.

All of the above is relative to the current price of gold or silver. When gold remained under $300 dollars per ounce, many mines stopped production as the cost of mining exceeded the recovery.

Also entering into the equation is the ratio of gold to silver. With gold being worth approximately 60 times the price of silver, a high silver ratio can quickly reduce the chance of recovering the costs incurred in the mining operation. i.e. Gold $470.00 per ounce, Silver $7.25 per ounce = 470/7.25 = 64.8 the ratio of silver to gold.

In Canada and Mexico and most of the rest of the world, the metric system holds court with results stated in kilograms, grams, and milligrams. The term troy ounce is little used.

Below are two definitions each in it’s own way explaining what an assay ton is.

Assay Ton:
A specialized unit of mass used by mineralogists in assaying (testing) ores for the presence of gold, silver, platinum, uranium, or other valuable metals. One assay ton equals 29.1667 grams. The assay ton is actually a sample size. Since there are 87500/3 = 29166.7 ounces in a short ton (2000 lbs), the number of milligrams of a precious metal in a sample of one assay ton is numerically equal to the number of troy ounces of that metal in one ton of raw ore. In Britain, the assay ton is based on the long ton (2200 lbs), and thus equals 98/3 = 32.6667 grams.

Assay Ton:
In the United States, a unit of mass, approximately 29.167 grams. The number of milligrams of precious metal in one assay ton of the ore being tested is equal to the number of troy ounces of pure precious metal in one 2000-pound ton of the ore. i.e. a bead is obtained that weights 3 milligrams, thus the precious metals in the bead would equal three troy ounces to each ton of ore with the understanding that this varies considerably in the real world as the amount of precious values in each ton of ore varies considerably.