Which weighs more, a pound of feathers or a pound of gold? A pound of feathers is heavier since feathers are weighed by avoirdupois weight while gold is weighed in troy (or apothecary) weight and 1 lb. avoirdupois = 1.21528 lb. troy. The precious metals, such as gold, silver, platinum and palladium, are usually quoted in troy ounces while most of our measurements (in the U.S.) are in the avoirdupois system.
In the troy system, the grain is the same as in the avoirdupois system and the pound contains 12 ounces, 240 pennyweights, or 5,760 grains. In the avoirdupois system, the pound contains 16 ounces or 7,000 grains. Note that while an avoirdupois pound weighs more than a troy pound, a troy ounce weighs more than an avoirdupois ounce. See the conversions below.
The following list gives some of the common conversion values. From the CRC Handbook of Chemistry and Physics, 61st Edition. The following abbreviations are used: av. (avoirdupois), t. (troy), dwt (pennyweight), oz. (ounce) and lb. (pound).
How to Mine and Prospect for Placer Gold, Bureau of Mines publication, Information Circular 8517, by J.M. West gives the following definitions of gold sizes. For information about mesh sizes, see our Mining Glossary.
For a more complete list of conversions see Mike Higbee's Conversion Table (formerly Tom Ashworth's site). For an interesting brief history of these measurement systems see WEIGHTS :- A Measure of Your Success.
Gold Chart is a printable troy weight conversion chart used by the club.
The following table shows the official U.S. price of gold from 1786 to 2001. This is helpful when reading old reports which give dollar amounts for how much gold was found but not weights. Many times an average amount found per miner is given in dollars and cents which is not meaningful unless one knows the prices at that time. The following data is summarized from What Was the Price of Gold Then? by Lawrence H. Officer, Economic History Resources, EH.Net, 2002. The following only gives the official U.S. prices and not the free market prices since this is only for historical information and until recent years, the prices quoted in documents were usually the official price. The link noted above also gives free market prices as well as background information.
| Years | U.S. Official Price (dollars per fine ounce Troy) (end of year) |
|---|---|
| 1786-1791 | $19.490961 |
| 1792-1833 | $19.393939 |
| 1834-1836 | $20.689655 |
| 1837-1933 | $20.671835 |
| 1934-1971 | $35.000000 |
| 1972 | $38.000000 |
| 1973-2001 | $42.222222 |
Note that the free market price was essentially the same as the U.S. Official price until 18 March 1968 when a two-tier market was established. One for governments selling gold to each other and the other a free market for gold. Effective 31 December 1974, the prohibition of private citizens to hold gold was repealed.
As can be seen in the chart below, the high specific gravity (relative density) of gold as compared to other minerals with which it may be found forms the basis for all gravity methods of separating the gold, including panning, sluicing, dry washing, gold wheels, etc. Specific gravity is a relative density, the ratio between the weight of a substance and the weight of an equal volume of water at 4° C. Below is a list of many common minerals, including those most commonly found associated with gold, along with their specific gravity and hardness. Notice that gold has one of the highest specific gravities and many common minerals that may be found in the field are much lower. As can be seen in the table below, minerals which might be confused with gold such as pyrite, chalcopyrite and mica also have much lower specific gravity. Mica, which forms in sheets, will try to float out of the gold pan. Mercury is listed because the old timers used mercury in separating the gold, losing some in the streams as a consequence. The mercury will be amalgamated with gold and should be kept (being very careful as mercury is toxic).
Another distinguishing property of gold is its malleability. Gold will flatten when struck while pyrite and chalcopyrite, both being brittle and harder than gold, will shatter. As a guide for hardness, the fingernail is a little over 2, a copper penny (not the newer zinc pennies) about 3, a steel knife just over 5, glass about 5.5 and a good steel file about 6.5.
The following table is based on data from the "Manual of Mineralogy", 19th Edition by Cornelius S. Hurlbut, Jr. and Cornelis Klein, John Wiley & Sons, Inc. 1977.
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Gold in the pure state has a specific gravity of 19.3 but the presence of other metals results in the range of values above. Most gold contains about 10% of other metals, silver being the most common, resulting in a specific gravity of slightly over 18. For example one source gives an average of 18.6 for the quartz gold of California. Another source states that gold from the Weaver placers in Arizona has a specific gravity of 18.53.
Lead is included above to show that it has a fairly high specific gravity, especially as compared to the minerals found in black sand and other common minerals. This is why lead shot, used in hunting, or lead fishing split shot is used in gold panning practice. Being very soft it can also be easily flattened or pounded into various shapes to mimic small nuggets. However due to the toxic problems with lead, it will probably end up being banned for hunting and fishing.
From BLM Technical Bulletin 4, Placer Examination Principles and Practice, the heavy minerals found in black sands consist of the following minerals in order of frequency: magnetite, ilmenite, garnet, zircon, hematite, chromite, epidote, olivine, limonite, rutile, pyroxene, monazite and the Platinum group metals (Platinum, Palladium, Iridium, Osmium and Rhodium). Of course this will vary by area. How to Mine and Prospect for Placer Gold, Bureau of Mines publication, Information Circular 8517, by J.M. West gives a slightly different list of black sand minerals which includes magnetite (magnetic iron oxide), ilmenite (iron-titanium oxide), hematite (nonmagnetic iron oxide), marcasite (an iron sulfide), rutile (titanium oxide), scheelite (calcium tungstate), wolframite (iron, manganese tungstate), tourmaline (boron and aluminum silicate), zircon (zirconium silicate), chromite (iron and chromium oxides), and cinnabar (mercury sulfide).
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This page was last updated on 6 Febrary 2008.