We get lots of questions about the purity of placer gold and how gold is weighed. Figured we post up some info. Purity is ALWAYS debated and varies GREATLY not only on your region of the world, but also the source from where and when the gold was formed. Gold size will often impact the purity of gold, but that’s not a 100% rule. MOST times finer gold will be more pure, but that is not a FIRM rule.
Regardless of SIZE…the AVERAGE prospector and miner finds placer gold that is in the 75% – 92% purity range.
(That’s the simple answer.)
Atomic Symbol: AU
Atomic Number: 79
Atomic Weight: 196.967
Specific Gravity: 19.300
Melting Point 1,945 degrees F
Boiling Point: 5,371 degrees F
MOH’s Scale of Hardness: 2.5
Units of measurement and purity
The most common solid gold hallmarks are the art stamps; 8k, 9k, 10k, 14k, 18k, 21k, 22k. Other hallmark stamps include; 333, 375, 417, 583, 585, 750, 875, 916, 917, 999. These all represent the permillage gold or “fineness” in the alloy of the item, 333 being 33.3% or 8/24ths gold (8k), 417 being 41.7% or 10/24ths gold (10k) etc.
One troy ounce [ozt] is exactly 31.1034768 grams
One pennyweight [dwt] is approximately 1.555174 grams
(20 pennyweights in an ounce)
One grain [gr] is exactly 0.06479891 grams
(There are 15.4324 grains in a gram)
One carat [ct] is exactly 0.2 grams. The carat is divisible into one hundred points of two milligrams each.
One troy pound [troy] is exactly 12 troy ounces [ozt], 5,760 grains [gr], or 373.2417216 grams [g]
One troy ounce [ozt] is exactly 20 pennyweights [dwt], or 480 grains [gr], or 31.1034768 grams [g]
One pennyweight [dwt] is exactly 24 grains [gr], or 1.55517384 grams [g]
Purity of Gold
Purity can be roughly assessed by the nugget color, the richer and deeper the orange-yellow the higher the gold content. Nuggets are also referred to by their fineness, for example “865 fine” means the nugget is 865 parts per thousand in gold. The common impurities are silver and copper. Nuggets high in silver content constitute the alloy electrum.
10 karat gold is 40% pure. (Some gold is alloyed with copper.)
14 karat gold is 58% pure and 42% alloy, generally silver.
18 karat gold is 75% pure.
24 karat gold is 100% pure and is attainable only through processing and refining.
Natural placer gold, “generally speaking” … 80-92% pure, is 21-23 karat
Purity testing from Alaska University
(Excerpt from the report)
Gold fineness values for Alaskan placer deposits were calculated using mint return production records and the following formula for gold fineness: Fineness = (Au/(Au + Ag)) x 1000. Past gold and silver production records from individuals and mining companies for the period 1900-1974 from 800 creeks in Alaska were examined and 550 creeks with production in excess of LOO troy ounces of gold were selected for data analysis.
The data are summarized according to 41 mining districts and six regions. The overall mean fineness for the 550 samples is 889, the standard deviation of the mean is 28.57, the 95% confidence interval for the mean is 880-898. The mean gold fineness values for the six regions studied are: Fineness No. of Districts Seward Peninsula 908 9 Upper Yukon-Tanana 884 11 Chandalar-Koyukuk 898 2 Lower Yukon-Kuskokwim 880 9 Copper-Susitna 886 8 Southeastern 893 3
The values for individual placers range from 567-995. One way analysis of variance among the six regions and the 41 districts shows that the regions and districts can’t be distinguished on the basis of gold fineness. The Kantishna district is anomalous and has the lowest mean value of 789, the lowest individual sample value of 567 and a coefficient of variation of 16 versus the average coefficient of variation of 4.33. The Rampart and Council districts have a mean fineness of 915, the highest mean values of the districts. Several districts, Kantishna in particular, have bimodal distributions of fineness, suggesting different sources of gold or different processes affecting deposition. We were unable to relate the gold deposits to particular host rocks or to discern clearly the relationship of intrusive rocks to the placer deposits.
The expression of the relative quantities of gold and silver in a mineral deposit can be described by two methods; first as the simple ratio of gold to silver and second as the “true fineness” which is the ratio of gold to gold plus silver multiplied by 1000 (Boyle, 1979, p. 197). Note that “fineness” as generally defined is the parts gold per thousand parts alloy, which could include base metals, However, as used in this paper, “fineness” is the ratio of gold to gold plus silver times 1000. Sample calculation of gold silver ratio and true fineness, Au gold Ag silver Gold to silver ratio = Au/Ag True fineness = (Au/(Au+Ag)) x 1000 EXAMPLE Production record Gold Creek Au 975 troy ounces Ag = 105 troy ounces Gold to silver ratio = Au/Ag = 975/105 = 9.29 True fineness = (Au/(Au+Ag)) x 1000 = (975/(975+105)) x 1000 = 903
The advantage of using the fineness value rather than a simple ratio in statistical reduction of data has been reviewed by Koch and Link (1971). Although the use of fineness values is preferable in data reduction and analysis much of the literature deals with the simple Au/Ag ratio. In this discussion both expressions will be utilized since much of the referenced literature deals with Au/Ag ratios. Boyle (1979) has extensively reviewed the literature on the Au/Ag ratios of the various types of gold deposits and a summary of his conclusions are listed in Appendix I.
The major conclusion bearing on this paper is that gold placers always have Au/Ag ratios greater than 1, In this investigation production records from approximately 800 creeks in Alaska were reviewed. These production records were primarily mint returns reported by individuals and mining companies up through 1974.
These records are not inclusive of all the production from Alaska nor do they include all of the production from a given creek. The records report the number of troy ounces of gold and silver produced and the last date of recorded production. Fineness values were calculated for each creek with a record of at least ten troy ounces of silver. By this method a sample size of about one hundred ounces was insured thus increasing the reliability of a given sample. Fineness values for 550 creeks were determined and are listed in this report.
These fineness values represent past production records and should not be used as a basis for determining the fineness of current production on a given creek. Previous Investigations on Fineness of Gold From Alaskan Placer Smith (1941) discussed in detail the fineness of gold from Alaskan placer deposits.
The data base included 1534 samples from 84 different creeks or areas in 41 separate mining districts. The analysis of the data only included a determination of the ranges in values and the percentage of values within selected ranges. The values range from a low of 565 to 970 fine. Twenty-three percent of the records had fineness above 900; forty-two percent were between 850 and 899; twenty-six percent were between 800 and 849; and nine percent were below 800 fine. Smith (1941) did not attempt to interpret the significance of the data nor did he attempt to present data from every creek in a given mining district.
More about GOLD….
Gold is a chemical element with symbol Au (from Latin: aurum) and atomic number 79. In its purest form, it is a bright, slightly reddish yellow, dense, soft, malleable, and ductile metal. Chemically, gold is a transition metal and a group 11 element. It is one of the least reactive chemical elements and is solid under standard conditions. Gold often occurs in free elemental (native) form, as nuggets or grains, in rocks, in veins, and in alluvial deposits. It occurs in a solid solution series with the native element silver (as electrum) and also naturally alloyed with copper and palladium. Less commonly, it occurs in minerals as gold compounds, often with tellurium (gold tellurides).
Gold’s atomic number of 79 makes it one of the higher numbered, naturally occurring elements. It is thought to have been produced in supernova nucleosynthesis, from the collision of neutron stars, and to have been present in the dust from which the Solar System formed. Because the Earth was molten when it was formed, almost all of the gold present in the early Earth probably sank into the planetary core. Therefore, most of the gold that is present today in the Earth’s crust and mantle is thought to have been delivered to Earth later, by asteroid impacts during the Late Heavy Bombardment, about 4 billion years ago.
Gold is resistant to most acids, though it does dissolve in aqua regia, a mixture of nitric acid and hydrochloric acid, which forms a soluble tetrachloroaurate anion. Gold is insoluble in nitric acid, which dissolves silver and base metals, a property that has long been used to refine gold and to confirm the presence of gold in metallic objects, giving rise to the term acid test. Gold also dissolves in alkaline solutions of cyanide, which are used in mining and electroplating. Gold dissolves in mercury, forming amalgam alloys, but this is not a chemical reaction.