Placer Examination - Principles and Practice
Technical Bulletin 4 Bureau of Land Management 1969
Table of Contents
GENERAL NOTES ON PANNING WITH SUGGESTIONS FOR IMPROVING TECHNIQUE
New pan - preparation for use: The film of grease or other rust preventative found on a new pan must be removed before use. This is best done by passing the pan over a gas stove burner or another suitable flame until the metal turns blue. Although this process is sometimes called "burning", care should be taken to avoid excessive heat. Blueing a pan not only frees it of grease but equally important, the resulting dark color. makes fine specks of gold much easier to see in the pan. The "burning" process should be repeated as often as necessary to keep the pan free of body oil films that accumulate on a pan in normal use.
Pan factor: Gold pans are made in a variety of sizes but the size generally referred to as "standard" has a diameter of 16 inches at the top, 10 inches at the bottom and a depth of 2 1/2 inches. A typical pan will hold 336 cubic inches or 0.0072 cubic yards. The number of pans representing a cubic yard of material in place (bank-measure) is called the pan factor. Pan factors vary according to the size and shape of the pan, the swell of the ground when excavated, and the amount of heaping when filling the pan. In practice, factors for a 16-inch pan range from 150 to 200 but an approximate figure of 180 is often used. This is based on a struck pan (i.e., level full) and an assumed 20 to 25 percent gravel swell.
Recommended pan size: The average panner should not use a 16-inch pan but instead should use the so-called "half-size" pan which has a top diameter of 12 inches, a bottom diameter of 7 1/2 inches and a depth of 2 inches. The half-size pan level-full weighs approximately 9 pounds compared to about 20 pounds for the standard 16-inch pan. Halving the sample weight by use of the smaller pan not only reduces fatigue, particularly when much panning is to be done but the fact that it is much easier to carry in the field and to use in a small stream or tub is an important consideration. The pan factor for a 12 x 7 1/2 x 2-inch pan is about 400, assuming a 20 to 25 percent gravel swell. Experience has shown that two half-size pans can usually be washed in less time than one full-size pan.
Use of perforated pan: Panning, at best, is a tedious, back-breaking job and anything done to speed the operation or improve working conditions will be repaid many times over in the form of more reliable results. The beginner and experienced panner alike can profit by use of a sieve (sometimes referred to as a "grizzly" pan) made by drilling 1/4-inch holes in the bottom of a pan of the same size and shape as the one used for panning. To use the sieve, place it inside of the regular pan and then fill with gravel and submerge in water in the usual way. When the material is thoroughly wetted, lift the sieve slightly and twist it back and forth (under water) until all minus 1/4-inch material has passed into the regular pan. The plus 1/4-inch-material is discarded and the fines which dropped into the regular pan are washed in the usual way. Aside from speeding the overall panning operation, the use of a sieve enables the engineer to conveniently inspect the plus 1/4-inch rocks and to estimate the proportion of coarse material.
Use of safety pan: Allowing the pan tailings to fall into a second pan generally referred to as a "safety" pan will guard against losing the sample by accident and will greatly expedite repanning where this is called for.
Panning large samples: When a large multi-pan sample is to be washed, rather than complete each successive pan, it is best to reduce them only to a rough concentrate. The rough concentrates are accumulated and are eventually combined for finishing in the usual manner.
Stage panning: Where a large amount of heavy black sand is encountered, a stage-panning procedure can be used to advantage. This is done by panning and repanning to successive high-grade concentrates without attempting to make a complete saving of black sand or values at any one stage. As the proportion of heavy minerals decreases with each successive repan, it becomes progressively easier to make a high-grade concentrate with a low-grade tailing. Usually, two or three repannings will make an acceptably clean tailing.
Supplemental data: When panning a sample the experienced engineer will note a variety of things among which are: Its amenability to washing, particularly where clay or cementing materials are present; the proportion of coarse to fine material; any evidence of unusual muddy water problems; the composition and angularity of rocks; the relative ease of concentration; the quantity and composition of black sand; indications of valuable or potentially valuable accessory minerals; the size, shape and other physical characteristics of the gold including "rust", tarnish or other factors which would affect its amalgamation. Any of the foregoing could he important factors in a placer mining operation.
Use as a geologic tool: Although the miners' pan is normally associated with gold deposits, it can be profitably employed when investigating a variety of heavy minerals such as monazite, scheelite, magnetite, ilmenite, cassiterite, chromite, cinnabar, etc. In general, it should be borne in mind that with few exceptions the pan can be employed in the study of either lode or detrital-type deposits containing finely divided minerals of relatively high specific gravity. The use of a miners' pan as a geologic tool has been studied and reported in detail by Mertie (1954) and by Theobald (1957).
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