"Well, did we find any gold?" is the cry of prospectors down through the ages. We got answers about this from the July meeting's speaker, Carl Jensen, Assayer, and principal of Jensen Technologies, a local lab which performs precious metal assays. Carl has a degree in analytical chemistry from the Colorado School of Mines and much industryu experience. He was accompanied by assayer Lee Swink.
As mostly placerers, we are mainly concerned with "free gold" which is not bound up in other material. Free gold, if there is enough of it, can be seen, for example, in a gold pan. Most lode gold, however, and some placer gold, is not free gold and therefore cannot be seen. The definitive test for ALL the gold in a sample is the fire assay.
Talk about venerable, the fire assay has been around for thousands, yes thousands, of years. It is still essentially the same, although certain procedures, such as weighing and splitting, have gone high-tech. The fire assay will give amounts of all the noble (precious) metals including silver and, with further testing, the six platinum-group metals.
Carl took us through the procedure for performing the fire assay, but what is also most important is the sample prep work. The sample must be ground to a powder (called a "pulp") of less than 200 mesh. This is much finer than white flour used for baking. The sample, about 15 grams, is then fired with lead, which forms a button of lead and precious metal, if there is any. The button is then fired again in a ceramic "cupel" to remove the lead. The result is a bead from which the noble metals can be separated. The lower limit of detection for gold is about .01 ounce per ton.
Mr. Jensen also introduced us to the recent development of a field procedure called the "torch assay." This method employs a jeweler's acetylene torch, which is of course much more portable than a lab oven. While not as precise as the standard assay, there is an obvious benefit in time--about twenty minutes versus a few hours. And of course an immediate result. If you can learn to do this you will become very popular!
Unlike most labs, Jensen's does not have a minimum charge, so you can walk in with just one sample and not get "hung out to dry." The prices are reasonable.
Log on to Technologies LLC (technologiesllc.com) for more information.