This section is from the book "Leaching Gold and Silver Ores. The Plattner And Kiss Processes: A Practical Treatise", by Charles Howard Aaron. Also available from Amazon: Leaching Gold And Silver Ores.
108. Collecting The Gold. Unless when working very rich ore, or for special reasons, the gold is not removed from the tub until several precipitations have been made. When a clean-up is desired, the liquid is drawn off as closely as may be done without loss of gold as described; the remainder, with some of the gold, is allowed to flow through the faucet into a small tub. The greater part of the gold remains on the bottom of the precipitating tub, and is taken up with a scoop, and carried in a suitable vessel, such as an enamelled kettle, to the filters. The bottom of the gold tub is washed by means of a small stream of water from a hose, and a whisk broom. The washings are added to the drainings in the small tub, and are either allowed to settle again, and again drawn off, or taken at once to the filters.
109. Washing The Gold. When drained in the filters, the gold, if nearly clean, which is known by its having a brown color, not too dark, may be washed at once on the filter, with hot dilute sulphuric acid and salt, or dilute hydrochloric acid, and afterwards with hot water alone; if very impure, it is transferred to a large porcelain dish, and boiled on a sand bath with slightly diluted acid. After adding water, it is again thrown on the filter, drained, and washed with hot water until the latter comes through tasteless. The filter bags, to which some gold adheres, are kept under water in a suitable vessel, to prevent rotting, until wanted again. When worn out, they are lightly sprinkled with powdered nitre, dried in an iron dish, and touched with a live coal. They then burn like touchpaper, and the ashes are added to the precipitated gold. Too much nitre must not be put on the bags, or the combustion will be too violent, and gold may be lost.
110. Drying The Gold. The washed gold is pressed by hand in the filter bags, transferred to an iron dish, and almost, but not completely, dried. While drying it is mixed with a little powdered borax and nitre. If several ounces of nitre are used, on account of impurities in the gold, it is advisable to add about half as much clean quartz sand, to protect the crucible from the action of the nitre during the subsequent melting of the metal. The purpose of allowing the pulverulent metal to retain a little moisture is, the prevention of loss by dusting while putting it into the melting pot.
111. Melting The Gold. If the gold is very clean, it may be melted in a black lead pot, but I have always preferred a sand pot, on account of the tendency of plumbago to cause contamination of the gold by reduction of the base metal compounds which may be present. There is, however, some slight risk of the sand pot breaking, nothwithstanding the precaution of adding sand with the nitre, and well drying the pot before putting it into a moderate fire. To guard against being obliged to wash the gold out of the ashes in case of breakage, with some risk of loss if the bottom of the ash-pit is rough, a black lead dish, made of the lower part of a crucible, is placed under the sand pot.
112. The fuel used for melting is charcoal or coke. English coke is so much better than that from the gas works as to compensate for the extra cost. A common portable assay furnace is very suitable for the melting of moderate quantities of gold, and a sand pot about ten inches high is large enough for several thousand dollars worth, although a larger crucible is needed than for metal which is in a more compact form.
113. When the crucible is red hot, it is taken from the furnace and gently tapped, to ascertain that it is sound, then filled with the gold, by means of a scoop, replaced in the furnace, covered, and gradually brought to a strong red heat. As the partly melted gold settles, the pot is refilled, without removal from the furnace, by means of the scoop, and, if required, a sheet iron funnel. It is important to refill the pot before the contents are quite fused, and while they are in a pasty condition; otherwise some of the gold may be thrown out of the crucible. The handling and melting of the gold are greatly facilitated if the precipitated metal is pressed into cakes, and heated to redness in a muffle, as is done in the mint with the finely divided gold from the refining. When the entire charge is in, a white heat is maintained until the gold and slag are thoroughly melted. An addition of more borax may be required if the slag is not sufficiently fluid. The mass is stirred with a red-hot iron rod, which must not be kept in too long, or it will make the gold base. Some metallurgists use a strip of black lead for the purpose.
114. When the slag is quite liquid, it is skimmed off the melted gold, by means of a piece of nail rod, which is turned at one end to a flat spiral, and bent to a suitable angle for convenient use. This is dipped, cold, into the melted slag, quickly withdrawn, and pressed upon a cold block of greased iron, so as to flatten the adhering slag, then just dipped into cold water, and again into the slag, repeating the routine until the latter is all removed, and the melted gold is seen in the pot.