The silver content of the sample can be determined on a single channel X-Ray fluorimeter or other device, and if found to be less than 0.30-0.36%, the bullion is considered refined. The chlorine injection is terminated, the pipes removed and the residual chloride layer bailed off. The metal surface in the crucible is finally cleaned by thickening the last traces of chloride with bone ash and skimming the surface clear. After heating the refined bullion to 1,100 oC, the furnace is tilted and the bullion poured into a sided cubic transfer ladle, lined with a crucible. This ladle is transported across the refining room to a holding furnace (e.g. 1,000 kg capacity) into which the molten bullion is transferred. The holding furnaces are similar in construction to the refining furnaces but are fitted with a larger unlined crucible. These furnaces can be rated at 200-250 kW and are equipped with a hydraulic tilting system. Some equipments have two coil voltages, and increasing the temperature of the molten metal and 1,200 V for melting ingots or bars. It is usual to fill the crucible with two refining charges before bar casting commences.
Most of the output of the refinery is in the form of bars, each of whose nominal mass is between 12.50 and 12.75 kg, and whose fineness is a minimum of 995.0 parts per thousand, with an average of 996.0. Cast iron moulds of suitable shape and design are used for the casting of the bars. Prior to use, these moulds are preheated to 100 oC by means of gas burners and then smoked internally using dieseline smoke. It is important to have platform scales of 100 kg capacity in front of the furnace, and is provided with mass pieces equivalent to 12.6 kg. a previously smoked mould is placed on the position on the scale arm. In some operations, 15 kg of molten gold are poured from the holding furnace into a preheated pouring crucible. A pair of dip samples are taken for assay, each pair usually represent 12 bars.
The metal is poured manually from the crucible into the mould until an increase of 12.5 kg is indicated on the scale. Before every twelfth pour z pair of dip samples is taken for gold assays. Although all the bars poured from the holding furnace have the same fineness, this system provides that up to eight pair of samples are taken from each charge, giving a good average assay value. A soft gas flame is played over the surface of the molten metal in each mould to ensure that the bar cools from the bottom upwards thereby preventing the formation of shrinkage cavities. The flame also polishes the surface and drive small particles of refractory dust to the edge of the bar. Solidification takes place in about 45-50 seconds after which period the red hot bar is tipped out onto a copper carrier and quenched in a water tank. The mould is returned to the circuit for reuse. The quenched bars are removed from the tank and numbered in a series code. The series code is changed every 10,000 bars. After numbering the bars are cleaned and inspected for defects in pouring. Each bar then has its mass determined on a scale to within 0.025 ounce troy. Each bar is accounted for on a content based on its gross mass and assay, and stamped with the name of the refinery and the assay of the bar. Such ingots are called good delivery bars and are acceptable for purchase in the world gold market.