At the London mint about 2,75o oz. (86 kilograms) of gold are melted in a pot, but larger crucibles holding nearly 6,000 oz. (i88 kilograms) are used for silver. The amount of metal to be melted in mints is small and its value great, so that small charges in the furnaces are convenient. The necessity for perfect uniformity in composition, discussed in the sequel, is also in favour of small charges. The cost of fuel is a small item and any loss of metal a serious matter. So crucible melting retains its place in mints.
The silver-melting furnaces employed in the London mint are cylindrical in shape inside and are heated by ordinary illuminating gas. The charge consists of refined silver bars with the required amount of copper, together with scrap silver from former opera tions and old coin if available. When ready for pouring, the cru cible is lifted out of the furnace by means of circular tongs which are suspended from a travelling electric crane. The crucible is placed in a pouring cradle which can .be tilted by gear wheels worked by a hand crank. The contents of the crucible are then stirred by a rotating mechanical stirrer in order to ensure proper uniformity of composition and a portion is dipped out for assay. The limits of error allowed by law in the composition of gold and silver coins are narrow. In British gold coins the allowance or "remedy" for fineness is 2 per i,000 of gold above or below the standard of 916.6. In most countries a divergence of only i per I,000 of gold is allowed and little difficulty is experienced in keep ing coins within the limits. Silver-copper alloys, however, even though perfectly mixed when molten, become of different com position in different parts on solidification. The only alloy of uniform composition when solid is that containing 720 parts of silver and 28o of copper. This is the "eutectic" alloy (see ALLOYS) and is used for coinage in Holland and Mexico.
By repeated passages through the rolls the bars are hardened, and to facilitate further reduction they are usually softened by annealing bef ore being passed to the finishing rolls. In some mints the strips are annealed. In the United States mints the use of very carefully refined metal has made it possible to discontinue strip annealing. When the strips are reduced to the correct thick ness they are examined by the "tryer," who cuts out one or two blanks from each strip with a hand machine and weighs them on a delicate balance. If the blank is too heavy the strip is again passed through the rolls.
The degree of accuracy required is indicated by the tolerance or "remedy" allowance in weight, which is different for each coin, and is the maximum difference from the standard weight which is allowed by law. In the sovereign it is 0.2 grain or about 1.62 per i,000. As the mean thickness of a sovereign is o.o466in.
the remedy in weight corresponds to a difference of less than +'000,2 in. in the thickness of the strip. Remedies in weight and fineness are intended to cover accidental variations.
The blanks are then passed to an edge-rolling machine, by which they are thickened at the edge so as to form a rim to protect the finished coin from wear. The operation is called marking, because originally the edges were not only thickened but were also marked with an inscription, as is still done in some mints. The letters are sometimes sunk and sometimes raised.
Like the graining or "milling" on the edges of coins, the inscrip tions were intended to put a stop to the practice of clipping and filing coins, which was prevalent in the i6th and 17th centuries.