Silver is used in various applications ranging from tableware, coins and jewelry to photo processing and electronic gadget components. Generally, all finished products such as jewelries and coins begin their existence from casted silver bars. These bars of silver are manufactured and sold by refining companies to silversmiths according to weights and purity. Casting is the process which mined silver nuggets and/or scrap silver is melted and poured into prepared molds to form silver bars. There are many different casting techniques used in making precious bars and each are utilized for to its specific advantage. Some of them are quite simple and can be done in a small workshop using basic tools while others are quite complex and need technical knowledge to utilize properly.
The most basic method of casting silver bar is the open casting method. This method is similar to making gelatin dessert or ice cubes. The scrap silver or silver nuggets are put in a carrying crucible and heated in high temperature in an induction furnace. Once the silver turns to liquid, it is then poured in molds, covered to avoid shrinkage and absorption of oxygen, and allowed to cool to solidify.
Closed casting on the other hand is done by pouring molten silver in a closed mould using a gating system and then closed to allow the silver to cool and harden. Sometimes water is poured on the mould to accelerate hardening. This is one of the most widely used procedure in making silver bars.
The vacuum pressure casting is done with sophisticated machine that is specifically designed for the purpose of casting silver. The initial procedure of liquefying silver is similar with other methods but the sequence of pouring molten silver in the mold is automatic and dependent on the machine's specification. The advantage of vacuum pressure casting machine is reflected on the quality of the finished product it produces. This machine is usually utilized in making fine silver bars and coins that have delicate designs.
The molds that are used in casting silver bars are usually made of stainless steel. This material may last a lifetime but is quite expensive and is generally used for large scale production of precious bars. An alternative material used by many small scale manufacturers is graphite and ceramic. Graphite or ceramic mold, although may last only after a few rounds cost far cheaper than stainless steel mold. They are also commonly used by big manufacturers as test molds before making a final mold of stainless steel.
Since 2008, more and more refineries utilize open-cast when making silver bars and began modifying their method of melting. This was due to safety and health concerns brought up by the London Bullion Market Association. The organization also insisted that only open-cast bars will be accepted as London Good Delivery starting from January 1, 2008. According to the organization, they are produced by closed-casting (or gated molds) have sharp edges on the part where the sprue was removed; this pose as a problem when handling. The method of pouring water to accelerate the cooling of the mold also produces some cavities on the bar that could trap water inside. The trapped water within the silver bars may cause a very dangerous steam when the bar is remelted and may sometimes cause a molten metal explosion.
Aside from open-cast method in making silver bars, the LBMA also implemented some basic requirements in order for a refinery to gain the accreditation of London Good Delivery. Included in the requirements is the overall appearance of silver bar. This means, no surface cavities, excessive shrinkage and bubbles; and the bar must have no sharp edges for easy handling, and can be stacked safely.
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