The general method of examining the purity of silver is by mixing it with a quantity of lead proportionate to the supposed portion of alloy; by testing this mixture, and afterwards weighing the remaining button of silver. This is the same process as refining silver by cupellation. It is supposed that the mass of silver to be examined consists of 12 equal parts, called pennyweights, so that if an ingot weighs 1 oz., each of the parts will be 112th oz. Hence, if the mass of silver be pure, it is called silver of 12 dwts.; if it contain 112th of its weight of alloy, it is called silver of 11 dwts.; if 212ths of its weight be alloy it is called silver of 10 dwts, which parts of pure silver are galled 5 dwts. It must be observed here that assayers give the name cwt. to a weight equal to 24 real grs., which must not be confounded with their ideal weight. The assayers' grs. are galled fine grs. An ingot of fine silver, or silver of 12 dwts., contains, then, 288 fine grs.; if this ingot contain 1288th of alloy, it is said to be silver of 11 dwts. and 23 grs.; if it contain 4288th of alloy, it is said to be 11 dwts, 20 grs., etc. Now a certain real weight must be taken to represent the assayweights; for instance, 36 real grs. represent 12 fine dwts.; this is subdivided into a sufficient number of other smaller weights, which also represent fractions of fine dwts. and grs. Thus, 18 real grs. represent 6 fine dwts, 3 real grs. represent 1 fine dwt., or 24 grs.; 1 1/2 real grs. represent 12 grs.; 1 32d of a real gr. represents 1/4 of a fine gr., which is only 1752d part of a mass of 12 cwt. 
