The Micro-Examination of Metals. 5 
If the operation has been carried out correctly, especially as 
regards temperature, the resulting bead consists of practically 
all the precious metals in the original charge of ore and no other 
metals or impurities. 
Cupellation beads of pure silver, when examined under the 
microscope, show considerable variation in structure, but the 
common types are fairly large crystal grains with definite 
boundaries and definite centres of crystallization. The very 
beautiful 'appearance of the latter is illustrated in fig. 2. 
The micro -examination of cupellation beads of pure gold 
generally shows distinct crystals, and in some cases it can be seen 
that crystallization has been from a definite centre and has 
developed until retarded by interference from adjoining crystals. 
Fig. 3 illustrates the regularity of development just mentioned. 
The appearance of cupellation beads of gold-silver is found to 
vary more or less in accordance with the proportions of the two 
metals, beads of high silver content approaching in appearance 
that of pure silver and vice versa. 
It has been known for some considerable time that platinum 
and its associates could be detected in cupellation beads by the 
surface crystallization and other surface effects visible to. the naked 
eye. The surface of a silver cupellation bead containing 1 * 5 to 
1 • 6 p.c. and upwards of platinum, for example, has a frosted 
appearance detectable by the naked eye, but when smaller 
quantities are present the surface appearance is quite normal, 
although some assayers who are experienced in the assay of 
platiniferous materials claim to be able to detect a difference. 
C. 0. Bannister and the present author* have shown, however, 
that the microscope gives more certain results than the naked eye, 
and that by microscopic examination of cupellation beads as little 
as 0’3 p.c of platinum can be detected when alloyed with silver 
by the corrugations on the surface of the metal. 
With 0*4 p.c. of platinum there is a tendency for the crystal 
boundaries to show distortion and the slightly corrugated surface 
visible whem 0*3 p.c. is present to be more pronounced. An 
increase in the platinum content results in an increase in the 
number of corrugations until with about 1 p.c. the whole surface 
of the bead is covered. This is illustrated in fig. 4, taken from a 
bead containing 1*2 p.c. of platinum. 
The effect of platinum on silver-gold beads is not so marked as 
on silver beads, and the effect of this metal on gold beads is even 
less marked, the corrugations not being as numerous or as well 
defined. 
Palladium, which has been largely used in conjunction with 
silver in various alloys, does not alter the appearance of the surface 
* Trans. Inst. M.M., 1913-14, Yol. 23. 
