| Methods
of melting and casting | |
The ancient process of lost wax casting has always involved
rendering metals molten. Modem technology in the form of induction heating is
now employed to carry out this function, as opposed to gas torch melting or electrical
resistance systems. The melting points of various metals vary widely; for example,
tungsten has a melting point of 3,400 C and tin a melting point of 232 C. Melting
points of various pure metals which interest us are as follows:
- Gold (Au)
1,063 C
- Silver (Ag) 964"C
- Platinum (Pt) I 773-C
- Copper (Cu)
1,083 C
- Zinc (Zn) 4190C
- Nickel (Ni) 1,4500 C
- Palladium (Pd)
1,587 C Pure metals melted together in various proportions produce alloys
which are formed in accordance with specific physical laws. Pure metals have precise
melting points - for example, gold melts at 1,063 C and at 1,062 C it is still
a solid. By
contrast, alloys have no precise melting point but a melting interval
(or
range). Melting, thus, commences at the lower point of this interval and
is
only finally accomplished at the higher point (figure 50). Thermal
flow differs from the melting phase to the solidification phase. 
Some
examples:
White Alloy Au 750
Ni 140
Cu Zn 110 - Melting interval
- 910-9550C - Red Alloy Au 585
Ag 90
Cu 325
- Melting interval
- 850-8350C - Yellow Alloy Au 585
Ag 205 Cu 210 - Melting interval
- 830-8350C The exceptions to this are the socalled "eutectic alloys" which
do not have a melting range/interval but a set melting point as have pure metals. For
example: Cu 28.5% Ag 71.5% Melting Point 770 C Producing gold alloys is
best done by adding pre-alloys (e.g. Cu/Ag or Cu/Zn) to already molten gold. The
energy source employed for heating and melting should contaminate the metal as
little as possible with either gases or solids causing a consequent deterioration
in metallurgical and physical properties. Electromagnetic induction is,
we firmly believe, the source that best fulfils this requirement. Figures
51 and 52 illustrate this. All metals have induced within them "eddy currents"
when placed within the influence of an oscillating magnetic field such as that
produced by an induction coil. The metal's electrical resistance to these currents
provokes a temperature rise and eventual melting. The principal advantages of this form of heating are: -
Unchanged elasticity, homogeneity and density - Negligeable shrinkage -
May be carried out in a reducing atmosphere (to limit formation of oxides) -
Short melting time - High purity due to lack of contaminants - Capable
of achieving temperatures up to 2,500 C - Silent and clean | 
