Correct temperature control is the sixth essential
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| Temperature control is very important. Since time immemorial the experienced eye has
been the thermocouple most used in the small workshop. The practiced eye is regarded as
being very reliable. Today we are able to fast track to a practiced eye with the aid of
small digital readout pyrometers. These establish accurate parameters to which we can
adhere and from which the inexperienced eye can learn. |
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As the alloy is approaching it's pouring temperature, the metal needs stirring to be
sure of a homogeneous mix. If using a stirring rod, this is best done with a soda glass,
or Pyrex rod, which will also collect any excess flux. |
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| My preference for stirring is to pick up the crucible with a pair of tongs and gently
swirl the metal until the majority of flux is deposited around the sides of the crucible.
This also has the benefit of making sure all components are melted, as little lumps will
be observed just below the surface if the melt is not complete. The excess flux I melt and
pour from the crucible at a later time. Pouring of the molten metal is done when the
operator is sure all components are molten, the resultant alloy is homogeneous, and the
surface is cleared of extraneous flux. |
| Care must be taken to ensure the metal does not overheat. The recommended pouring
temperatures for precious metal alloys are usually between 75°C – 100°C above the
liquidus. Ingot moulds should be preheated to approximately 150°C. Granulation water
should be cold. Casting moulds usually range between 200-650°C this depends upon the
pouring temperature of the metal and the physical proportions of the item to be cast. |
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| In discussing these six essential points of care, I have used as an example the making
of a master alloy to which is added fine gold and fine silver to create 9 ct, 10 ct, or 14
ct yellow gold alloys. I believe the making of master alloys to which fine precious metal
is added is the best practice. This minimises the possibility of a non-homogenous alloy.
Master alloy is efficient to make in larger quantities and economic to hold in stock. The
best equipment to use is that which successfully accomplishes the task and with which the
craftsman feels comfortable. In my small workshop I use an Oxy/L.P. gas micro torch,
70-100 mm clay crucibles (scorifiers) always preglazed with boric acid. My melts do not
exceed 300 grams. For yellow gold alloys I use a nozzle with 5 x 1.1 mm holes, this
produces a soft reducing flame. When pouring yellow gold ingots, I prefer to coat the
mould with carbon black (soot) as a release agent.
There are two reasons I do not use silicon as a scavenging agent in higher carat rated
alloys, such as 18 ct, 22 ct, etc. The antipathy of gold to silicon means that only
insignificant quantities can be used without detrimental effect; also, gold does not
readily attract oxygen, consequently scavenging agents are not needed if due care is used
in melting.
So far I have only spoken of yellow gold alloys. White gold alloys require other
considerations. Nickel white gold alloys have been used in the past. The inherent
metallurgical limitations and current understanding of allergenic problems preclude them
from consideration here. I have always produced palladium white gold alloys in all carat
ratings. Recently we have added platinum-palladium alloys to our range.
For platinum and palladium alloys, I use a single 1.5-mm orifice nozzle which will
produce an oxidising flame and attain the higher temperatures required. I use the same
type of clay crucible. 18 ct platinum/palladium white gold alloys should not be cast into
gypsum bonded investment moulds. Due to the higher casting temperatures required, the
gypsum (calcium sulphate) releases sulphur dioxide with harmful results. With platinum and
palladium white gold alloys, the ingot mould should be free of carbon, as it can have
harmful effects on the metal. These alloys can be produced in the small workshop using
similar procedures and equipment. Due to the higher melting point of these alloys,
protective glasses with appropriate gas welding filters must be worn.
A master alloy of copper and zinc is made, in similar manner as previously described.
That is, place the zinc first into the crucible, add the copper, with boric acid. Using a
reducing flame, apply heat to the copper and zinc. Be patient, take every care to control
and minimise the formation of zinc oxide. |
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The palladium and platinum are each rolled to a 0.1-mm shim and alloyed with the gold.
The palladium is placed first in the crucible with a little of the gold and the platinum
on top. Bring the metal temperature to where the palladium begins to alloy with the
platinum, add the rest of the gold, piece by piece, being careful not to allow the melt to
become so cool that the platinum stops alloying with the rest of the melt. As soon as you
are satisfied of a homogenous melt reduce the temperature a little, then add the
previously alloyed copper and zinc. |
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| After pouring, if you are not sure the alloy is homogeneous, remelt! With
subsequent melts, the use of scrap is beneficial to assist in the alloying of the platinum
and palladium and is added with the shim. Indeed a little scrap added at the beginning of
new melts will assist higher melting point metals to alloy. |
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| The periodic table is my main reference. This table details the eight precious metals
and some alloying elements; note especially the melting points, boiling points and heat of
vaporisation columns. These have been a guide to more predictable results.
The alloys I have described were formulated and made using gas torch melting and have
proven working characteristics, that is they will produce consistently good results either
as cast or wrought metals, provided due care and attention to detail as described, is
taken. They will remelt and recast retaining their good working properties.
My metal formulations have always started in the scorifier under gas torch. My initial
trial sample is often no more than a one-gram button, which I test for colour, ductility
and malleability. If it shows potential I make a larger sample for evaluation by casting
and as plate and wire. Alloys formulated in theory do not always work in practice, but one
must start somewhere. Formulate, alloy, trial, record, and fine-tune; my progress has
followed this path. Every alloy we produce began with this process; they include all carat
ratings of gold, in yellow, white, and red alloys, and a special palladium-gold alloy
which retains it's spring in the as cast state and shows a marked improvement in tensile
strength when heat treated.
Formulating alloys, searching for new properties to provide the characteristics my
customers require, has been a challenge. The special palladium-gold spring alloy which is
so useful for cast catch tongues and flat springs, the silicon-gold alloys from 1964 as
well as other precious metal alloys have been rewarding achievements.
Traditionally jewellers prepared their own metals. Today we are fortunate to have
precious metal suppliers who supply most of our needs. However, there are often special
qualities in a metal a jeweller would like to achieve. These are not always available in
stock alloys. Thus today's artisan needs the skills of his traditional predecessor. In
conclusion, I quote from an ancient metallurgist, Edward F. Law, who wrote in 1909: -
"It has long been known that the temperature at which metals and their alloys are
poured has an important influence on their mechanical properties, and the most suitable
casting temperature for any particular alloy has been determined by practical experience.
Deoxidisers should only be used to free the metal from the unavoidable oxidation, which
takes place even during the most careful melting. The knowledge that oxidation can be
wholly or partially cured should not hinder the strictest precautions being taken to
prevent oxidation during melting. Excessive oxidation caused by too rapid melting and
consequent overheating of the metal, or by overcharging the crucible, is often
incompletely remedied by the addition of deoxidisers.
In the manufacture of metals on a large scale it is not always easy to produce a
mixture of uniform composition even with careful stirring, and in practice it is often
considered desirable, if not necessary to remelt the metal a second time. The difficulty
is greatest when the metals to be alloyed have widely different melting points, and is
still further increased if one of the metals is volatile. In order to reduce this
difficulty to a minimum the pure metals are not melted together, but previously made
alloys (master alloys) whose composition is known are used to make the final alloy."
Finally I would remind you of the essentials:
Safety, good housekeeping, accuracy in weighing, melting order, fluxes and scavenging
agents, temperature control.
References
Mark F. Grimwade, "Basic Metallurgy For Goldsmiths", Gold Technology, 2, June
1990
E.F. Law "Alloys and their Industrial Applications", Charles Griffin and
Company, London, 1917
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