Check this out! Microwave melting
A domestic microwave oven melting metal at 1000 degrees Celsius
Research is nearing completion on a system that will allow the
melting and casting of bronze, silver, gold, and even cast iron,
using an unmodified domestic microwave oven as the energy source. A
potential foundry in every kitchen !!
MELTING METALS IN A DOMESTIC MICROWAVE
David Reid
The first part of this Foundry Note describes a technique for
using a domestic microwave oven to melt and cast, to accurate
shape, small quantities (up to a quarter of a kilo) of bronze,
silver, white metal or iron. The technique has been used to cast
pieces from ceramic shell moulds up to about 18cm high, and is an
accessible alternative to other small-scale melting set-ups, for
example the flask casting of jewellery. The second part of the
note describes thoughts and tests which led to the procedure. It
offers guidance and some warnings, to anyone making
investigations into metal heating by microwave.
Some pictures of the process
Background
The microwave work was triggered by a short reference to the
refining of rare earth metals, at Illawara Technology Centre,
which was mentioned by a visitor to the Central Saint Martins
foundry, Dennis Glaser. Since these metals melt at temperatures
above 800 degrees Celcius, it seemed possible that the method
could be adapted to melt and cast small objects in the workshop
or studio. If this could be done a domestic microwave would,
effectively, become a cheap and accessible furnace. Trials were
begun which simply aimed to melt metals such as silver and bronze
in open crucibles. However, it soon became obvious that casting
to shape could also be accomplished by adapting the Reid
Technique (RT) - a simplified ceramic-shell procedure for the
casting of non-ferrous metals, patented in 1990. RT was first
developed to avoid the problem of heat loss, which makes the the
pouring of small melts very difficult - these difficulties arise
however the metal is heated, and while the microwave technique
set out here can be used for heating small amounts of metal in
open crucibles, its greatest potential lies in its use as a
flameless furnace in processes such as the Reid Technique. The
crucial discovery, made during extended tests with various
susceptors - materials which heat up when exposed to microwaves -
was that two substances, graphite and magnetite, working together
were required to achieve the kind of heating we were looking for.
The Method in Brief
A wax object is prepared, attached to a hemispherical wax cup. A
second blank wax cup is prepared.
Both waxes are coated with a patent ceramic shell slurry
containing some graphite.
These are then stuccoed with a magnetite sand.
Further layers of normal ceramic shell slurry are applied and
stuccoed with molochite grain to build up the shells.
Both shells are dewaxed, by rapid heating in a flame.
The shell cup, containing enough metal to fill the mould cavity,
and a lump of carbon, is glued with ceramic paste to the mould.
An insulating, but microwave transparent, ceramic fibre block is
placed around the cup area of the mould.
The assembly is placed in the oven chamber, and the timer set
for a specified time. The firing time depends on the type and
mass of metal to be melted e.g. 330g of sterling silver would
need 17 minutes in an 850 watt microwave.
When the beeper sounds, the mould together with the insulation,
is removed from the chamber and inverted, allowing the metal to
run into the casting cavity without loss of temperature.
When the mould has cooled, the shell is removed to reveal the
casting.
Equipment and Materials
The Microwave Oven
Domestic microwave appliances are based on the magnatron; an
electronic device which converts electrical energy to microwave
energy, which is fed via a waveguide to the cooking chamber.
Since the conversion is somewhat less than perfectly efficient,
the magnatron has to be cooled by a stream of air from a fan.
This air is then led to the oven to help remove steam produced
during cooking. Once in the chamber, the microwaves are
reflected by the metal walls until they are absorbed, (usually by
water-containing food), their energy being converted to heat.
Should absorption not take place - if, for example, the oven is
activated when empty, some energy will re-enter the waveguide and
cause over-heating of the magnetron. Usually a safety switch
turns the machine off when this happens. Note that the reflecting
walls and the constant frequency of the microwaves set up
standing waves in the chamber. This results in some areas being
much more active than others and is the reason why food must be
rotated through the varying field to cook evenly.
To be of use for metal casting, a domestic microwave oven rated
D or E (850W or 1000W) needs two slight modifications: the
rotating glass plate must be removed and the holes which admit
air to the cooking chamber must be taped over (masking tape works
reasonably well). The air from the magnetron cooling will then be
re-directed to the exterior. No other modifications should be
made. Microwaves are potentially dangerous and the uninitiated
should treat the oven with respect.
Insulation
This is critical. Microwave energy transformed into heat within
the shell must be contained if the temperature is to rise to the
point at which it will melt metal. Insulation also protects the
walls of the oven. Ceramic fibre wool, in various forms, proved
to be a very useful insulating material. Working Towards a Method
It must be stated that, at the out-set of these experiments, the
researcher was completely ignorant of microwave technology. Much
was learnt along the way, mainly by cautious empirical
investigations which took a serious view of possible dangers. A
very early purchase was a microwave leakage detector. As anyone
who has left a fork in a microwave oven knows, metal with sharp
projections placed directly in a microwave field will cause
arcing. This spectacular abuse will burn the interior walls and
over-heat the magnetron. Although most modern ovens are protected
by heat sensitive cut-outs, arcing will eventually ruin the oven.
However, if the microwaves are absorbed and their energy is
converted to heat before they meet the metal, no such damage
should occur. When food, (containing water, a very efficient
absorber), is placed in a microwave field having a frequency of
2.4 5GHz, virtually all the microwave energy is converted to
heat. So, the problem was to find a substance which, when put in
a microwave field and in contact with a refractory container,
would absorb heat (be a good 'susceptor') and raise its
temperature to about 1200 degrees Celcius, thus allowing alloys
within it to melt and become castable.
Early experiments using carbon as a susceptor were discouraging.
The uninsulated crucibles barely attained red-heat and after
running for 5 or so minutes the machine shut off. Insulation
around the crucible helped, as did the realisation that the
cooling air from the fan could be redirected. But it was obvious
that a more efficient absorber had to be found. The literature on
absorbers mentioned both silicon carbide and ferrites as
susceptors, so an SiC paste was mixed with clay and applied to a
thickness of about 8 mm and dried on the inside of a ceramic
shell crucible, which we knew from earlier tests to be
non-absorbing. After drying, it still didn't show red heat after
10 minutes in the microwave field. Powdered ferrites proved very
difficult to obtain until it was realised that they were just
modified iron oxides. A quick visit to the Ceramics Department
gave samples of red, yellow, black and granular iron oxides.
Similar sized samples of these were simultaneously put in the
oven and fired for a couple of minutes. They all showed warming,
but the granular substance (magnetite) was hot enough to burn the
finger.
Another crucible was prepared, lined in the same way, but this
time with a clay-ferrite paste. 50 g of sterling silver was
added, and the crucible was capped with a carbon lined shell.
Although it became very hot the silver had not melted after 10
minutes of firing. The carbon-lined cap was replaced by one lined
with magnetite/clay and the test was re-run. After 15 minutes
firing the silver was found to have melted. A very exciting
moment! Although the methods were a bit crude, a temperature of
900 degrees Celsius had been reached. Using a similar crucible,
with a small mould attached, the first castings were made soon
after. However, the process was not efficient enough to be really
useful. Some simple calorimetric calculations showed that very
little of the energy entering the chamber was actually getting
to the metal. Much of it was being absorbed by the crucible, and
the walls of the chamber were getting quite hot, showing that
less than perfect absorption was taking place around the metal.
Closer examination of the absorbancy of various materials
followed, and attempts were made to formulate more efficient
ferrite susceptors.
How thin could an absorbing layer of magnetite be made in order
to reduce its thermal capacity but retain its heating qualities?
The granularity of the magnetite suggested it could be applied as
a stucco in a ceramic shell build-up. How many layers would be
necessary? Two were tried for a start, then one. Both crucibles
heated well when placed close to the wave-guide port, but the
heating was by no means even. Small areas of the shell would
'light-up', getting hot enough to melt the refractory. Test
shells, after cooling, that were replaced in the oven, sometimes
fired up and sometimes did not. Again, much testing was
undertaken to try and solve this problem.
Parallel experiments going on in the foundry involved heating
glasses of various compositions in a microwave oven. It was
found that although these were transparent to the microwaves when
cold, they would absorb microwave energy when just below red
heat. Maybe magnetite had to be above a critical temperature to
absorb well. The carbon coated shells, we already knew, would
always heat from a cold start. A double susceptor - a carbon
(graphite) loaded primary coat stuccoed with magnetite sand - was
tried with some confidence. The crucible, with a mould attached
was fired and the temperature climbed steadily from cold, to
something sufficient to melt the silver which was then cast.
Further experiments led to temperatures high enough to melt
small amounts of cast iron. This proved to be a limit - any
increase above it caused the magnetite to flux and destroyed the
shell. The hunt is on for susceptors that can take the
temperature of the crucible beyond the magnetite limit. It may
then be possible to cast even higher melting point metals, such
as steel, by the microwave method.