Platinum casting is an object whose shape is obtained by transferring liquid platinum into a cavity or mold and allowing it to freeze, thus taking the form of the void inside the mold.  The mold, or flask as it is commonly called in the jewelry industry, can be prepared by first creating a wax or plastic replica of the original object, then encapsulating that wax or plastic replica in a refractory material such as investment or shell molding.  Lastly, the wax or plastic replica must be lost from the refractory material by either heat, as is the case inside a burn-out oven, or high pressure steam (autoclave,) as is the case in shell casting.

Development of Modern Platinum Casting

Platinum, is grayish-white in color, malleable, ductile and has an extremely high melting temperature.  Only four or five other metals in existence have a higher melting point.  Platinum is about twice as heavy as lead.  It is the most important of a group of metals called the platinum group.  The other members of this group are  ruthenium, rhodium, palladium, osmium, and iridium.

Chemically, platinum is a noble metal.  It does not react to air, water, single acids or ordinary reagents.  It will dissolve slowly in aqua regia (royal water), only after being alloyed down one part platinum group metals to four parts nickel or copper.  It was not until the early 19th century that the aforementioned process was perfected in England.  In this way platinum was first separated from other platinum group metals.  For the first time in history platinum was refined to a pure state.

It is said that platinum was first discovered by the ancient Egyptians nearly 3000 years ago.  Platinum metals were probably used in alloyed forms in ancient Greece and Rome.  Pre-Columbian natives used platinum in alloyed form hundreds of years ago, long before being conquered by the Spanish in the 16th century.  In the beginning, the Spanish who brought this metal back to Europe had mistaken it for silver, thus dubbing it platina, or, "little silver." At that time the metal's worth was not fully comprehended.  Later it became the metal of choice for the royalty of Europe.  It was first crafted into fine jewelry in 1780 at the court of Louis XVI of France.  In the 1800s Russia's Karl Fabergé made remarkably beautiful and complex jewelry from platinum.  This jewelry is renowned the world over.  Many of the world's great jewelers, including Cartier and Tiffany, followed suit with their legendary jewels designed in platinum mountings.

It was not until mankind was in the Iron Age that man could possess the heat required to melt platinum group metals in alloy form.  This kind of heat is required for smelting platinum group ore.  Once the raw material was smelted and platinum group alloys were created, only then could man begin to use platinum group metals.

It was not until the 1800s that platinum group metals were first separated by means of aqua regia.  This method of refining removes all impurities from the platinum to achieve four nines purity.  It was also during this era that bottled gases were first introduced.  This technology set the stage for platinum casting to fully come of age.  Toward the turn of the 20th century, bottled gases were at the state at which they are today.  Acetylene gas was the most common gas available in bottle form.  This was because acetylene can be dissolved in liquid acetone while under pressure.  In this form it is much less hazardous and less likely to explode.

Hydrogen gas produces the hottest flame of all bottled gases.  Hydrogen can be regulated to 30psi, which is double that of acetylene.  This makes it ideal for melting and stirring platinum very quickly.  It is important to note that hydrogen is highly explosive and must be treated with respect.

Modern, up-to-date methods include plasma-arc torches and induction melting, where electricity is used to heat and melt platinum.  These new methods are much safer and require much less experience on behalf of the caster.  The level of skill involved in operating a torch and using the flame to stir the molten platinum to a uniform temperature is quite high.  The learning curve is drastically reduced by a factor of years verses days because of the new methods. Induction is by far the best way to melt platinum.

Platinum Casting Theory

Platinum is unique in that, theoretically, it expands very slightly when heated.  When it's reaching its melting point, it liquefies into approximately 10% more volume than in its solid state.  Upon solidification the opposite occurs in that the platinum decreases approximately 10% in volume.  In platinum casting practice, inside the mold the liquid metal that is touching the surface of the investment solidifies first.  As it shrinks, more liquid metal is supplied by the centerline of the casting, which fills the voids that occur during solidification.  As the pattern solidifies, the centerline starts to freeze and draws more liquid metal from the feeds.  After the pattern is completely solid, only then can the feeds (gates, risers, sprue, etc.) solidify.  Only after all the feeds are solid can the button solidify.  If everything occurs in this order the pattern will be very dense with no porosity at or near the surface of the casting.  The feeds, however, will be less dense since they solidified after the piece.  The button should be full of holes, as it solidified last.

Casting Problems Defined

The casting problems, as any other, may be divided into general parts:

Design Problems

The Design and Production of the Mold or Flask

(A) The Mechanism and Rate of Metal Solidification
For the sprue gates and risers to be placed and proportioned properly, the crystallization pattern and shrinkage in volume accompanying solidification must be known.  These factors vary considerably, depending upon the chemical composition of the metal and the thermal gradients in the mold.  If you want to make plate or rod, for example, you would use a cold mold, which is water cooled.  If you want to cast rings, you would use a heated flask that was burnt out properly.

(B) Heat Transfer During Solidification (Risering)
After the solidification process is understood one can proceed to study the control of shrinkage porosity by application of heat-transfer principles.  If the volume of the liquid metal is greater than that of the solid metal, voids will occur unless steps are taken to change the natural thermal gradients.  We need to know what can be done to change the heat-transfer conditions.  The use of liquid-metal reservoirs (risers) and chillers can be beneficial to the solidification process so that the pieces solidify before the risers and sprue gates.

(C) The Flow of Liquid Metal
The problems involved here are illustrated by the following questions:  What are the calculations leading to the selection of proper pouring temperature?  On one hand we must avoid overheating the metal to the point where porosity becomes an issue, and on the other, we must prevent solidification of the metal stream before the mold is completely filled.  How will the channels for delivering the metal (the gating system) be designed?

Sprue Gate Design

The objective of a sprue-gating system is to permit distribution of the metal to the mold cavity at the proper rate, without excessive temperature loss, free from objectionable turbulence and entrapped gases.  Any good gating system is the result of considered engineering compromises.  Metal and mold compositions affect the choice of design for a gating system.  The characteristics of a heating ceramic mold such as is used in investment (or "lost wax") casting permit variations in the gating system customarily used with cold molds of other compositions.

The gate should enter the heaviest section of the ring.  It should be a single, heavy sprue one and one half times the size of the cross-sectional area it is feeding.  It should have fillets at the joint, flaring the gate slightly (no 90º corners).  This will slightly increase the surface area at the junction and will minimize any turbulence when the metal enters the piece.

Riser Design
Risering is a process designed to prevent the formation of shrinkage voids in the casting upon solidification.  The point to recognize is that contraction can take place at the constant temperature or over a narrow range and is the result of a density change accompanying the transformation from liquid to solid.  Shrinkage porosity can be solved by controlling the solidification pattern and thermal gradients so that the voids are produced outside the body of the casting proper.  To produce a sound casting, the riser or reservoir of liquid metal which is to compensate for the shrinkage must satisfy two independent requirements.

(A) Riser Size
If the riser is to supply liquid metal to feed the casting shrinkage, it must freeze after the casting.  The ratio of the riser must exceed that of the casting.

(B) Riser Placement
For an alloy with high centerline feeding resistance, a casting will require a closer spacing of risers than for other alloys.  In other words, the effective feeding distance of a riser in a wide-freezing band alloy is smaller than in a narrow-band alloy.

Centerline Feeding Resistance

We can define a term to quantify the ease of feed to a casting.  We know that once the centerline has solid crystals, difficulty occurs.  We also know that the greater the percentage of total solidification time during which these centerline crystals are present in a casting, the more difficulty there is in feeding.

Steps of the design and production of the mold (flask);

1.  Design of piece to be replicated.
2.  Model-making, correct location and size of gate and risers.
3.  Wax injection mold (rubber mold.)
4.  Injection of the wax.
5.  Investing.
6.  De-waxing.

Process Problems

Effect of Mold Temperature
In lost-wax casting, the temperature of the mold or flask varies greatly.  This is due primarily to the nature of the casting method.  A secondary reason for temperature variation is what you are trying to cast.  The higher the temperature (inside the mold or flask,) the longer a mushy condition, i.e., liquid + solid, exists.  The lower the temperature, the more rapid the freezing occurs.  The casting must freeze at the surface first, then gradually solidify inward.  The centerline must freeze last.

Melting, Refining and Transfer of Liquid Metal
(A) Gases in Metals
During melting, porosity in casting is often produced by solutions of gases in liquid metal which are less soluble in the solid metal and therefore precipitate as bubbles, leading to holes and porosity.  The gates can contribute to this by introducing turbulence and entrapping gases during the casting process.  The open-air system of melting the metal can also contribute to gases being absorbed into the metal.  Another source of gas is from the flame of the torch or furnace.  If not adjusted correctly, the flame can adversely affect the metal by absorption of excess gases, especially reducing gases such as CO, H2 and CH4.  The reducing gases will change the characteristics of the metal and cause embrittlement and cracking.  Reducing gases or getter gases can also create havoc on silica (SiO2) by changing its form to silicon (Si), which can be also absorbed by the metal in the charge.  Since most refractory materials used in platinum casting are made of silica, there lies a potential for contamination if mishandled during use. 

Platinum can be refined to some extent by melting the metal and allowing it to begin to solidify, causing a snow cap, and then remelting the metal again.  By doing this several times, you can de-gas the charge and also remove contaminants by allowing them to gradually float to the top where they solidify with the snow cap.  Later, as the charge remelts, the cap rolls over to one side and the contaminants are removed by sticking to the crucible.  Induction heating is ideal for this process because the charge is heated from the bottom upwards to the cap of the charge, unlike a torch, which is heated from the flame downwards.

(B) Control of Common Elements
When service tests have shown a metal of a certain composition and color to be most desirable, we need to determine what combination of scrap, furnace conditions, temperature/soak time and casting machine is necessary for reliable, economical production.

The best way to get repeatable and consistent results is to use the same amount of metal in every melt and apply heat to the point where the metal is 100% liquid.  At this point, remove the heat and allow the charge to cool to the point where the metal is starting to freeze.  Next, reheat the metal to its liquidus point and continue to heat for a fixed interval in time (soak time) until transfer of metal occurs.  This method ensures homogeneity of temperature as well as alloy mixture at the point in time when transfer occurs.  Metal should transfer quickly and smoothly so that differences in metal temperature do not occur due to cool off during transfer.  The metal that leaves the crucible first should not be hotter than the metal that leaves last.  If this happens, the metal that reaches your rings will be hotter than the metal inside your sprues and solidification will not occur in the correct sequence.  It is extremely important that the metal in the charge be the same temperature throughout.  If it is not the same temperature, the hotter metal will flow out before the cooler metal.  Remember also to ensure that the crucible hole is sufficiently large to transfer the charge without any cool-down inside the crucible. 

If you are using a torch, you must mix the metal with the flame of the torch carefully to ensure none of the metal leaves the crucible's dish.  With the proper skill, this technique can produce acceptable results.  This technique, however, requires considerable experience in order to perfect.  The transfer of the metal to the mold can be accomplished several different ways and the method chosen depends upon what you are trying to make.

Material Problems

Rate of Solidification of Different Alloys
The solidification of alloys differs in three principal ways from that of pure metals:
1.  Usually, the freezing of alloys occurs over a temperature range.
2.  The composition of the solid which separates first is different from that of the liquid.
3.  There may be more than one solid phase crystallizing from the liquid.

Effect of Alloy Composition
It is an experimental fact that some alloys are far easier to feed than others, given the same flask temperature.  In general, those with the smallest liquidus—to solidus interval are easiest to feed, while those with long freezing range present difficulties.

Causes of Porosity (Holes)
There are four major causes of porosity:

1.  Overheating of the metal.
2.  Carbon residue after burnout.
3.  Improper gate location, dimensions, or shape.
4.  Too high a flask temperature. 

V3N3

Platinum Casting Theory and Techniques
Peter Romanofsky
Romanofsky

This is an abbreviated version of the original work. For full technical details, please consult the original paper.