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A rose engagement ring. Coming up with a design for this piece was quite tricky. I wanted to make something with strong rose influence without being too cheesy. The ring needed to be light and elegant on a fairly small finger, but durable enough to last a lifetime. I came up with some initial sketches, discussed them with the client


and we landed on this one, but without the leaves. 

After an long, winding journey through metal casting, I ended up with this piece. I'm very pleased with it.


The choice of metal was a tricky one. We wanted a hard wearing ring with a bright white lustre. The obvious choices are white gold or platinum. White gold is strong and hard, but most white gold alloys are somewhat yellow, and are generally plated with rhodium to improve their colour. As an engineering solution, this sucks. Rhodium plating is expensive, and wears off over time. A ring worn constantly generally requires "cleaning", or replating, every few years. There are some new white golds which do not require plating, but they must be cast at much higher temperatures, and are generally more difficult to use. Platinum is a good option from the client's point of view, except the high price, but it is much more complex to cast. Since this was my first casting project and there was a pretty tight deadline, I wanted to avoid metals with very high melting points.

Solution: Platinum Sterling. It's a brand new alloy of sterling silver, platinum and palladium, invented in 2003. It has the hardness and strength of  white gold and the whiteness of rhodium, but is less expensive than white gold of platinum. It casts at similar temperatures to sterling silver, so I wasn't too worried about building the casting infrastructure.

The stone is a 5mm brilliant cut African garnet, a clear choice for a deep, rose red.

Lost Wax Casting

The flowing, organic form of the ring was more suited to casting from a soft, easily sculpted wax positive than fabrication from wire. Furthermore, the grade of Platinum Sterling I wanted to use is only available in casting grain. Lost wax casting is the most common technique for jewellery castings. It allows one to reproduce arbitrarily shaped combustible objects with faithful reproduction of detail and excellent surface finish. Generally a positive master is carved/formed/welded/filed in wax. This wax positive is then covered with a fine plaster investment material, which is fired in a kiln to produce a slightly porous, heat-resistant negative mould. The wax burns away during the firing process, leaving a cavity that can be filled with liquid metal. The porosity of the investment allows the air to be forced out under the pressure of the liquid metal flowing into the mould.

I built several tools for the casting process including a kiln , vacuum table and centrifuge. The making of these tools will be documented separately, along with an acccount of casting successes and failures. The rest of this page will describe their use in producing the ring, without going into the details of their individual construction and operation.

Making the Model

I started by turning an aluminium mandrel on the lathe. The wax model would later be built around it, so its outside diameter determined the ring size.
I built up the model from 2mm wax D-profile wire. First I curved the wire around to one side and used the welding tool to attach a second piece to form a Y shape. This would become the top two prongs of the setting; the leafy parts of the bud. I then wrapped the stem of the Y around the mandrel
and welded it into a closed loop.
Next, I used various carving tools to refine the shape of the model. I tapered the prongs, and carved a valley at the Y junction to suggest overlapping leaves.

I welded on a lump of wax and formed it into the shape of a thorn. This formed the third prong of the setting.

Next I set the stone into the model, bent the prongs over it, and tweaked them into a perfect fit. 

The lost wax technique can produce a very accurate reproduction of the wax surface. Making the model's surface as smooth as possible reduces the amount of finishing and polishing one must do to the metal casting. I had heard of two techniques for polishing the wax - mechanical polishing and chemical reforming. The former is pretty obvious - the same basic approach as polishing any other material. I liked the sound of the chemical approach. The basic idea is to dissolve the surface of the wax with a solvent. The liquid surface area is minimised, removing the scratches, and when the solvent evaporates out you have a hard, polished object. Apparently some commercial solutions are available, but I didn't have any to hand. I tried white spirit.

It worked to some extent. The wax dissolved, and not too much, but the white spirit and wax residue formed a slurry that accumulated between the model and mandrel. When the model was removed from the mandrel, this residue left a rough surface. Perhaps if I applied solvent without the mandrel in place, the crud would be able to run off, and a better finish would result. I cleaned off as much of the residue as possible, and dealt with the rest in silver.

Next, I welded on the sprue and riser. I was careful to produce a nice flared interface between the sprue and model, to ensure unimpeded flow of silver into the mould. I attached their other ends to the rubber base and the piece was ready for casting.

 Preparing the mould

I weighed out investment powder and water and thoroughly mixed them using a food blender
I poured it into a cup and loaded the cup into the vacuum chamber for a couple of minutes of degassing.
Then the investment was poured over the model, and the mould degassed for another few minutes.
After about 10 minutes, the vacuum was turned off and the mould left to set. A couple of hours later the rubber base was peeled from the mould, leaving a perfect pouring cup, complete with a hole for the riser vent.

The mould was loaded into the kiln . I programmed it for a 5 hour burnout profile. This was a hybrid somewhere between the 10+ hours recommended by the investment manufacturer and the ~2 hours recommended by Fred R. Sias in his book "Lost Wax Casting".
Here the kiln is just starting the first phase of the burnout - ramping up to 270 celsius over 30 minutes. 


The peak of the burnout is 730 celsius for two hours.. After that, the kiln ramps down to the desired flask temperature for casting. I set it to the high side of the recommended range for platinum sterling, 630 celsius, to account for the small size, and hence small heat capacity, of the flask.

 At one point in every project, I'm involved so deeply that documentation goes out of the window. This time, it happened at what is probably the most exciting part - the casting itself! I have heaps of photos from failed vacuum casting attempts, but none of the centrifugal casting process which was used for the finished product.



 Here's a picture of the centrifuge.The white part is the crucible in which the metal is melted. The flask is loaded in the cradle between the crucible and the balti dish (safety shield!). Metal flows out of a small nozzle in the end wall of the crucible when the centrifuge spins.

I preheated the crucible and casting grain, getting everything hot enough for the metal to just melt. I added a little flux powder to keep it clean.  Then I took the flask out of the kiln, loaded it into the centrifuge, heated the metal a good way past its melting point and turned on the centrifuge.

  I let the mould cool for 20 minutes or so, then quenched it the rest of the way in cold water. By this time, it was too cool for all of the investment to crack away from thermal shock, so I carefully dug it out of the mould with a teaspoon.

Here's the finished casting. Came out absolutely beautifully. Don't think I even needed the riser, but after spending all that time on the model, I didn't want to risk an incomplete fill. The corrosion resistance of Platinum Sterling is superb. Ordinary sterling silver comes out of the investment black. This photo is of the casting simply rinsed under a cold tap , no pickling had been done at this point.


Next I trimmed off the sprue and riser
and filed the stumps down to match the profile of the ring.
The casting ready for prepolishing.
Cleaning up the insides of the prongs.
Prepolishing with fine wet and dry paper.
I polished the intricate details with carbide-impregnated silicone wheels of various grades and profiles. I used a mop and rouge to polish the more exposed areas. To get to the inside of the setting, I used some strips of old sock and liquid polish in a "flossing" action.
  Setting the stone was a fairly straightforward business. I simply bent the prongs outwards with a pair of pliers, put the stone in place and bent them back. I used pieces of cardboard to protect the metal surface from the jaws.
Finally, a blast in the ultrasonic bath to clean off the polish.

The finished item



I'm very pleased with the end result. The flowing, organic form turned out precisely how I had intended. The deep red of the garnet complements the bright white platinum sterling beautifully. I finally ironed the bugs out of my casting setup, and I'm now able to produce consistent, high quality castings. 


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