The following article reprinted from AJM
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Casting with the Stones
Already Set in the Waxes
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| Stone-in-place
casting can offer dramatic savings in stone setting labor costs, once a few
common questions are answered. |
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By Ajit Menon
Stone-in-place casting has been done in the United States for
more than 10 years. In the beginning, though, this process was used very
discreetly, since it conveyed tremendous economic benefits and an edge over
competitors. In recent years, interest in the process has grown and information
has become more readily available. In this article, we answer some of the most
frequently asked questions about stone-in-place casting.
How does setting stones in wax differ
from setting stones in metal?
The most obvious answer is that it takes much less strength
to set the stones in the wax than to bur and move metal around a stone. Because
great pressure is not applied to the stones while they are set, there is much
less stone breakage.
In addition, when stones are set in wax, the metal remains
unworked. In traditional channel setting, for example, the metal of the channel
is burnished or pounded with a hammer, which can cause the metal to become
brittle or cracked.
Are special materials and/or special
equipment needed for casting stones in place?
No. Gemstone casting can be done using conventional casting
equipment, although it is important that the equipment be capable of accurate
temperature control. In addition, vendors do offer investments, casting metals,
and even casting equipment specially designed for the gemstone casting process.
Alloys have been formulated to give a better casting efficiency at low
temperatures, and additives can be added to the investment to protect the
gemstones, allowing flasks to be burnt out at a higher temperature.
When using centrifugal casting techniques, there may be some
flashing of metal over the stones if the force of the metal going into the flask
is very high. This usually can be corrected by changing the orientation of the
wax pattern in the tree. The pressure of the incoming metal can be reduced by
increasing the angle of the wax pattern toward the horizontal plane of
the center sprue.
What stones can be set this way?
The stones most commonly set in wax for casting are diamond,
sapphire, ruby, and garnet. (Ruby and sapphire are second only to diamond in
hardness.) Cubic zirconia and laboratory grown colored stones can also be used.
Gemstones should be flawless and of high quality, since those
with inclusions tend to become milky or frosty during the casting process.
There are many stones that can be cast in place, but success
is determined by the type of setting and the care taken in investment removal
and in monitoring temperature controls during casting.
What types of settings can be used for
casting stones in place?
Numerous types of settings can be used for casting stones in
wax. In flush mountings, prong settings, or bead settings, the model is made so
that the setting's dimensions are fractionally smaller than the stone's girdle
diameter. The modelmaker must also calculate the shrinkage factor. If the
setting dimensions are not correct, the stone may be lost or
dislodged during casting.
Gemstones are often set by gently pressing a heated wax pen
tip on the top of the stone. The gemstone is heated and slips easily into the
setting. The process works great for most settings.
In prong settings, the prongs must be pre-notched in the
model so the waxes have the appropriate notches. In flush mount settings, if the
seat is too small, the wax may bulge when the stone is set, and excess wax may
be pushed onto the top of the stone. In this type of setting, the seat must be
cleaned all the way through the wax to create a brilliance hole. A brilliance
hole will also hold the stone in place during investment and casting.
For channel settings, the model should have a notch or edge
cut for the stones in the channel wall. As with other types of settings,
the channel wall must have dimensions slightly smaller than the stone to
be set. The stones are slipped into the channel, where the notch will hold the
girdle securely. Slight adjustments to the way the gems sit in the channel may
be made with a heated wax pen. The stone setter must be careful to keep the
stones from overlapping or touching each other, since this may cause them to
chip or crack.
Channel setting can be simplified even further. Instead of
being installed into the wax patterns, the gemstones may be set directly into
the rubber mold, and the wax injected around them. In this method,
sometimes referred to as the "gemstone in mold" technique, the gemstones will be
automatically placed in the wax.
Sorting gemstones into the rubber mold is
beneficial for channel setting styles that have a lot of closely arranged
stones. The process takes less time and is excellent for high volume
production.
For this process, gemstones must be properly set in the
metal model and a rubber mold made from that model. Gemstones are then set
in their proper seats in the rubber mold, and the wax injected into the
mold. Once the wax pattern is removed from the rubber mold, minor
adjustments, such as aligning the gemstones, may be made with a
heated wax pen tip. |
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Strict measurements must be taken by the modelmaker for this
procedure to be successful, and smaller stones of a carat group must be used to
ensure there are no large voids around the stone in the rubber mold. Stones must
also be set with a slight space between them so they are held securely by the
rubber when the wax is injected.
In addition, the modelmaker must also take care with the gate
location. Parting lines should not occur over stones, and the stone
setter must be able to place stones in one side of the rubber mold. The gate
should be thick and located on the side of a ring to prevent non-fills and
shrinkage porosity.
Other types of settings can also be used as long as
the modelmaker keeps in mind stone size and shrinkage, and carefully determines
the gate size and location.
The wax stone setter should not begin setting stones until
waxes are inspected, defects mended, and parting lines cleaned. This will help
ensure that stones will remain clean of any excess wax that could cause the
stones to become loose.
Stones should also be inspected once they have been set in
the wax. The stone should be straight and secure in the seat. If minor
adjustments are made with a heated wax pen tip, be sure to check the bottom or
culet of the setting. Heated wax can surround or cover this area, and the
investment will not hold the stone in place.
How do you keep stones from falling out
of place once the waxes have been burnt out?
If the stone is not secure in its seat, the stone will move
during casting and become embedded in another area of the item. To keep stones
in place, stones should be exposed at both the top and the bottom. Remember, the
wax areas will be replaced by metal, while all the areas exposed to you will be
filled with investment. When the wax is burnt out, the gemstones are held
securely in place by the investment.
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What special wax burnout procedures are needed?
Any conventional burnout procedure may be used. Care must
be taken, however, that the peak temperature does not exceed a safe level.
Although other authorities recommend that the casting flask
temperature be limited to 850°F, we have successfully cast diamonds at
flask temperatures up to 1,1 50�F, and CZs and other synthetic gemstones
at regular flask temperatures in the range of 1,250�F to 1,300�F.
We recommend 1,050°F for diamonds, 950°F for
lab-created opals, and 1,000°F for other gemstones.
A longer burnout time is used to compensate for the low
peak burnout temperature. I recommend that flasks be held at the peak
temperature for a minimum of six to eight hours prior to casting. In
addition, steam dewaxing for waxes with gemstones is highly recommended.
Although my own practical experience suggests that steam dewaxing is not
necessary, as a metallurgist I would personally recommend the steam dewaxing
process, since it can help produce a more complete burnout. |
How large can casting trees be?
Casting trees can be as large as 10 inches long. Flasks can
be as large as 1 foot long. Our experiments have shown that casting efficiency
starts to decrease when trees are longer than 10 inches.
What special precautions must be taken
during the actual casting process?
Care must be taken that the flasks are not handled
roughly when moving from the oven to the casting equipment. Do not overheat the
metal, and make sure the metal is clean and free from dross. Pour the
metal as soon as possible if it is hand poured. Let the flask sit for at least
two hours at room temperature prior to removing the investment. The flask must
not be quenched in water.
Doesn't the lower temperature lead to
non-fills and shrinkage porosity?
It can. Even though other factors contribute to such defects,
lower flask temperatures are largely responsible for nonfills and shrinkage
porosity. Proper spruing techniques, including location and thickness of the
gates, should diminish the chances of shrinkage porosity. Proper casting
technique should eliminate non-fills. In addition, there are alloys specially
formulated for this process that have higher fluidity and long-range freezing
parameters.
If flasks can't be quenched, how should
investment be removed from around the tree?
One of the most important and difficult aspects of the
gemstone casting process is removal of the investment. Flasks should never be
quenched in water. Quenching causes sudden contraction of the stones and metal,
which leads to shattering or cracking of gemstones.
Once cast, flasks should be allowed to cool for two hours.
With a rubber mallet, gently tap around the flask to loosen the investment. With
more tapping on the flask, the bottom will release the cast trees.
The button of the newly released tree should be immersed in
cold water for five to 10 minutes. When the casting is cool enough to be
handled, it may be sprayed as usual, or soaked in any commercial
investment removing chemical solution.
High pressure washing can be used to remove investment,
although the technique can sometimes damage or crack synthetic opals, bend
prongs, or knock diamonds from their channels. However, when stones are knocked
from their channels, I usually find that the stones were not properly set in
the first place. Extra care must be taken to protect opals and prongs while
using high pressure spraying.
Some gemstones may be sensitive to certain acids, so test a
sample stone prior to pickling the castings. Tumbling equipment with very
small media, such as a magnetic tumbler with fine steel media, can be
used to shine the castings. Most gemstones are not affected by this process, but
they should be tested to be certain the media will not scratch them.
I tried casting diamonds in place, and
the diamonds turned milky. Is there any way to correct
the problem? Can the diamonds be re-cut?
Diamonds will lose their crystal structure and turn milky if
they are heated above 1,200°F in an oxidizing environment, such as that found
during burnout. The diamonds become amorphous (non-crystalline) and lose their
optical properties.
As far as I know, these diamonds cannot be re-cut since the
milky appearance is not a surface phenomenon only. The entire crystal
structure of the carbon atoms is lost—theoretically it is not even a diamond
anymore. I do not know of any process that will change the milky diamonds back
to their original state.