This is my first posting. I am not a bench-trained jeweller but am
partly migrating there from sculpture. I have taken short courses
on stone setting, but mostly I just rely on books and tinkering. I
used to cast small to medium (2 - 15kg) sculptures in bronze some
years ago using plaster-based investments. But I now exclusively
use ceramic shell as the investment material.
I have been struck by the number of threads dealing with casting
problems, and would like to share some of my experiences. First of
all, I have very few problems casting sterling silver for items at
least 2mm thick. I think the maximum was perhaps 20mm thick (a
solid silver seahorse, which I later set with small ruby eyes). In
particular, I never seem to have a problem with firescale, and
rarely have problems with burnout. This I do in the open air using
a gas torch, and I stop only when there are no more visible flames,
which takes about 5 minutes for a jewellery item. Carbon deposited
on the outside of the shell doesn’t bother me; it is a good reducing
agent.
Also, and this may interest you, water may be helpful during the
casting process. The ‘sealed bomb’ system advocated by David Reid
means that a sealed crucible, along with a charge of metal and a
small piece of charcoal, is heated to a suitable temperature to melt
the metal. I found that water in the shell seemed to have the
effect of displacing all air with steam, albeit at over 1000=BAC, and
this seemed to give an improved surface finish on both bronze and
silver, though occasionally with some interesting patinas. To
understand this you need to know that the ceramic shells are very
porous to gases but not to liquid metal.
It is generally accepted that steam dissociates only to a negligible
degree below 1700=BAC, so I don’t think there is any chance of free
oxygen. However for those using plaster-silica-based investments,
the plaster will normally decompose at casting temperature to give
significant amounts of sulphur dioxide, which can be extremely
reactive.
Over the past 6 years, my students have made something like 1500
silver, bronze and pewter items varying in size from minute finger
rings as thin as filigree to 200mm miniature sculptures. Some of
the items had stones set in situ. Is there anyone else 'out there’
using ceramic shell for jewellery casting?
I use investment, but I would like to give shell a try
To those of you who have been discussing ceramic shell casting; is
this the same as, or similar to core sand casting. When I was in
graduate school, hard to believwe, 40 years ago, some in the
sculpture department were doing what they called core sand casting.
They were making models using styrofoam and then covering the model
with a thin layer, maybe 1/8" to 1/4", of a mixture of sand and a
resin and catylist. There was no need for a burn out as when the
bronze , or other metal, was poured, the foam model disintegrated on
contact with the molten metal. Is this similar to or the same
process as ceramic shell casting?
Over the past 6 years, my students have made something like
1500 silver, bronze and pewter items varying in size from minute
finger rings as thin as filigree to 200mm miniature sculptures.
Some of the items had stones set in situ. Is there anyone else
'out there' using ceramic shell for jewellery casting?
I use ceramic shell for bronze sculpture and have tried doing a bit
of jewelry (small key chain units in white and “gold jewelers
bronze”) with some problems. I didn’t attempt but about 6 castings
with this and part of the problem was the metal types (both
temperature variation “intolerant” - easily overheated, especially
the white alloy). At this point, I would use centrifugal or vacuum
casting in investment verses ceramic shell just for the ability to
get metal “pressure” into the casting void via vacuum or centrifugal
force verses relying on just gravity to fill the voids.
Again, this is all based on a rather short (6 tries) and potentially
problematical (alloys) so not really a good try. But with MANY hours
in each shell in wax work, treeing and using brand new crucibles with
relatively expensive alloy (compared to Everdure Bronze) I just
couldn’t continue experimenting. I do love the shell process for
regular art castings however…
John Dach
MidLife Crisis Enterprises
Maiden Metals Foundry http://www.MLCE.net for some examples of my wife’s work and my casting.
... I used to cast small to medium (2 - 15kg) sculptures in bronze
some years ago using plaster-based investments. But I now
exclusively use ceramic shell as the investment material.
Excuse my ignorance but could you offer, or direct us to, more
on shell casting? I’ve Googled around a bit and found
this, but it’s only a teaser really:
“The ceramic shell technique begins by dipping the gated wax into
vats of slurry followed immediately by a bath of sand. This process
builds a very thin wall of silica around the wax. When repeated
approximately 9 times, allowing for drying time in between dips, a
hard ceramic shell, about 1/2” thick, forms around the wax." (see Bronze Casting Process - lost wax — ModernArts)
I’ve also seen reference to this technique in Sylvia Wicks’ “Jewelry
Making Manual” (p.138-9). Mention is made there of using “colloidal
silica” and “molochite” neither of which appears in your everyday
jewellery supply catalogs.
I for one would certainly welcome more particularly
suppliers.
While the process was used during WW2 in a somewhat different
form the base process used today was developed in the 60’s as an
industrial casting process to make high value complex parts. It is
very widely used today To cast both common and exotic metals- see:
The foam casting is NOT shell casting or really anything like it. It
has some fun and very functional aspects but is just not the same.
Foam “can” be shelled but I personally would not, not the detail
available in foam as when using wax models.
As for more you might try http://www.ArtMetal.com and
look in the archives. Also questions can be put out to the list but
I think if the question is not specific, you will be directed to the
archives as there has been much discussion over the past few years.
Also look at http://www.remet.com and also Ransom Randolf, both
manufacturers of ceramic shell materials. Also look in Thomas
Register for Ceramic Shell Manufacturers/suppliers.
If you have specific questions, I will answer them if possible.
Also could get an idea of the process by going to a local art
foundry or industrial foundry using the shell process. It as
originally developed to cast turbine blades and the like. It has
been around for 40-50± years in one form or another.
Molochite is a “grog” added to porcelain clay bodies to increase
their strength, so it should be available through ceramic suppliers.
if you buy it on line, you’ll probably have to buy a lot, so I would
check with a local store.
I’ve used silica in ceramics, too, but I’ve never heard of
"colloidal silica"–maybe a ceramics supplier would know about it.
Also, the UK books often refer to things that are more common in the
UK than the US–I think the process I used for firing small thin
porcelain pieces in a saggar filled with silica sand was one I read
about in a UK book, but never saw elsewhere (although, given how long
ago it was, I might be mistaken about the origin of that particular
library book).
Molochite is a "grog" added to porcelain clay bodies to increase
their strength
Grog is fired, ground up clay, and it is added to clay bodies to 1)
make them stiffer when wet, so they will stand up and not slump and
2) to reduce shrinkage (since the grog has already shrunk when it
was fired the first time).
In 20-some years in ceramics, I don’t remember coming across the
term “molochite”. But then, my memory isn’t what it might be…
Several people have requested ceramic shell casting details, so here
goes:
I mix up fine molochite powder, 200 grade, with colloidal silica
solution to make ‘shell liquid’ to paint the wax model. The wax has
already been treated to prevent it repelling the water-based
solution by spraying lightly with hair spray, or painting with
debubblizer. The wet surface is immediately dusted all over with
medium molochite sand (60’s grade). Using a cold fan helps speed up
the drying of the layer to about 1 hour in warm, dry conditions.
Another layer of shell liquid and sand is applied which dries in
perhaps about 40 minutes. These two layers catch the detail on the
wax, the second layer could be omitted if the first layer was
already perfect. Then two or three more layers are applied, but this
time dusting with coarse molochite sand (16/30’s grade). These dry
in perhaps 30 minutes each, and are used to build up the thickness
of the shell. This total of 5 layers (about 4mm thick) is amply
strong enough even for 15kg sculptures; a minimum of 1 layer fine
sand and 2 layers of coarse sand will do for jewellery if you are
practiced and have a delicate touch.
Although it is time consuming to do these layers, normally one would
batch several waxes for the same process. I usually add graphite
powder to the shell liquid for the first two (medium sand) coats; it
makes the investment easier to remove and improves the surface
finish of the cast.
I keep the shell liquid mix in a strong rubbery bottle (a kind of
thick resilient plastic), so that I can THUMP it on a bench while
rotating it regularly (with the top on, of course). This usually
breaks the contents up into a usable state after less than 5
minutes, and has to be done whenever the contents are left
undisturbed for more than an hour. This technique works perfectly
even if the bottle hasn’t been disturbed for a fortnight. The bottle
has to be replaced often since it gets rather roughed up.
Once the layers have been applied the wax can be burnt out, either
by putting it straight into a red-hot furnace which simultaneously
hardens the shell and melts / burns the wax; or by using a gas torch
played on the outside, working from the exterior openings to the
hidden cavities, so that expanding wax always has somewhere to go
other than by breaking the shell. The shell is so porous that this
is a relatively rare event, since molten wax can be forced through
the shell.
In the UK I now have to buy colloidal silica solution in bulk (25kg)
at around =A33 / litre, since the St Martins foundry, my previous
supplier, closed down. I would be very surprised if you couldn’t get
it in the States a lot easier and cheaper. Molochite (also referred
to as mullite) is just fired (or ‘calcinated’) china clay and is
offered in a variety of particle sizes from powder to coarse sand by
ceramic suppliers (for pottery glazes etc.)
Molochite is made by firing raw low-iron kaolin to very high
temperatures to bring about maximum conversion of the clay
crystal to crystalline mullite (usually 95%+). The latter has
high mechanical stability and resistance to thermal shock.
Molochite is available in a wide range of sizes (from 8 to 325
mesh) and in dedusted form. It is a very uniform materials. It
can be used as a very white firing porcelain grog and
aggregate material. However, its chief use is in the
investment casting industry, where successive coats of
increasingly coarser molochite slurry are applied onto wax
models. After drying, the wax is melted out and the molten
metal poured in.
Since molochite is used for mechanical purposes in most
applications, its chemistry is not usually a consideration.
Jim Binnion
James Binnion Metal Arts
Phone (360) 756-6550
Toll Free (877) 408 7287
Fax (360) 756-2160
@James_Binnion
Member of the Better Business Bureau