Sand casting

Jewelry Discussion
1

Dear Riccardo et al.

I think, I’ll go ‘Orchid’ on this subject, as I have already got a
few mails about it and it obviously is of major interest to a larger
forum. So here is the answers to your questions:

Sand casting is a method that has a lot of interest for me. I have
experimented with it a bit and find the directness and immediacy of
the method quite rewarding.

A couple things that I would appreciate hearing about from you aRe:

  1. What do you use for frames? Here, there is usually just one small
    frame that you can get commercially. I am thinking to make my own set
    from welded angle iron, but there may be better ways of doing this.

I normally use either cylindrical frames made of steel or some cast
iron frames in the shape of a bottle, the latter ones being of East
Indian origin (so my supplier tells me). I have also tried some
homemade wooden frames, but not with the same success.

  1. What kind of sand are you using? I can get all types of sand from
    foundries in the area, but you have to buy it in 100 pound sacks. Is
    there a particular type of sand that you use and like? What about the
    Delft Clay?

I use a type of ‘sand’ that I buy from one of my tool and metals
supplier. It has a dark red colour and an oily feel. I heard that it
consists of finely crushed bricks mixed with oil, and I have tried to
put it under a microscope. It looks exactly like some crushed brick,
which I tried to make just as a test.

  1. Do you have any written guidelines for the way you work?

Nope, but Martin Niemeyer of The Netherlands just sent me a thorough
description, which I - and I hope, I have your permission, Martin, -
will copy heRe:

"Description of the Delft clay casting method.

  1. Place the ring with the inner connecting rim on top, on a flat
    surface. 2) Fill this ring with the Deft clay, and ram this stuff in
    with a hammer. 3) Wipe the surplus of with a ruler. 4) Put the model
    half way down in the clay. 5) Cover the filled ring with talcum
    powder, and brush it with a soft brush, all over. 6) Place the second
    ring, with the marks toward each other. 7) Fill this second ring with
    clay, ram it , and remove the surplus. 8) Remind the marks and take
    the two rings apart. 9) Remove gently the model. 10) Push with a pin
    (round 4 -5 mm) from the model print, to the outside of the casting
    ring. Also make 2 -5 ventilation holes with a 1.5 mm pin. 11) Make
    with a pocket knives a casting cone around the 4 mm hole. 12) The
    casting cone should directly connect to the model print, and the
    ventilation holes should connect to the, most far away sides of the
    model. 13) Place the two ring together, reminding the marks. You are
    now ready to cast. 14) The melt should be heated during casting. Take
    enough material so that there will be enough force to push the metal
    in the mold. Also the meld should be as hot as possible so that there
    will be no blocking of frozen metal. 15) The casting clay will burn 1
    mm in, remove this and the rest can be reused.

… and may all the gods of good casting be with you.

Kind regards from Denmark, where the autumn is colouring the leaves.

Niels Loevschal

2

was: Casting in tuffa

My advanced class and I have been having a great time exploring
primitive casting methods-- sand

Noel, or others: Can you explain a little about sand casting. I
imagine the sand needs to be prepared somehow, probably with oil, to
keep the form while pouring. I am aware of the commercial casting
sands, but there sure is more to explore…

Thanks, Andreas

3

Pick up a good book on foundry work. The Lindsay catalog has a bunch
of them. Bottom line: It’s not hard, as such things go, but for best
results you need sand prepared for casting. Doesn’t have to be oiled.
Sand with the right amount of water works just fine.

RC

4 
Noel, or others: Can you explain a little about sand casting.

I have been experimenting, but with methods, not materials. I use a
sand casting kit bought from Indian Jewelry Supply. It includes a big
can of treated sand that is pretty sticky. Also a crucible, “parting
powder” (talc, or something like it), boric acid as flux, and a steel
two-part frame with a pouring spout opening. Rio also sells a kit, a
“Delft casting kit” with similar sand and a tiny round mold that I
find nearly useless.

You pack sand into one side of the mold and press your model into
it. The model must be able to stand up to having the sand hammered
down on it, and must have no undercuts. It can be a twig or other
natural object, a metal model, a baked polymer clay model, etc. One
student cast Barbie shoes. The model is pushed in halfway; the sand
is dusted with powder, then the other half of the frame added and
filled with sand. The more firmly the sand is packed, the better the
detail of the result. I use a hammer to compress the sand.

The frame is opened and the model carefully removed. A sprue opening
is carved in both sides of the sand along with vent openings from
extremities of the cavity to the sides of the frame. The frame is
bound together with binding wire, set upright, and molten metal is
poured.

I have learned that the larger the sprue, the greater the chance of
success. I have had good results from pre-heating the whole mold to
300-400 degrees to slow the solidification of the silver. Higher and
the sand begins to burn. I have even filled the mold successfully in
a centrigugal casting machine, but I cannot promise that if you try
it, the sand will not fly all over (along with the silver, of
course).

The sand exposed to the hot silver has to be scraped off and thrown
away, but the rest is re-usable.

This method will not work very well for some large models, small or
thin models or areas of models, but then, if it doesn’t work, you
re-cast the silver and all you’ve lost is a few minutes. And it does
not harm the model, which is great.

Noel

5 
... Rio also sells a kit, a "Delft casting kit" with similar sand
and a tiny round mold that I find nearly useless.

I use the Rio kit a lot, the sand’s excellent for detail and the
rings are fine as mold containers. I had some more made at a local
engineers shop, same size and bigger, from steel pipe. I’ve even
altered the Rio rings to allow a sideways pour into the mold. Great
for casting that prefer a sprue to its side.

You don’t have to pound on the clay to make a good casting, teaching
the process to 15-16 year olds I let a few go through that were only
lightly tamped. They worked, to my great (inward) surprise.

The method is only good for two-part molds but I’ve done quite
complicated shapes that needed a fairly wavy partline.

The parting agent is just that talc you puff all on your baby.

Oh, and I’ve worked out that for vents you only need the finest of
channels, like you get pushing a earwire-sized piece of stg wire
(0.8mm) into the sand. Several if needed.

I never pre-heat the molds, but you’re right No=EBl about the sprue
size: bigger is better. About 5mm dia.

Brian
Auckland NEW ZEALAND

6

One can buy foundry sand too. its cheaper and works well if you want
a textured outcome as its a bit coarser than the delft clay. The
cope and drag otherwise known as the 2 part molds can be made
inexpensively main thing to remember is to make a mark with a saw on
the outside of the frames to always have a point of lining them up
as one will be opposing the other - i make an offset handle, or just
attach a desk hardware pull (sort of like an ovoid cup)to the frame
of the drag to lift it away cleanly. If they fit too perfectly
parting them is less than foolproof.

Another thing to do to avoid the binding wire or other time consuming
stabilization methods is to route 4 holes on each frame’s outer side-
a4 “c” clamps can then be easily tightened down to hold the whole
together. You can actually make your own clay as well; bentonite or
green clay is mixed with oil until it remains firm when squeezed in
ones hand and doesn’t crumble- if too wet add more clay. too dry more
oil. It is also available in a 15lb. bulk bag from JSP or Grobet
affiliated suppliers for about 12 dollars US.

the more detail in the model, the more tamping is necessary though
the previous posters disagree- and that’s OK- we can disagree. It’s
all actually related to the sand’s humidity and the ambient temp.
one is working in. However, everywhere i have lived on the planet,
good compression is essential for consistent results… Starting from
the outside corners begin tamping with any wooden implement that is
level and has a straight edge and a rounded edge for maximum effect.
I have seen people use a piece of angle iron too…I prefer wood as it
allows the operator to feel a certain hollow thud when it is
adequately tamped down all over and working from outside to the
centre seems like what I learned a long time ago from master casters
in India.

I have always done it that way as the sand compresses much more fully
with no outer edge gaps as sometimes occur when a jeweler/caster
works from the centre outwards…

Another thing - after embedding the model and tightening the cope
and drag and then parting the two,and dusting with jewelers talc - is
to take a fine wire or scribe and make some gas vents in the clay
which will help assure even filling when the molten metal is poured-
particularly with gold. the sprue can be attached to the wax model in
particularly before embedding it. Otherwise te sprue should be hefty
relative to casting in any other method, and a conical pouring gap
half the width of the model assures the metal will create enough
pressure to force it down into all the recesses, details and azures
that may be present in your model. I only pre-heat when there is a
great deal or detail in the model or you are casting many things on
one “tree” - rings for instance. calclating the metal necessary is as
easy as weighing the model and multiplying pi x the weight of the
model plus 1 gram for each 3" of 8gauge sprue. When working in Pt the
formula is a bit different,multiply pi x and add, as the metal is
softer but heavier.

Don’t be too sparing with the talc,nor too heavy. If you have
aligned and compacted correctly and allowed for separating by setting
a handle of sorts in the plan when constructing the molds yourself.
After pouring, if the used clay becomes too dry enliven it in a
closed container with a bit of water and oil, leave in the sun and
when condensation is just visible it’s rehydrated. It does loose its
effectiveness though after three or four pours. When it aggregates to
cement like clumps that are no longer friable it’s used up- work it
into garden soil that needs to hold water more than it does for a
water loving plant or, more specifically - for roses… I could
expound but won’t. If anyone needs more questions answered feel free
to contact me.

RER

7

If you want an introduction to do it yourself foundry work, order
Dave Gingery’s book on the charcoal foundry from Lindsay
Publications.

About half the book is devoted to sand casting for absolute
beginners, including how to make most of the tools you’ll need.

RC