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Delft casting in two parts?


#1

Question for anyone with casting experience:

I have purchased a Delft casting kit. My first attemp was a thin
pendant, square, around 1.5 inches on a side. I tried a couple of
times but was unable to get a full cast. I have seen some of the
suggestions (get the metal hot, use borax, keep the model level,
etc). They seem helpful but I’m still having problems. I’m wondering
it it would be possible to make two sprue channels and cast it in two
pours. Would the cast turn out the same as if it had been cast all at
once, or would there be a visible line or weak spot in the cast,
where the two pours meet?

On a related note, I have another piece that I am going to cast. It
is a bit smaller than the first one, but still thin in the center
2/3 of the model (oval shape in this case). Around the edge, it is
thicker, around 3.5mm. Should I put the sprue in the center, or at
one edge, where it is thicker?

Todd Welti
Living Color Opal and Intarsia


#2

Hi Todd,

my experience has been to use a crucible to get the metal hotter,
rather than a shallow melting dish.

Make your sprue about 1/4" or fatter.

Sprue always to the thickest section.

Use more vents and then reduce the number gradually when you get
good results.

Two pours will leave you with a cold shut which will pull apart. I
found it ok for 2mm and above but I have seen thinner done
successfully,

regards Tim Blades.


#3

I have had better luck getting sand casting to fill if I pre-heat
the mold to about 500 degrees before pouring. I have even put the
mold in the centrifugal machine with no ill effects!

Noel


#4

you might try 2 or 3 sprues around the sides got to get more metal
in faster

Don in Idaho


#5

With the “delft clay” material, you need to vent your mold. I make my
vent risers by poking with a piece of 18 or 22 gauge wire. In the
case of the piece described above I’d vent at the corners. The risers
are easy to nip off.

This type of oil bond sand is denser than the coarser grades of
casting sand, so the venting is more necessary. As counterintuitive
as it sounds, sometimes having the end of the mold opposite of the
gate slightly elevated helps with some shapes (it keeps the molten
metal from running out in a thin, but not filling sheet).

Ron Charlotte – Gainesville, FL


#6

Although this info is about Petrobond sand casting, you might find
it useful for your application, http://www.granthams.com/Repair .
There is more related metalworking stuff at
http://www.granthams.com/Projects

Have fun!
Rod


#7

Well, I tried twice more and still no sucess. Perhaps someone has
some suggestions:

The wax: around 1 x 3/4 inches, thin in the center (perhaps 1.3 mm)
and thicker around the edges, aruond 3 mm. There is some moderately
fine detail in the wax. Note that i am using Argentium casting
grain, from Rio. It this case it is around half new grain and half
leftover silver from the last attemp(s).

Attempt 1: sprue around 4 mm wide at the narrowest, where it meets
the model. I sprued it at the edge of the model, where it is
thickest. 4 air vents on opposite side. I used around 1.3 times the
calculated silver weight, to allow for the silver in the plug and
air vents. Heated it using propane and oxygen, and a melting
tourch-tip, using a bit of borax added right before pouring. The
result was around 1/3 of the model filled.

Attempt 2: basically the same sprue and vent arrangement. I added
more silver this time, over 2 x the weight of the actual model. It
took around 7 minutes to melt the silver this time. I heated it up
more than the first attempt. It was definately liquid, not really
glowing, though the crucible under it was a bit. I swirled it around
and continued to heat for around a minute after it was liquid. This
time I saw silver in the air vents and thought it had worked. What
actually happened was it filled the outer thicker part, but not into
the center (thinner) part of the model.

I also noticed (and have noticed on other attempts) that the part of
the model that does fill doesn’t have the full detail of the model.
It looks like it has been sanded down. So, I wonder if the metal is
hot enough. How does one know? i dont want to overheat either.

Is the Delft system only good for larger items like rings? I am
interested in pendants right now, and they cant be too thick, or
else they are too heavy. Perhaps it wont work on thinner items. I do
have a miling maching, so perhaps i could cast it thicker and plane
down the back after using the milling machine.

Any more suggestions?

Todd Welti
Living Color Opal and Intarsia


#8

Hi Todd,

you seem to be nearly there, use a larger sprue, 6mm is normal, vent
the centre of your pattern, the metal flows around the thick part and
then the air in the centre has nowhere to go. Don’t worry about
getting the metal too hot, if it is not filling it is unlikely you
are doing that.

A can lined with ceramic fibre,with an inlet for the flame and a
hole in the top makes a great furnace, it will speed up your melting
considerably.

regards Tim Blades.


#9
Is the Delft system only good for larger items like rings? I am
interested in pendants right now, and they cant be too thick, or
else they are too heavy. Perhaps it wont work on thinner items. I
do

Hi, Todd. I had to look up Delft… Your pieces are too thin. Your
process is sand casting, in general - imbel a model into compound,
remove model, pour. It has been said, truthfully, that you can cast
anything if you sprue it right, first off. You have two strikes
against you in the sand casting method, though. First is that your
flask is cold - the metal/crucible heat differential is so high that
it means a lot. Second is the gravity pour. You have no push behind
the metal. The real solution is to go to lost wax and centrifuge or
vacuum, but assuming that you are doing this for a reason ($?)-- You
are exceeding the limitations of your process. You have to either
sprue much more or thicken your models. If you use spruing then spue
to the places you’re not getting until you do get them. It might be a
lot of spruing. And you may never get fine details, especially if the
flow is at an angle to the details, i.e. if you cast school ring
where the push is towards the letters, it will pack in there. If the
metal comes from the side and has to make a right turn into the
letters, they may not fill - an example. 1.3 or less mm is awfull
thin for a gravity pour - if you have piercings it is doubly so - I’d
think more like 3-4mm for a 1" across pattern, likely. Finally, your
metal should be “swimming” and no more. Overheating the metal is the
very definition of counterproductive. Gravity pour into a cold flask
is only going to get you so much in results. It’s good for big solid
men’s rings, something like a belt buckle, etc. Someone recently cast
a real bee into silver with a centrifuge, though, and that’s simply
impossible with the methods you’re using.

http://www.donivanandmaggiora.com


#10
The wax: around 1 x 3/4 inches, thin in the center (perhaps 1.3
mm) and thicker around the edges, aruond 3 mm. There is some
moderately fine detail in the wax. Note that i am using Argentium
casting grain, from Rio. It this case it is around half new grain
and half leftover silver from the last attemp(s). 

I’ve cast similar sized items in Delft molds in pewter, classic
sterling and bronze. Honestly, I’ve never worked with the Argentium
stuff, so I really don’t know how it behaves on a gravity pour.

You can try moving the gate sprue to the thinner section at the
center. It’s more of a headache in terms of post-cast cleanup, but
that will normally do the trick for a thin center/thicker edge
object. Simply putting a couple of vents in the center region may do
the trick.

Ron Charlotte – Gainesville, FL


#11

Todd

your last email had these statements----

I heated it up more than the first attempt. It was definately
liquid, not really glowing, though the crucible under it was a bit.
I swirled it around and continued to heat for around a minute after
it was liquid........" and "......I wonder if the metal is hot
enough. How does one know? i dont want to overheat either. 

Which led me to respond to your question…when I originally read
your post I thought…“hmmmmm…sounds like the metal isn’t staying
hot enough long enough…” These are my impressions-

When you say it is liquid, is it at a “rolling sphere” liquidity? (
it will look like a rolling orange mercury marble) If your metal
"freezes" you could add a little boric acid powder -a sprinkle- and
a stir with a graphite rod…keeping the flame on it as well of
course…

Are you pouring from the crucible very closely… right at the
opening in the sand? (you need to keep your torch on the lip of the
crucible while you pour so that the metal has no “cooling time” until
it fills the cavity in the mold) and be sure that you have plenty of
vents! it will take a few minutes to clean them up but worth every
last detail that they preserve…

Other than that- perhaps it is an argentium question? In that case
Cynthia Eid could give her well researched opinon- (I am more
familiar with casting sterling and golds)

  • I really love having the Delft casting available- great for design
    experiments and quick “referance duplicates” of pieces I want to
    have professionally cast.

Hope this helps- Happy casting!-

ciao- Maureen BZ (happy to have Tucson temps finally go below 80
degrees!-see you at the gemshow!)

Don’t worry, B. Zappy


#12

Thanks for the many suggestions. I’m going to try once more. Of
course, a vacuum or centrifical casting system would work better,
but I was hoping to avoid the initial outlay of $.


#13

Hi Maureen,

The melted silver is a rolling sphere, though not orange, it looks
more like mercury. I sprinkled in the borax and swirled the crucible
around 'till it dissolved. I don’t have a graphite rod. I did indeed
keep the torch on the silver as I poured it, and the distance as
short as possible. I also wondered about whether it has anything to
do with the Argentium. To complicate matters, there are two types
that Rio sells. I have to dig up my old receipt to remember which
one I ended up with. I am going to try putting the air vent(s) over
the thin part to see if that helps. I have wondered about making two
sprues, but dont know if splitting the flow of silver when pouring
will cause porosity or faster cooling or other problems. I guess I
should just try it.


#14

Todd

A good substitute for a graphite rod is the ordinary pencil. It might
ignite, jus be careful. Yesterday I finally set up my casting
equipment: a small dental inlay furnace and a small Torit vertical
casting machine. I waxed and invested a ring with 14 year old never
opened cristobalite (the manufacturer said it had a shelf life of 2
years!). Casting came out great. Used what is called technique
metal: it does not roll when ready to cast, you have to poke it with
a pencil to verify full melt. Anyway it worked.

Charles Friedman DDS
Ventura by the Sea