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How large should a sprue hole be

Hey i posted about 2 weeks ago about some casting problems i was
having. I’m still trying to troubleshoot but i think my main problem
is not having an oven to bring up the temperatures of the castings
high enough. I’m getting totally incomplete castings. I use a
propane burner with a flowerpot setup and my guess is its simply not
hot enough perhaps.

One question i hade was how big should the sprue hole be at the
buttom of the flask? Is 8 gauge large enough or should it be bigger?



I have used the propane and flowerpot many times…though I prefer
an open coil electric hot plate. Be sure the porpane tank is full
and, at full on, it should run for about 2.5 hours. Make sure the
flower pot is covered by at least two layers of heavy aluminum foil.

Re the sprue…8 gauge is much too large and remember, you can only
do one flask at a time and it cannot be too large. I usually use old
tomato paste cans as a flask. If the sprue is too large, the metal
will sagg down into the sprue and go into a semi-hardened state,
blocking the remaining metal from entering. I normally use two or
three (sometimes even 4) narrow strips of 20 ga wax sheet placed
closely together (about 1/2 mm between them). That way, the metal
will not loose its surface tention and sagg.

An alternative way to heat the flask is to use a charcol ‘starter’.
This is a stainless device obtainable at most ‘outdoor’ shops. It is
thin metal, about 4-5’ around with a piece of screen inside about 1/4
up from the bottom and a wood handle on the outside. Place about a
dozen briquettes into the device then the flask, then fill it the
rest of the way wit briquettes. Light this off and wait about 2 to
2.5 hours. Does a great job.

Cheers from Don at the Charles Belle Studio where simple elegance IS
fine jewelry!

I found that casting small like that I ran into trouble when I over
crowded the flask now I do 1 ring at a time in 2 inch flasks the hole
is 3- 8 gauge wires wide at top this is working well with my potato
steam casting.

I can do 6 flasks a day(tiny oven) and while they burn out do other
things so it is increasing my production and I enjoy it immensely. my
Goals are to do at least 6 pieces a day so to cast 6 makes things
less stressful at the bench.

Silver & Cameo Heritage Jewelry

Sayed, I gather from your post that you are using steam casting.

I don’t think the problem lies with your burnout method,

The problem may be with your need for multiple sprues----especially
as the 18 Ga. are so small.

With steam casting, you need the small sprues, but you need several
of them to make sure that your molten metal is reaching all areas of
your model.

Make sure you have sprues leading to each area of your carved item.
With Steam casting it is better to over-sprue rather than

After heating the flask you should see no blackness (carbon) around
or from the sprue hole. If you see this you are not done burning
out. You haven’t said what you are using for casting. For centrifigal
make sure the metal is completely melted. It should look like it is
bright mirror after fluxing. For vacuum you may not have enough pull
from your pump. If you are using another simpler type of casting the
problem could be you need a larger opening. Sand casting larger
pieces requires a larger sprue. Please give us a little more info.

Steve Ramsdell