Casting flask blowout

Jewelry Discussion
1

Hi orchid,

I’ve been learning how to cast silver for the last couple of months
and after lots of trial and error, mostly error, i achieved some
small successes casting 60-70 grams at a time in 3x3 flasks, so i
decided to move up to 3x4 flasks with 100- 150 gram charges. the
problem is that I guess i tried to cram too much into these flasks
and when I had finally melted all that metal with my "little torch"
casting tip, my partner got the flask onto the vacuum pad, turned on
the vacuum, the gauge wouldn’t go past 17, there was a horrible
sucking sound, and it became obvious something was wrong. I guess 1/4
inch of investment above the top of the tree isn’t quite enough?

So now that I have a bunch of investment in the filter and hose of
my vacuum line, and it appears some fine white powder has also made
it past the filter and into the line going to the "cast/invest"
switch on my VIC 9 casting unit, is cleaning it out of there and
changing the oil going all I have to do to deal with this?

Should I install another filter in the line since the stock filter
doesn’t seem adequate? what about the metal that was probably
overheated and abandoned in the crucible when it became clear that
the flask had blown? is it still usable? should I treat it like an
old sprue button and mix it with fresh grain?

I switched investments recently from westcast caster’s choice to
satincast and it seems like satincast is a bit softer, maybe that
has something to do with it?

Also, I’m thinking about a bigger torch for melting cause the “little
torch” is just too little. any suggestions? still learning the hard
way,

Douglas

2

My dear Douglas,

To prevent the casting flask blowout following suggestions should
work.

  1. Proper water powder ratio is very important if you use more water
    then recommended then the investment will have more water in it and
    while improper burnout the steam will crack the plaster from inside
    and the result is blowout while casting.

  2. The wax pattren should be minimum 18mm below from the top edge of
    the flask, and 12 to 15mm from the wall of the flask.

  3. Burnout very slowly. In your case of 3x3, 3x4 inch flask or 4x4
    flask,if you put one each. Then put the flask in the oven after one
    and a half hour of investing preheat the furnace(oven) at 100
    degrees centigrade,load the furnace hold for one hour. Raise the temp
    to 150 DC(Degree centigrade) hold it for one hour, third step is to
    raise the temp to 200 DC hold it for one hour, fourth step is raise
    the temp to 250 DC hold it for one hour, fifth step is to raise the
    temp to 300 DC and hold the temp for an hour, sixth step is to raise
    the temp to 720 DC hold it for 2 to 3 hours or over night if you are
    going to cast in the following morning, then drop down the temp to
    your required casting temp between 400 to 650 DC depending up on the
    thick and thin castings.

Try this and you will be able to prevent the blowouts.

Best of Luck

May God bless us all with total health, Mental, Physical, Spiritual
and Social.

Be happy
Umesh

3

Hello Douglas,

we use the same vacuum machine with great success, casting up to 300
grams, but you would need a different torch, Uniweld Casting torch
with a rosebud tip from Otto Frei would be fine. The strange sucking
sound could be caused by a bad seal, the top of the flask could dirty
or uneven. Check you investment mix (very important) and your
casting temaratures (very very important).

Good luck
Hans
www.hansallwicher.com

4

Dear Douglas,

Sorry about the mess!

I would leave at least a half inch at the top of a flask and between
the sides of a flask and waxes on the tree.

You’d be better off with an Acetylene/Atmospheric torch for casting
that size flask / quantity metal, using a brazing or melting torch
tip. Of course if you want to do even bigger castings you may want to
invest in a oxy-acetylene or oxy-hydrogen set-up.

As far as cleaning-up your vacuum caster, that’s no fun. I once had a
student blow a flask and continue to pour pewter into the colleges
vacuum casting machine. I replaced all the hoses, the oil, the
filter, and picked little bits of pewter out of the pump. I was
surprised when the vacuum pump still worked. I installed a second
filter after that incident and it did save the day when a second
student blew out a flask about 6 months later.

Your metal can be salvaged by getting it molten again and pouring it
into a bucket of cold water to make shot, than you can easily add it
to new casting grain a little at a time. Good Luck!

Nanz Aalund
Associate Editor / Art Jewelry magazine
21027 Crossroads Circle / Waukesha WI 53187-1612
262.796.8776 ext.228

5

A good way to prevent metal from entering the vacuum pump is to put
a trap in the vacuum line.

Cut about 4 inches off a 2 inch diameter copper tube. Get two caps
for the tube. Solder one cap on one end of the tube. Drill two holes
in the second cap the diameter of your vacuum lines. solder this cap
on the other end of the copper tube. Place the line from the vacuum
pump in one hole and place the line from the vacuum table in the
other hole. The tubes should extend about a half inch into the copper
tube. Seal around the cap and two vacuum tubes with a caulking
compound.

For added protection a copper scrub pad can be packed into the
copper tube. This will definitely impede the metal from flowing into
the line to the vacuum pump. Unfortunately any metal sucked into the
copper tube will combine with the copper pad.

Either way if metal is sucked into the vacuum line it will be
trapped in the copper tube and not sucked up the line to the vacuum
pump…

A drawing is available upon request.

Lee Epperson

6

Hi all,

thanks for the good advice. This week I followed the rules for
investing and burnout down to the smallest detail, I measured the
height of the sprue tree in the flask to the millimeter, weighed the
investment to the 10th of a gram, etc… I still had some Westcast
investment left so I invested one flask with that and the rest with
Satincast, and believe it or not the Westcast casted perfectly, but
the rest with Satincast blew out again. Now that I think about it
both of the canisters of Satincast I ordered had broken seals when I
got them from the supplier. I disn’t think it would be a problem
since they were in plastic zip bags… Could it be that it absorbed
enough water from the air to make it that fraglie? anyway, despite
the frustration of having spent a lot of time sprueing all those
blown flasks I’m kind of relieved to find that it may not be my
fault… has anyone else had problems with the 2 kilo canisters of
Satincast?

Thanks Orchid for the moral support. If this was easy i guess
everybody would do it.

Douglas

7

sorry to hear about your casting problems I,ve packed over a 120
pieces into a 4x6 inch flask came.25 -.5 inch from the end, so you
cut it close but play it safe. something to keep in mind when
investing the water to powder ratio has this effect less water more
dense, less poris, stronger more water more pores air escapes easier
if your close to the top of flask add a little more powder. every
time you invest swirl a little bit of investment on top so you can
see how thick it should be. some day this will save when you have
done your math wrong. for now follow the instructions A few tips to
help with casting I use centrifugal casting, I’ve cast hundreds of
pieces with a vaccum and there is nothing wrong I just prefer
centrifugal for smaller flask. buy a bigger torch this is not an
option it must be done! coat your flask with boric acid powder. a
good glassy layer to clog the pores pre heat the flask to glow, how
hot depends on how much metal (this is very important) dump your
metal in sprinkle a little boric acid powder grab your flask, keep
in mind that fire blowing into your flask could cause problems, not a
factor if your using vacuum casting at this point the metal should
be starting to melt sprinkle a little more boric now stir with a
carbon rod (crystal rods chip to easy) or move your crucible in a way
that moves your metal. this helps get the metal off the bottom and
also give you a viscosity gauge very important. you can over heat
your metal but it is more likely that you will under heat it. You
could do an experiment and get your metal melting and when you think
your ready to cast hold for 30 seconds before pouring you should
notice that the metal will become more viscous (liquid) think of
syrup compared to honey if you have any more ? email me. I dont look
at the ganoksin site to often