Back to Ganoksin | FAQ | Contact

Investment cracking


#1

The posting from jagman425@aol.com was right on. I’ve been casting
for about thirteen years now and have had a recurring problem with
breakouts and cracking. I’ve gotten lots of advice on the subject and
have read several casting manuals looking for a solution. Many
included too much water as the problem but nobody ever mentioned too
little water.

Many years ago I met a goldsmith at a Jewelers Convention in
Newfoundland. He said he always mixed by “feel” and said to mix the
investment to the consistency of sour cream. I know now that that
would make the investment short on water. In the last few months I
have been mixing my investment and very carfully measuring the
ingredients. I use a very large injection needle to measure the
water. The result has been perfect castings every time. The mixed
investment has the consistency of whipping cream (unwhipped).

I think that books on casting place too much emphasis on what can go
wrong with your investment. They should simply say measure the
ingrediants exactly to the manufacturer’s specs. and if you have
problems, calibrate your measurement equipment. If you still have
problems, then you can rule out improper mixing of the investment and
look somnewhere else.

Now that I am more confident I have gone back to my investment scale
and use a pyrex beaker for measuring the water. I use the hypodermic
needle to fill exactly between graduations. You cannot imagine the
relief I feel now when I have a half dozen custom waxes cooking and
don’t have to cross my fingers at quenching time.

One of the tips I received from an Orchid member was to harden the
investment by adding Boricx Acid. This helped, but didn’t eliminate
the problem. The draw back was that removing investment from the
castings took quite a bit of time. I still use the Boric Acid but in
much lower douses. It seems to give the castings smoother surfaces. I
use about 3/4 teaspoon per pound of investment.

I hope this saves someone the agony that I have gone through over the
years.

Cheers!
Robert Hood


#2

One of the biggest problems with investment cracking is not letting
the wet investment vent the steam before it forms. The slow rise in
temperature is important and then holding it at plateaus until it is
well dried and burned out.

The use of Borax in the mix is new to me. I have not tried it but it
sounds good. How much do you use. I usually add a small amount of
debubblizer to the mix. It helps the air to remove itself from the
mixture. I also paint my wax with it and then with a strainer or
piece of nylon stocking put over a hoop, I slowly sprinkle dry
investment powder on the wet surface. It adheres very tightly and
gives me an exceedingly smooth surface and reduces the amount of fire
scale I get.

Have fun and keep up the good info. It is really great to have such
an interesting network out there.

Walt


#3

Modern investments have debubblizer already in the investment adding
more is a waste of time and can cause set up time problems.

Use de-ionized or distilled water and weigh both the water and the
investment powder with an accurate scale. This will solve most all
investment cracking problems. Cracking is most often caused by too
much water or too little water or moving, bumping, or otherwise
disturbing the invested flasks before one hour has passed after gloss
off.

Investment with water in it will not exceed approximately 212
degrees F until the water is gone from the investment it can’t, no
mater what the oven temp is. If you put a thermocouple in the
investment you will see that the temp stays at 212F until all the
water is gone then it will start to rise. Also the water will come
out at about the same rate whether the oven temp is 300F, 500F or
1000F so the idea that lower temps slow the release rate is not
really true as long as the oven temp is greater than 212F. It has
more to do with the thermal conductivity of the investment and the
shape of the flask than the oven temp. Follow the burn out schedule
provided by the manufacturer they do test this kind of thing.

Jim
James Binnion Metal Arts


@James_Binnion
Member of the Better Business Bureau


#4

I absolutely totally agree with this statement by Jim. Water can only
evaporate at 212 oF …and because of this, if you have gone through a
proper burnout cycle, there can be no water left . Now, if you put
your flask in for one hour… which is not going to work as you can’t
get to 1350 that quickly, there could be moisture left for a short
time but this is pure fantasy ! Daniel Grandi