One of the things I learned through experience in casting is the
need to keep the investment dry by storing in a well sealed container
and of precision in the proportion of investment to water. The
manufacturers of my investment, Gold Star Ultra specify an investment
to water ratio of 38% to 42%, the thinner mixture for intricate
castings and the thicker for heavy ones. I usually split the
difference by making the water 40% of the weight of investment. For a
85 by 110mm flask I add 750g of investment powder to 300g of water,
in my case rain water. I measure both the investment powder and water
by weight using digital scales that read to one gram. This is a more
than adequate level of precision.
I mix my investment in 2 litre ice cream tubs which fit comfortably
in the bell jar for debubblising. I add investment to water and mix
with a spatula until the mixture is smooth and free of lumps. It
should look and feel like a crepe batter I start a countdown times
set to 8 minutes as soon as I add the investment to water. I
debubblise in the tub for around two to three minutes. Watch for the
mixture to firstly rise and then subside somewhat.
When it bubbles look a bit like cooking porridge then the mixture is
I then pour the investment into the flask and debubbilise the flask.
It is important to keep an eye on the timer and ensure the flask is
out of the vacuum and the top of the investment smoothed over by the
time 7 minutes have elapsed. A handy feature on my timer is a three
minute warning beep.
As a note of warning remember that the silica powder in investment
is potentially carcinogenic so wear a mask with proper dust filters
when handling investment particularly when it is dry. Disposable
paper masks are really not good enough.
I keep my soak temperature when burning out below 700C, usually 650.
I find this consistently gives me a clean burnout. The manufacturers
of the investment I use specify the maximum soak temperature as 735C
but to me this uncomfortably close the point where the gypsum in the
investment starts to break down (around 750C)
After all this you just have to decide what incantation to casting
goddess works for you as you pour the metal.
All the best