Vacuuming investment

Hi all,

I’m wondering if there is a problem with vacuuming investment for
too long? I have heard that if one vacuums until the investment
"boils" that it can actually put air back into the investment. Is
this true? Most people who have commented on casting say they vacuum
for 1.5-2 minutes. If I do this the investment is definitely boiling
away, so I’m not sure how to judge the vacuuming time. Most of my
flat castings are bubble free, but occasionally I have bubbles in the
castings. I’m using a Kerr vacuum system with a small bell jar.


Jim, Running a vacuum on your investment till it “boils” is not
usually a problem. By letting it boil you are not putting air into
the investment but taking it out instead. Where you do run into
problems is letting it boil too long. This has been shown to
actually take some of the water out of the mix, allowing it to dry or
gloss off sooner than expected. This means that you actually have
shortened the actual working time. Lets say that you have a working
time of 8 minutes on your investment. If you run the first vacuum
for 5 minutes then release the vacuum and pour the investment in your
flask and vacuum again you are probably running very close to your
maximum working time of 8 minutes. If you did your first vacuum that
long you may have taken out some of the water making your actual
working time closer to 7 or 7.5 minutes. This is cutting it very
close. By reducing the water content of the mixture when you pour
it into the flask, your mix is now thicker and may trap air on or
next to your model. (This is where the myth of adding air comes in.)
Actual working times vary according to the type of investment you are
using and the type of models your are investing. Check your actual
mixing directions for specific

A rule of thumb for investment with an 8 minute working time.

  1. Vacuum investment, it should raise, fall, and then vacuum for
    about another 30 seconds to 1 minute. ( +/- 3 min)

  2. Release vacuum ( +/- 15 seconds )

  3. Pour investment into flask ( +/- 15seconds )

  4. Vacuum flask ( +/- 2-2.5 minutes )

  5. Total working time ( +/- 6 to 6.5 minutes )

  6. Set flask in a place where it will not be moved for at least 1

Water temperature can also play a roll in how fast it sets up as
well. Another tip, …take the temp. of investment (DRY), subtract
from 156, and that would be what your water temp should be!

I hope this helps. If you have any further questions please feel
free to contact our technical support department and we will try to
assist you with any issues you may have.

Phillip Scott G.G.
Technical Support & Sales
Rio Grande

Hi Jim, Here is the link to kerr’s investment/technical support
page. This should let you know exactly how to mix their investment. If
this does not solve your problem, then , you may want to use a wax
dip which will help reduce bubbles . There are also many well written
articles and ideas in the archives about investing. Daniel Grandi
Racecar Jewelry Co. Inc.

My casting reference warns about boiling the investment, however, I
ignore this warning and boil my investment every time I prepare for
casting. I let off the vacuum for a short time each time the
investment boils . When I am within a minute of the working time for
the investment I let the vacuum off and continue tapping the
investment table to free any remaining bubbles. Of course, if you
boil the investment beyond its working period, you’re going to have
troubles. R.H.