In our lab at the college, we’ve always mixed investment by “feel” to
teach students how to determine the appropriate consistency.
Basically, our approach has been to use about 75% of the flask’s
volume of room-temp water, then adding the investment and mixing
until it’s the consistency of yogurt/sour cream/pancake batter.
Vacuum for 3 minutes, pour into flask and vacuum flask another 3
minutes. The more experienced students can do this in their sleep
and know exactly the consistency to look for.
Needless to say, though, with beginners among us (and even some of
the old-timers), there has been a decent amount of waste. Since all
of the students chip in to pay for the investment, it only seems fair
to do what we can to level the playing field and keep “beginner
waste” under better control.
Hence the introduction this semester of “The Chart” - the official
mixing chart that comes with each batch of investment that tells you
how many pounds of investment you need to how many mL of water for
each size of flask. We have a good (and new) investment scale and
well-marked graduated beakers. The problem is that if you follow
the chart, you get a very THICK investment that sets up WAY too
quickly. It’s the consistency of a very thick pancake batter when
mixed and you can vacuum it for about 1.5 minutes at each step - at
the most - before it sets up. We had some of the most experienced
students do their flasks this way and follow the chart so that we
could test it and see what the results were.
SO. do any of you know any sources of GOOD and accurate investment
mixing charts? I’m reluctant to spend the time it would take to
thoroughly test and revise this one, but am tempted at this point to
hide all but 3 or 4 flask sizes and work out the proportions for them
and post them. However, if I can get a good chart, it would
definitely help me avoid that step and we could maintain our