Just curious. At what point does investment get really hard. Does
this take place during the burnout period, or can it happen prior to
burnout? If the latter, about how long after it has been poured into
the flask will it become fully hardened--too hard to scratch into?
Depends on a few things. One and probably most important to set up
time, it the temperature of the water and the investment. the hotter
the faster. Also new investment verses old, possibly improperly
stored investment, the new will be faster than older investment and
if there are hard pieces in the powder, it may not set up at all.
How long you stir it has a role in set up time too. There are some
brands that are a bit faster than others and the chemical make-up of
your water (we use distilled so the water is always a constant).
Careful and correct measurements of both powder and water are
important, a little more or less of one or the other CAN make a
difference. We will get out invested cans into the kiln within an
hour of pouring but we burnout differently than many/most but it does
and has worked for us for many years.
Once you get things to work well for you, stay with it. If you want
to try new investments or methods, do so on "dummy cans and dummy
setups" so you don't loose work that you have a lot of time invested
if the experiment fails. Get it to work well for the "test pours"
then start using it on real work.
Most gloss off ia about 9 min. After that it hardens somewhat
Try scratching it with your fingernail after 10 min then 20 and so
Burnout drives out the water and cures it. SD
If your definition of "hard" is "too hard to scratch into", the
answer is never.
It acts much like plaster, but it really isn't plaster at all, and
it doesn't act *exactly* like plaster in all respects. (which is why
we use it for situations that plaster would never tolerate.) One of
the differences being ultimate strength. Even when fully set, it's
not all that hard. You can carve it with a butter knife. (Which is
what I use to trim my flasks.)
I'm not sure exactly what you're asking, but I suspect you're trying
to figure out at what point it'll be too hard to scratch casting
info into? Never.
It's got a shock sensitive stage from about 9 minutes after the
water hits it until about half an hour later, so do *not* handle it
during that phase, but after that, you can carve whatever info you
want into the base of the flask, using pretty much anything. Pencil
works well, or an old nail.
Hope that helped.