Burnout cycle of larger flasks

Jewelry Discussion
1

I would like to know if a larger flask will take a longer or shorter
burnout time in the kiln than a smaller flask?

2 
I would like to know if a larger flask will take a longer or
shorter burnout time in the kiln than a smaller flask?

Longer. Slower ramp speeds (change of temperature rate), and longer
dwell times at each step. for larger flasks. Investment is a pretty
good insulator, so temperature changes travel somewhat slowly from
the outside of the flask to it’s interior. While burning out, you
don’t want radical differences in temp from the inside to the
outside, or you risk stresses that can crack the investment. Thus
larger flasks need to change temperature more slowly (the ramp rate
on your controller if you have one, or just how you set the kiln if
not. And the larger flasks, taking longer to internally reach the
external temp, plus taking longer for oxygen to reach the oxidizing
wax and the combustion products to escape, need longer times at each
stage as well.

3

They take a little longer; it’s not so much the size as the time lag
to get the whole to a uniform temp. This time lag can also appear
when you have flasks invested with different investment powders, some
conduct heat faster than others (we’re not talking noticeably faster
here, but it is something to bare in mind none the less).

I’ve found in the past that if you’re in doubt giving the flask a
longer soak time doesn’t hurt things, so long as you don’t over heat
the flask (had a student take a gypsum bonded invested flask up to
1650’F once the smell of sulphur was very noticeable!), but holding
an extra 10-15mins at 1450’F before going down to the required
pouring temp most certainly won’t hurt things.

One thing to keep in mind investment casting is somewhat of a black
art, some things that work for me won’t work for you…

Cheers (and good luck), Thomas.
Janstrom Designs.

4

A larger flask always requires a longer burnout than a smaller flask.

Shannon

5

It takes longer, and there is probably a chart about this available
from the manufacturer of your investment.

M’lou