Cleaning rusty casting flasks

I had a problem getting the investment cleaned out of my flasks, so
decided to let them soak in water to soften the investment.
Unfortunately, left them in the water too long and they accumulated a
lot of rust. It is just on the surface of the flasks, and they are
still unpitted and smooth. However, I would like to get rid of all
the rust, particularly that which is on the inside of the flasks. I
have some rust remover and have been wondering about using it, but
don’t want to runof risk of any residue of the rust remover
interfering with future casting. What is the best way to deal with
the rust on casting flasks. Alma

If it’s just surface rust staining on a smooth surface, try CLR
( ). It’s cheap and effective,
and rinses clean enough to be food safe.

Sanding is how I treat rust in my tools

Hi Alma

Unless the flasks have some significant corrosion I can’t see the
need clean off the rust. All of my flasks have shed their shiny new
stainless steel finish on their first experience of burnout. However
their superficial surface rust doesn’t affect their structural
integrity or have any impact of the investment process. I always
clean out all the investment from my flasks straight after quenching
and removing the casting. I start by cleaning out all the investment
in the quenching bucket with a stiff toothbrush or just my fingers,
rinse the flask under a tap and put on top of the burnout kiln to
dry. I usually have enough time between melts to do this. I have had
some of my flasks for 7 years and they look about as rusty as the
newer ones and they all work just fine.

May the casting goddess look kindly on your efforts


Alma- Bar Keepers Friend. It has oxalic acid in it that will remove
the rust. Wear gloves and rise well after.


Hi Alma,

If you’ve got the room in your kiln, just stick them in your next
burnout. (empty) Or do them one or two at a time.

The real oxide will flake off, and then just quench them, and
that’ll blast off anything loose. Dry, and they’re back to normal.


Thank you all for your recommendations on cleaning my rusty casting
flasks. It is good to know that surface rust is not a real problem
and that I can get them clean, and not have to invest in new ones.

Alma, I have a steel spatula, like a larger version of what you
could use to spread peanut butter. I use that after every quench to
scrape around the inside edge of the flask to remove any remaining
investment, rinse in the bucket and then a few more spat scrapes
around the inside to remove any loose rust or crud. Then I rinse and
let them dry on a towel. Occasionally whenI grab a dry one from the
towel it will feel a little gritty and I just rinse and wipe with my
fingers and it’s good to go.

Here is a little bitof useless I did happen to note
that flasks seem to last about 100 burnouts before they get so thin
that I want to replace them. Mark