There must be something going on with the flask blow out described
in the posts.
I have invested and cast flasks from 2 1/2 inches to 6 inches for
thirty years. I have had only two blow outs that I can remember.
Something in the steps I take prevents blowouts or the casting
gremlins are busy elsewhere when I cast.
The frustrating thing about casting is that what works for one may
not work for others. Knowing this I risk telling what I do in
investing and casting my flasks. Some of the techniques will probably
leave casters thinking “NO WAY THAT WON’T WORK.” However I have had
success following these steps.
Maybe these steps will lead to clues as to why blow outs happen.
I try to follow the rule of allowing inches of investment above the
wax for every 1 inch of flask diameter. Note I said I try to follow
the rule. Some times on smaller flasks I violate this rule.
If the upper part of the wax forms a solid plane like the bottoms of
my large pottery I provide a much larger amount of investment above
I do not worry about the distance between the wax and the flask on
most of my waxes as long as the wax does not touch the flask. On the
larger pieces of pottery I try to provide at least of an inch between
wax and flask. The only investment holding the top of the investment
to the bottom of the investment is the ring between the flask and the
I do not use tap water but use only water bought from local water
Both the investment and water are at room temperature.
I use Ransom and Randolph investment. I store the investment in my
house. I have had investment sit for many months between usages.
Weight out 5 pounds of investment and measure the recommended amount
of water. I pour the recommended amount of water into the mixing
bowl. Pour the investment into the water. Mix the investment with an
electric mixer on high speed for 1 min 15 seconds.
Place the mixing bowl under a bell jar and pull a vacuum. The vacuum
is maintained for 1 minute after the investment pancakes.
Place the flasks to be invested on the rubber pad and pour the
investment through a kitchen strainer into the flasks. I find the
strainer will sometimes strain our improperly mixed investment.
Five 2 1/2 inch flasks can be invested at a time.
Maintain a vacuum for one minute after the first bubble appears in
If all goes well I will have enough time to invest a second set of
flasks with the remains of the first 5 pounds of investment. I try
to complete all the investing within a seven minute period.
Let the flasks sit for three hours.
I scrape the top of the investment with a curved trowel. This
gives a slight concave shape to the investment. I believe the vacuum
will be more uniformly distributed across the entire face of the
flask. The slight curve in the investment might, and I am just
guessing, give the investment a little more strength.
My oven holds 16-2 1/2 inch flasks on the first level. I will add
additional flasks if they are short. The oven will handle flasks up
to seven inches tall. The taller flasks should be placed in the
center and front of the oven. There is a danger of overheating the
heating coils if tall flasks are placed against the back and side
walls. If I cast very large flasks I place them in the center of the
I normally do not cast unless I have a full oven.
Place flasks in the burnout oven and increase the temperature from
room temperature to 250 degrees and hold for 2 hours. Increase the
temperature by 300 degrees per hour until the temperature of 1350 is
reached. If the oven is really packed the oven may not be able to
increase 300 degrees per hour.
I maintain the burnout temp of 1330 for 8 hours. I have found it
takes 8 hours to completely remove all the wax in the full oven. Then
cool the flask down to 860 degrees if silver is to be poured. The
flasks are allowed to sit for at least one hour before pouring the
The silver is melted in an electro melt furnace at around 1370
The flask is placed on the pad when the system is set to pull a
vacuum. It might be safer to place the flask on the pad then increase
the vacuum to the flask.
Pour the silver.
Place the hot flask immediately on to a wax covered pad. Throw wax
on the sprue and cover the flask with a larger flask then place a
solder pad on top of the cover flask. This if my way of preventing
fire scale on the silver casting. The process is better described in
Orchids “Tips from the jeweler’s bench.”
Allow the melt furnace to stand for around 5 minutes before a second
pour is made. This allows the upper ring of the graphite crucible to
heat up to pouring temperature. If the ring is not heated up again
before pouring the silver will chill as it flows over the ring.
I quench the smaller flasks in about 7 minutes after pouring and
around 15 to 20 minutes for the very large flasks.
Note: The gripping tongs normally used to hold the flasks do not
work well on the 4, 5 and 6 inch flasks. To solve this problem I
scrape some investment away form the top and bottom edge of the flask
so that I can grip the flask with vice clamp pliers. The flask is
removed from the oven by clamping the pliers on the non sprue end.
The flask is placed on a flat surface and the pliers are moved to the
sprue end. The pliers are left clamped to the flask as the silver is
poured. The flask and head of the pliers go under the surface of the
quench water when quenching.
Now that I have said all that the casting gremlins will probably
cause some blow outs when I cast four 4 inch flasks this coming
I will attempt to add some photos and drawings to illustrate the
above to my blog next week.