The basis for molding sand is clean silica sand. The
type used to mix concrete is just fine. However it must
be sifted through the finest window screen you can
find because there are a lot of large pieces in it. The
finer (smaller grain size) your sand the smoother the
surface and better the detail of your castings.
Some sandblasting sand comes in very fine grade and
will work for the molding sand. The place I bought this
from didn't know what mesh size it was (the higher the
number the finer the sand is) he said it was simply
quot;mediumquot; grade. So I decided to gamble and buy this
bag (100 pounds for $12.50). When I got back home I
opened it only to discover that is was more course than
the sand I sifted through window screen! Waste if
money... This sand is not typical sand. It is actually
black in color and the package says; quot;contains slag and
coal.quot; Yes this sand is slag from an iron foundry. I paid
12 bucks for some ground up foundry slag!
In the top left corner is the sandblasting sand (which
will NOT be used). In the lower left corner is the sifted
silica sand. On the lower right corner is the quot;gravelquot;
sifted out of the silica sand! And on the top right is
some sifted fire clay.
The clay requires sifting just as much as the sand
because there are chunks in it as big as the gravel sifted
from the sand.
After all the sifting I weigh out a quantity of sand and
drop it into my mixing tray. In this case I happen to
have 40-something pounds worth all sifted.
I moistened the sand slightly then sprinkled a percentage
(the percentage depends on formula you use and type of
clay you have) of clay onto the sand and began mixing it in.
Once it's all mixed in, moisten the mix 'til it has about as
much water as a rung out rag or sponge. Then perform the
squeeze test (squeeze an handful into a cylinder and see if
the sand retains the shape and shows details of your hand).
Let the molding sand sit for an hour or so because the clay
does not immediately absorb the water. Afterward perform
the squeeze test again, the sand should feel dryer and it
should perform better.
If the sand holds a cylinder shape after you squeeze it
and not much sand sticks to your hand then it's good
stuff. If a lot sticks to your hand then it is probably a
little too wet. If it doesn't even hold it's shape then
there is probably not enough clay. Add more clay 1 or 2
percent at a time, then retest.
Here you can see that the sand forms a clean cylinder
when I squeeze it in my fist. It also breaks in half
cleanly. This is good sand!
After all that I have 48 pounds worth of new molding
sand! The small hand shovel (also shown in the above
photo) is an aluminum casting of mine and was done
with a technique called quot;coping downquot; detailed in the