Jim Wilkins wrote:
"Terry Coombs" wrote in message
...

It's been a while since I molded and cast anything , so I decided
to get a little practice today . First melt was an old cast aluminum
decorative skeleton key . Wall hanging doodad , about 2 feet long
and 4 or 5 lbs of aluminum . Next was to mold up a disc that'll be
an end cap on a ball mill cylinder . That went well so I cut up a
couple of ingots snd fired the furnace back up . I'm going to have
to address the hydrogen porosity I'm seeing with some of my stock .
I've tried chlorine pool granules and that works well enough , but
the fumes are dangerous so I'm thinking about bubbling some CO2 in
the melt to degas . Anyway , the pour came out well too other than
the H2 microbubbles , and that piece is in the lathe now for
machining .
--
Snag
As soon as I get the rest of my sand up here
I'll be ready to start casting shaper parts ...

The clerk at MG Stevens told me the porosity meant my sand was too
damp, though he gave me a small bag of broken degassing pellets.
http://www.mgstevens.com/

I might take my patterns to a local art foundry:
http://www.granitestatefoundry.com/

So far I've built up machine parts that could be candidates for
casting by welding, annealing and forging/jacking out the shrinkage
distortions. Unlike casting all the metal doesn't have to be heated
red-hot at the same time, and it gives the tensile strength of steel
rather than aluminum.
-jsw

My understanding of hydrogen gassing in aluminum is that it's from the
propane flame and dirty feed stock - and those degassing pellets are
basically the same thing as pool chlorine IIRC . What you're talking about
is probably surface porosity , which can be caused by excess moisture in the
sand . Too hot a melt can contribute to that too . What I'm talking about is
throughout the entire casting . After looking at where I've already machined
this blank , all I've got this time is surface defects most likely caused by
being too hot and the rough sand I used for my first batches of greensand .
The rest of my crushed olivine and bentonite is coming home the day after
Christmas . Then I'll have the materials to make finer-grained molding sand
..
--
Snag
And as much else of my stuff as will fit in the SUV-
The Wife has some items I'm supposed to find room for too .