Have been looking and reading for some time and now have a question
perhaps one of you may answer. What qualifies as “new” metal in
casting? Have been using offcut Sterling Sheet in 50/50 combo with
old buttons, sprues, etc. Am getting porosity particularly at point
of entry [more pronounced] as well as further into the piece [very
fine]. Is this an alloy problem [offcut sheet not “new”?], sprueing
problem [turbulence?], flask temp. problem or am I just not holding
my tongue properly.
For the best results, use new casting grain from a metal supplier:
this contains silicon which acts as a ‘sponge’ and removes oxygen
from the metal very efficiently. This can very successfully be
mixed 50/50 with old sprues and buttons. The offcuts of new sheet
do not have this property and while they will probably be OK on
their own, mixed 50/50, the chances of porosity are increased. The
only other thing that I would suggest is that you make sure that
your temperatures are as low as it is possible to cast at and this
should prevent porosity even using plain old sprues and buttons,
however, if you do this, you may have to ensure that the sprues on
your patterns are quite large.
Hope that this helps.