After no-fills (incomplete castings) porosity is the biggest casting
problem (not counting casting the wrong metal).
Porosity can begin with investing, burnout, melting, mold cavity
fill or in the cooling.
Discovering the cause of porosity is a matter of forensic sleuthing.
It begins with the shape, size and location of the pits. Here’s a
few from my exper -jagged pits. Most likely some particulate
inclusion, on the surface of the metal being melted, in the mold
cavity or n the crucible
-jagged or unsound pits filled with white powder. Usually this is
investmentthat has spalled or broken away from someplace in the mold
cavity. Often these begin as a small speck that grows and grows as
you file (and uncover more of the investment inclusion). Most often
these pits are accompanied by a complimentary positive or raised
roughened area in the surface of the casting. This is where the
-tearing. So called Hot Tears most often occur in thin areas between
thickerareas. This is the result of stresses that occur during the
cooling of the cast metal.
-round/spherical pits. Gas Porosity. Trapped gasses in the molten
metal massoften the result of contaminants or over heating the melt
or oxidation. If using old buttons to cast with, be sure to
completely clean off any pickle from the cleaning process. Another
cause of this is the overheating of the investment during burnout.
-Shrink Spot porosity: looks like "spongy areas of metal. Often
small spraysof tiny non-round pits that often show up during final
polish (of course). This is a result of cooling metal and less than
ideal spruing. Thicker areas which are the last to cool and fully
solidify are literally torn apart by theareas that cool first and
shrink, pulling metal from these liquid/semisolidthick areas. It’s
how the button is designed to work. It should be the thickest/most
massive area of the casting and will be the last thing to cool
andsolidify, providing a place to draw metal from as the casting
cools. This is “progressive solidification” which ideally confines
pits to the button.
-large or smaller pits on the surface surrounded by a bright ring or
halo. Usually rounded hemispheric depressions. The result of flux
which has gotten into the metal flow and occupied space on the
surface, hence the depression. The halo is from the flux preventing
oxides. Ideally, the metal slides out from under or above the flux
in the crucible: being heavier it centrifuges outahead of the
Hope that helps.
Please excuse any typos-- curse my clumsy digits…