Another steam factor is the hydrostatic type preasure created, from steam, it raises the mold, just enough to allow metal to flow out. Many mistakenly believe that the metal raises the molds, and allows seepage. Stream does raise the mold first. I use mold glue, helps prevent this. On larger molds , where the casting is far away from the edge, I use the trowel to just make a knife like cut. Thus helps seal the mold. Of course I also use weights and clamps.
Pictured is a mold I made this morning. I show the mold glue, and as applied, and the trowl, how it cuts. I also show vents thar are on top, and bottom, which do not go in the cavity, and the vent rod from the edge of cavity, out.
Steam from green sand is like steam in a cylinder of a steam engine .It pushes, only in this case it pushes back on the molten iron and raises the mold to allow seapage.
I pour a lot of iron with short sprues, but the amount of iron standing a couple of inches taller is not much. Velocity with a taller mold, gives the iron the nessessary flow to fill some molds.
The plaque looks very nice. I suspect you had very hot iron.
Steam damage resembles ice that is partially melted. Sometimes you see ice like it along a creek, that is hallowed out, and looks like some one melted part of it with a torch. Some times fingers with very thin, hard sharp points appear.
When I lived in Indianapolis, we would make large molds like flywheels,10 hp IHC, etc , and long boiler grates. We would leave the molds open for a day or two, and let them dry out some.
I do not like putting vents on top of small protrusions. Some times the steam collects there . It also does something that makes a hard spot. Not sure why.Especially on a flywheel ring. I like to make the vents on the inside edge, or make slots on the center out, and riser on each spoke, for venting, and feeding.