These castings are a spiral of which the pattern was made by coiling insulated 2.5mm2 ground wire to a 3" (7.5cm) disk. I used bronze / brass with high copper content. The fun was that after pouring and opening the mold the drag was intact and could almost be reused (last photo)... I made vent holes with a 6mm drill bit which was even partially filled with metal on the right side, but not on the left. And, yet, on both extreme edges the mold did not fill completely. I heated the metal to 1200 C, well above the melting point. What did I do wrong ?
Did the AL have to go through the coil or wash across? Maybe feed it from both sides and add some vents.
You could add some phosphorous copper shot to the bronze just before the pour to deoxidize the bronze and boost the fluidity.
UPDATE: I now made an extra canal to the other end and now the mold filled completely. But the phosphorus bronze is an idea as well, I tried it once by immersing a few grams of NaPO3 + SiO2 + Al powder (34:10:15 mass ratio) mixture wrapped in thin copper sheet into the metal. These chemicals generate phosphorus vapor and as it is 'under water' (under metal) it does not burn, but dissolves in the metal.
You can buy some phos copper brazing rods as a readily available source of phosphorous. Ideally the amount added gets entirely consumed by the dissolved oxygen and minimal traces are left in the bronze as it forms phosphorous pentoxide and gets skimmed off the surface. You then have minutes before the de-oxidizing wears off.
What would an addition of silver have on bronze? The most common brazing rods here(and that I have on hand...) seems to have 5% silver (and 5-6% P).
Here a try of the same casting in cast iron. Same mold as picture 1 in the OP (two castings at a time) with vent holes. One of the castings was incomplete, but the other one was smoother than the bronze one. See picture (left-right: bronze - pattern - cast iron) with an inch ruler. The pattern is a coiled 2.5mm ground insulated copper wire.
