Last week I made a brass casting of the figures '2' and '6'. Those are about 9cm (3.5") high, the patterns from plywood 10mm (3/8") thick (top right). I molded these patterns with a central sprue + gate and 4 risers at the tips of the digits. To smooth out the mold I used baby powder between the pattern and the mold, the same as I use to separate cope and drag. I melted about 120ml (1kg) of brass, filling a crucible similar to the left one. The risers filled quickly but the casting was very rough. I grinded them with a sanding disk on my angle grinder and then used files, a sanding machine and a Dremel to smooth them (top left). Then I did another attempt, using copper (old piping) and brass mixed to get a more orange appearence. But the second casting was also rough (bottom right). The first casting was actually too hot (1200 C) as the crucible when taken out of the furnace was belching green flames and zinc fumes, I had to use a respirator. This did not happen at the second casting (more copper, less zinc). Should I leave away the risers and only scratch air vents in the drag ? Sometimes that works enough. Photo 2 shows the roughness of the 6 compared to the pattern.
I would either say that your sand was too wet or that its clay content was low. I also personally don't think you need 2 risers per number. And a couple of vents in the sand should also help. You can make a vent wire from an old coat hanger, and that should work pretty well. Patrick
wet sand could definitely be involved, also overheating will be a surface finish issue, and 3/8 tall roughly 3/8 thick you dont need any riser unless you are VERY concerned about the finish on the rear of the part, at the worst it will dimple down, just a little, , many of my plaques get a little low spot in the center of the rear, who cares, that part is screwed to the wall V/r HT1
roughness is usually sand related but can also be influenced by horrible gating and metal temps. The usual issues are....too much moisture in the sand....sand grain not fine enough....not enough clay in sand...melt temp too high...mold not hard enough...there are more but these are the usual suspects.
