Mark,
If you are referring to the furnace, it is on casters. Acer is correct, the block of material I am pouring into is a graphite ingot mold I use to consolidate sterling silver and gold when I am refining. The graphite ingot mold is sitting on top of a cast iron cone mold used to pour "buttons". The buttons are a mixture of a lot of slag (borax, glass, etc.) and a small amount of scrap metal. The metal sinks to the bottom of the conical mold and is easily broken off the remainder of the slag after cooling. The metal is then remelted and poured into "shot" to start the refining process.
As far as a surface to work on (rather than the concrete) dirt is good. Or, if you go to anyplace that sells pottery making equipment, they sell ceramic plates in various thicknesses and sizes. I have one 18 inches square and 3/4 inch thick that I use. Easy to move around.
As for silver castings, yes, ideally, you should use no more than 50% recycled silver (i.e. previously cast) and 50% new silver. The difference is that new silver has the correct alloy amounts and little or no dissolved oxygen. Unlike gold, sterling or coin silver absorbs a good deal of oxygen in the molten state, if exposed to the air during the melt, and a lot of the copper and silver get oxidized to silver oxide and copper oxide. Some of this leaves the melt as slag but some stays in solution and contaminates the metal. The addition of new metal brings the alloy amounts closer to ideal and also includes small amounts of anti oxidants. You can reuse old silver but the more reused silver in the casting the more probable you will get a lot of porosity. This is often from dissolved oxygen trying to get out of the metal as it solidifies. (Porosity is also the result of poor gate designs that don't allow the body of the casting to "draw" more liquid metal as the part solidifies). And, speaking from experience, when you go to all the trouble of making a pattern, and a mold, and a furnace, and the melt, and the pour, and the clean up, and the....etc., the last thing you want to see is that the casting you have invested so much time and effort in is no good because its full of blow holes and porosity. Faster, cheaper, easier to use new metal. The new metal, processed in a commercial refinery, is usually melted by induction (very fast) and the furnace has an inert atmosphere (argon) so the new metal is protected from the absorption of atmospheric oxygen.
Lots of tricks and art to castings like this...and obviously it was done for centuries without all the technological wizardry, but then a lot of the old timer castings were really junk or just out in out scrap when they got done.
