My crucibles are silicon carbide, I've had them for years. Nothing wrong with graphite/clay, though. The size number designates the capacity of aluminum in lbs., so the # 8 will do 8 lbs (note...not kgs) of aluminum or 24 lbs of bronze.
24 kg (52 lbs) is a BIG crucible, what are you making?
I was making sculpture, the #8 would easily do a full size head & shoulders bust in one pour and the old furnace would do a melt from cold in about an hour. Lately, though, I have just been making small model engineering projects.
Right now I am melting Aluminum and building steam engines. I want to work my way up to steel but for that I need to make a new furnace the one I have now won't last long at those temp's. My clay crucible has crack in it but it seems to still be able to work. I remember in high school the crucible there was made of ceramic but they cost a lot. I just found out that the Silica flour that I bought can handle temp up t0 3116 f and I have fire clay that handle 2800 f the problems I been having with fire clay is cracking. I now know this is do to thermal shock. but the silica flour turns to glass once heated. This might fix my problem of trying to get high temp with out Thermal shock.