BCS A base block is a high temperature pedestal used to raise up a crucible to the heat zone of a furnace.
A base block is generally used in a gas fired foundry furnace to lift the crucible up so that the burner flame doesn't directly blast into the thin wall of a crucible. If the burner flame is allowed to directly strike the crucible it can cause errosion of the wall of the crucible thus shorting its life. The proper way to prevent this is to use the base block to raise up the crucible out of the burner zone.
Raising the crucible also allows it to be in the "heat zone" of the furnace. Although the burner flame enters the body of the furnace at the bottom the hottest zone is from the middle up to the top. It is in this region that the walls of the furnace are heated by the circulating gas most effectively. Having the sides of the crucible in this region promotes the best heating from the turbulent gas stream and by the heat radiation of the glowing furnace inner walls.
The base block should be tall enough to have the burner flame aligned with the top of the block. It's ok if the top of the block is higher than the burner inlet as well. What you don't want is to have the flame hitting the thinner sides of the crucible. It is also acceptable if the flame strikes the thicker bottom portion of the crucible as this part is not as suseptible to wear from the gas.