10 x Vermiculite Firebricks 9" x 4.5" x 1". to Suit Villager stoves

Doew anyone know roughly how much each block weighs?

can these bricks be used to insulated a small crucible for melting silver?

1200 celsius is the max temp, how long do these bricks last at this temperature?

How much gap shall I leave between the boards to account for expansion? Shall I fill the gap with fireproof grout or leave the gap?