A plug for the tap hole of a metallurgical vessel includes a canister containing refractory mortar between a pair of end plates, one of which is movable toward the other to squeeze or compress the mortar through an aperture in a generally radially outward direction to seal against the tap hole wall. A plunger is provided and has axially relatively movable sleeves. One sleeve connects with the plug, while the other sleeve has a pusher head engageable against the movable end plate of the plug. The plug is connected to the plunger and inserted into the opening. The sleeves are axially displaced to cause the disks to relatively move axially toward one another, displacing the mortar into sealing engagement with the tap hole walls. Upon location and sealing of the tap hole by the plug, the plunger is unscrewed from the plug and withdrawn for later use.