fade out material

with this script the object material should slowly become transparent but after time spent disappeared suddenly and not gradually. How can I fix it?

#pragma strict

var materiale : Material;
var tempo : float = 5.0;
var minimo : float = 0.0;

function Start () {

materiale.color.a = 255;

}

function Update () {


if(tempo > minimo){
 
 tempo -=Time.deltaTime;
 
 materiale.color.a = tempo/(minimo*2);
}
}

resolved, I did not notice that the variable minimum was 0. Sorry and thanks all the same