I have a brick fireplace that we are installing a set of gas fired logs into. I have to replace most of the fire bricks in the box and am not sure of the way to do it. I am not very skilled in masonary, but am in just about all the other construction fields. I have the bricks, fire clay mortor, whats the mix, do I need sand, how much between each brick, whats the drying time?????? John S.