Fire Brick Question

[mrpbjnance]

The only place I can find that sells "Fire Brick" cannot tell me if they are insulating firebrick, or low duty firebrick. They don't have specs or anything.
Is there a way to tell by look, weight or feel that the bricks they have will be acceptable for use building the oven?

Thanks
Mike

[DrakeRemoray]

They should weigh 7 or 8 lbs. The insulating firebricks are lighter.

Here is a link to the firebrick I used, It is medium duty, but it is all I could find:
http://www.alsey.com/fireplace_brick.htm

Drake