The first task is to determine a firing profile that will result in a brick meeting structural standards. 5 percent overnight absorption was chosen as the initial target. 5 percent absorption will exceed ASTM standards for freeze/thaw for bricks. Absorption tests were carried out by weighing the brick, then putting it under water overnight, then wiping off the surface and weighing it again. Absorption is the difference between the two numbers divided by the dry weight.
The dried bricks were subjected to the following standard profile:
2 minutes to 625 F
30 minutes at 625 F
4 minutes to 1832 F (this max temp varies)
30 minutes at 1832 F (this soak time varies)
The maximum temperature and the final soak time were varied while making pairs of bricks.
Here are the initial results:
The left scale is Absorption, the bottom scale is soak time in minutes
At 1868 F, the granules of glass were beginning to pop out of the brick. This temperature was considered to be too high. The initial firing profile to focus on is holding for 60 minutes at 1850 F.