In an attempt to push the envelope, we decided to try making reduced weight bricks with frogs. We cut a pattern and screwed it to the bottom of our mold:
It appeared to work well with our press mold. The bricks coming out of the mold were now about 1200 grams, but were the same overall thickness as previously, and looked like this:
We formed and fired a series, starting with the standard profile of 90 minutes to 1850 and holding for 30 minutes. The cold water absorptions of those bricks were as follows:
4.2%
4.8%
4.6%
4.5%
for an average of 4.5 percent. This was a little higher than the 4.1% average for the 1600 gram bricks, which was unexpected, but probably not statistically significant.
To push the envelope further, we began reducing the time at maximum temperature, so see how quickly pairs of these thinner bricks would fire.
At 20 minutes soak:
4.7%
4.7%
At 15 minutes soak:
5.6%
5.4%
At 10 minutes soak:
6.6%
6.4%
This average exceeds the minimum required to meet the Heavy Paving Brick spec., so we stopped here.
Taking the bricks to the University of Washington test lab for braking, two things were pointed out to me by Hanson Fong, the post-doc running the test machine:
1) The sharp corners of my relief were conducive to stress concentration, and would probably cause premature failure, and
2) The ASTM spec required us to break the bricks with the frog down, which was the weakest orientation.
We forged ahead, and got the following breaking strengths:
1388 pounds
1606 pounds
1555 pounds
On the last one we turned it over just for fun and broke it with the frog up, and got
1733 pounds. All of these were lower than required by the ASTM spec.
So that’s that, we failed with our attempt to further reduce the weight.
We are certain that weight reduction is possible, as is profile optimization for even better energy savings, but we need to move on to the energy calculations. Besides, optimizing the profile for a kiln that will never actually be used in production would be somewhat of an exercise in naval gazing.