Problem solving

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So we’ve established that we can make a brick with absorption of less than 5 percent by soaking for 60 minutes at 1850 F.

The profile has been one minute to 1100 F, then holding at 1100 F for 30 minutes to assure organic burn-out, then four minutes to 1850 and hold for 60 minutes.

The absorption has been good, and the brick surfaces look good, but there are cracks on the long edges of the bricks.  The assumption is that going from 1100 to 1850 in the absurd time of four minutes is simply too much for the clay/glass mixture, so it cracks.

The first thing we try is slowing the profile down to two hours from 1100 to 1850.  The cracks are much smaller, but about half of the bricks we make still have cracks.  Here’s a picture of a crack:

The vertical dimension is 1 inch overall

Since we are trying to prove that we can make viable bricks quickly with 50 percent glass, we can’t have cracks.  And once a satisfactory kiln profile has been established, the next step will be to test the bricks in flexure, so we want no cracks, which, even if they are small, will weaken the bricks both because they are voids in the brick profile and because they act as stress concentrators.

Even slowing the kiln down to a three-hour ramp from 100 to 1850 does not completely eliminate the cracks.  The ceramic literature talks a lot about the two stages of quartz inversion, so we further modify our profile to account for the quartz inversions, slowing to 180 degrees F per hour for 45 degrees on either side of 573 F and 867 F, the two quartz inversion points.  That doesn’t solve the problem.

We don’t want to slow the kiln profile any more, so we think about the actual possible causes for the cracking.  Maybe it’s not simply from going too fast.

Two other possible causes come to mind.  First. no kiln is perfectly uniform in temperature.  And this kiln has very hot elements mounted in only the top and the bottom.  The thermocouple is mounted in the middle of the kiln.  So maybe, through both temperature stratification and radiation heat transfer, the long sides are heating and shrinking faster than the short sides and middle of the brick, and hence cracking.

The bricks are being fired standing on the long sides for maximum strength during firing.

Second, during firing, we believe that the bricks containing glass get somewhat softer than bricks containing grog.  As described in a previous page, we are depending on lowering the viscosity in the glass to encapsulate the particles of clay and give the final strength rather than the chemical evolution of the clay.  An expected result of that process is a softening of the whole mass during firing.  So it may be that during firing the soft brick, standing on its side, gets "torqued" enough for a little split to open up.

This test kiln was built with quartz support tubes on 2-inch centers, because that is the dimension of the support tubes in a roller hearth kiln I saw at a tile manufacturer.  We wanted to simulate a roller hearth furnace with this project.  The owner of the roller hearth has since pointed out to me that the rollers in the roller hearth are always turning, which helps with the stress on the piece by never giving it a chance to concentrate in one place, so this kiln, if anything, should place greater stress than expected in a manufacturing environment.

So we try firing with the brick laying flat and it appears to have worked!  The first two bricks made laid flat have no visible cracks and an average absorption of 4.6 percent.  We will now try to squeeze down on the profile to find the fastest possible profile.

At the Brick Research Center at Clemson University a few years ago I saw an experimental roller hearth furnace that consisted of multiple layers of mullite rollers.  Bricks were being fired in single layers very quickly through this furnace.  It appears that the future of brick manufacturing is to get away from stacked furnace cars and go to much faster single layer firing.  That would be perfect for incorporating glass as a raw material.

This may have been more than you wanted to know about solving this particular problem.  The intent was to show that the problems we’ve seen in incorporating high levels of recycled glass into brick manufacturing appear to be solvable.  In the visits we made to ceramic manufacturers during this project several manufacturers said, in effect, “if you can add glass to my clay mixture and lower the firing temperature and nothing else changes, then we’re interested.”

Well, other things do change.  That’s life.  But if you can save half the time and half the energy, maybe the price you pay is in being a little flexible and solving some intermediate problems.

Want to see what glass bricks look like?

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Finding Equivalent Absorptions

We have decided to use ASTM C-1272-05a, "Standard Specification for Heavy Vehicular Paving Brick" as our standard
for making an adequate brick.  This is a good standard for this project because paving bricks are generally smooth, without hollow cores or frogs, and we can only make smooth bricks.  The two physical requirements we will try to meet are Breaking Load and Cold Water Absorption.

Cold water absorption gives us the resistance to freeze/thaw and a sense of whether the brick is fully fired.  High porosity often correlates to low strength.  The standard for cold water absorption is an average of 6.0 percent for 5 bricks.  In our initial absorption tests we determined that we can get below 6 percent absorption by firing 50 percent glass bricks to 1850F.  We also determined that we can prevent visible cracks by firing to 1850 in ninety minutes, then holding at 1850 for 30 minutes.

So our standard time/temperature curve looks like this:

Vertical Scale is Degrees F, Horizontal Scale is hours/minutes

So we make ten bricks using the profile above.  The absorptions of the ten glass/clay bricks are:

Average absorption is 4.1%, with a remarkably narrow variance.  It was also notable that there were no rejections, that is, none of the bricks made using this profile had visible cracks.

Ultimately, this project is about determining how much energy is takes to make a glass/clay brick vs. a grog/clay brick, so now we have to find an equivalent absorption using the same clay and regular grog.  In a screening test, the gradation of the glass we are using looks like this:

After testing a variety of grogs, we determined that a 50/50 combination of Christy Grog, 12M and 20M, pretty closely replicates the glass gradation.  And when we make a brick using the same Redart fireclay and the grog mix, we can form a brick using the same 16.5 percent water we used with the glass, so the gradation looks good.  Our standard brick not using glass is therefore:

800 grams Redart Fireclay
400 grams Christy Grog 12M
400 grams Christy Grog 20M
264 grams water

Now we form, dry, and fire a brick using grog and the same firing profile.   The brick looks good, with no visible cracks.  It's interesting that in a small-scale kiln using standard clay raw materials, one can fire a brick this quickly.  The absorption of the grog brick at the same firing profile is 10.4%, and the brick feels pretty whimpy.

Also, the color of the grog brick is kind of a salmon.  The glass brick fired at the same temperature has developed a nice brick red:

The brick on the left contains 50% glass, the one on the right 50% grog.  Same firing profile.

I don't know why the red would develop so nicely with the glass content.  I've always thought that oxides like iron develop specific colors depending on the temperature and potentially other oxides present.  In this case the glass seems to have in some sense "wetted" the iron to bring out a rich red at low temperatures.

Now we keep firing clay/grog bricks at higher temperatures until we reach the 6% absorption figure required by the ASTM specification.  This took more firing than I expected.  Here's the data:

So we had to fire to 2100 degrees F to get the same absorption with the grog that we achieved at 1850F with glass!

Now we can do the strength testing then begin the energy analysis.

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