Sand left in kiln after removal of bricks.
When I opened the kiln after my first firing, the bricks were...dark. Burgundy. Comparing this colour to the vibrant orange that I had seen before, I knew something was wrong.
First, the sand I am using changes colour. It must contain a lot of iron, or some sort of clay-ish rock, because it turns from brown to burgundy when fired.
Second, I had fired the first batch to 1100C, whereas for every firing prior to that one I had only fired to 1000C. I think this changed something in the clay-sand mixture- maybe the sand melted at the higher temperature.
When I made test bricks with 50% grog, they were much stronger than just with clay. I thought that sand would work the same way, but turns out that sand makes the clay a lot weaker. The bricks from the first firing are very brittle and crumbly. I have changed the proportion of sand in my mix, so that it's now not more than 25%. I can't give up sand as a mold release, though, so I'll have to work with a different sand if I want to get that orange finish that I like so much.
But, the second firing went much better. These bricks are stronger and more orange.
Second batch brick (top) and first batch brick (bottom). I can crumble the edges of the first batch brick.
Bricks stacked after being taken out of the kiln.