So I'm currently underway building my rocket powered pizza/cob oven and I managed to buy 80kg of calcined flint clay.
I thought I was buying flint or fire clay that I was going to use for my cob dome, but it turns out I didn't know what "calcined" meant.
For those of you who were like me, it means it has been super heated and turned into pottery and therefore won't react with water.
So I guess it's effectively grog or crushed fire brick. It has the consistency of fine beach sand.
So now I'm thinking I will just use the clay I dug up from around my house, but should I use this "grog" as the sand in my cob.
Or should I do a mix of sand and grog.