Building a fire pit

I decided to build a fire pit today in order to fire the core sample. luckily I had some materials available that MIGHT survive the temperature I need to try and get the clay to a low bisque ( only just ceramic and not clay- glowing red/ orange for about a half hour - it will probably need about 24 hours of heating though aaargh!) ) So I dug the pit and laid internal brick and rubble walls, then covered with roof tiles ( already fired clay) to support the work. I wasn't happy that I had to use a pick to dig the hole as it was full of rubble, but I used this in the end. I then sealed with clay to stop smoke/ heat getting out.