Fire clay in detail photo. Fire clay is a normal mud, simple as that, but a mud with higher Alumina (AL) content. Has usually whiter-lighter color. Whitish to yellowish, pinkish, light brownish. It’s also cheap as mud. Refractory or pottery suppliers sell it. Even if it comes in dry powdered form in bags, fire clay is still very heavy (physical properties calculators for various refractory materials.) You only need one bag per dome if you buy the clay in dry powdered form. Commonly Alumina content of fire clay ranges between 24% – 34% Al and Silica from 50% to 60 percent