Graphical charts used to determine the thermal diffusion in materials in the shapes of slabs, cylinders, cubes, and spheres. They plot the Fourier number on the x-axis and a dimensionless temperature on the y-axis, and are an extension of the (p. 186) Gurney–Lurie charts. They are named after American chemical engineer Hoyt C. Hottel (1903–98) who was professor of chemical engineering at MIT.