mat_model is the default material for models. All default material names are listed in the manual under engine objects->material.

In the template directory materials.c contains mat_model
ambient_blue = 200;
ambient_green = 200;
ambient_red = 200;
diffuse_blue = 255;
diffuse_green = 255;
diffuse_red = 255;
specular_blue = 0;
specular_green = 0;
specular_red = 0;
power = 0;
sun_albedo = 50;

There is also mat_metal but no clay or other defaults.

-Splashmaker