Fullers Earth (aka Bentonite) is a good way to achieve a dusty look either on costumes or on set pieces / props. Cat Litter provides a larger-clumped equivalent. Both are made from absorbent clay but can become slippery in large quantities.
There are reports of problems with singers / actors (and audience) inhaling the dust (it’s a lung and eye irritant) so this effect should not be widespread, or used to create a dusty atmosphere – it’s only for localised small effects.
Flour, corn starch, sugar or other organic products should NOT be used on stage as they are highly combustible in dust-form.
Real feathers can bring on allergies in asthma sufferers. Synthetic or fake feathers are a suitable replacement but care should be taken as they are most likely flammable.
Mud effects on scenery can be created using products such as Flints Mud.
Rubber Crumb could be used, along with foam chips / offcuts dipped in paint then squeezed out. The foam will need to be treated with flame retardant.
Polystyrene / Styrofoam can be used in chunks, which can then be coated with Rosco Foamcoat (to provide a hard-wearing flame retardant surface)
More information coming soon!