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Clay or earth render is inexpensive and easily made and applied to a straw bale house's internal walls. Not only is it non toxic, but it looks great, has a virtually non-existent environmental impact when compared to other methods that use lime and cement.
This 8 min video will show you how to make and apply a clay slip, and make and apply the base coat; a straw/clay plaster. A sand/clay plaster layer is applied 8mm thick on top of that and finally the top coat has the desired colour oxide in it.

Thanks to Rosalba and Mark, Nick Dunn and Andrew Richardson from http://www.strawbalehouse.com.au and all the workshop participants!