It is quite difficult to burn stacks of office paper too. You have to get the oxygen in somehow, and as the main additive is clay, when the outer layer burns it leaves behind a fire resistant ash. Without enough wind to blow the ash away, it keeps oxygen from getting in.
There’s now a steel designed on the same principle - it’s used on a bridge near us. It produces a very close grained solid rust which makes oxygen penetration hard, something like the way aluminium forms an adherent oxide that prevents further oxidation.
tl;dr the plaster on the straw bales is an important part of the fire retardancy.
You can buy hemp clothes here. Hemp is grown in the EU which has a very low cannabinoid component but which produces high strength natural fibres. It costs more than cotton but is a lot more environmentally friendly.
