I guess our rules are probably different (they usually are) but we can buy what is called "class A fire retardent timber". Now if you believe that I have a great bridge to sell you over in Sidney. It used to belong to my Grandma who only used it on Sundays.
I once demonstrated to a Fire Marshall that it burnt with the application of a cigarette lighter for a few minutes and then cntinued to smoulder he said "It's Fire Retardent, it says so." and wandered off.
I always belived that acrylic paint increased the fire resistance of timber without any additional chemicals but that
canvas and so on needed extra.
Everything I've learned tells me that what you had is Fire Retardant material. Fire Retardant material is not fire proof. If a flame is held to it, it should burn. But once the active flame is removed, then the fire will stop, you might get a little
smoke and a
bit of charring, but it will not flame up.
Now, moving on to Flame Retardant
Muslin. It is better to
purchase Non-Flame Retardant
Muslin, paint it, then Flame Retard it with chemicals you can buy from most theatrical supply places. When you paint over the retardant, two things can happen. It can affect the color of your paint, and if you're using a sufficient watered down version of paint, you may start to remove the retardant as well.
Paint helps a little
bit, but it's not a true retardant. Now, as far as use, whenever
pyro is in use around set pieces, then retardant should be used, proven many years ago by my undergrad professor when we were doing a version of Cyrano de Bergerac. We had flash pots go off in the scene and several borrowed pieces decided to catch fire. Next thing you see is the crew running on
stage with fire extinguishers. Because they were borrowed, the TD chose not to retard them, and it was proven to be the wrong choice.
But you learn a little
bit each year ya get older. That man is now the head of a department and continues to do well. Hope I helped a
bit.