I've tried RIO, manganese dioxide, and copper carb on greenware. They all brush off pretty easily after a cone 05 bisque without something to fix them. The terra sigillata is a very fine slip, and it acts to fix the oxides so they DON'T brush off at bisque, so you can brush glaze over very easily. The copper carb was brushing off bisque very easily too, so I usually use the copper carb/terra sig mix on bisque as well as green. RIO and manganese dioxide however you can simply apply to bisque and rub it back with a sponge, it sticks well enough to get fun effects!
HMMMMmmm, wait, its been ages since I started using the sigs with stuff on greenware, so maybe it was something else I was doing that was giving me fits. I just know I started using the sig and things started working! It might have been the Mason stains that I was trying to use on the greenware that were the problem, I normally use RIO and manganese straight, but only on bisqued stuff, and the copper carb was NOT behaving as well on the bisque. Mason stain with the sig works wonderful on greenware though.
Sigh, ok, disregard that first part, I'm a tad burnt out trying to get ready for a show!