Of course you knead the clay while your claying so you'll have clean sides to work with. And pick out any big pieces of dirt.
This is what I do Once both sides dirty, I heat the bar in a microwave for 15 seconds to get it easy to mold, then cut it in half once or twice to expose the inner layers Then shape into a ball and finish off by squeezing it back into a wafer This can be done up to 15 times and claying of up to 25 vehicles Sure it may be easier to just buy more clay but there isn't a need to so quickly If the bar has gone all stiff, the microwave for 10 to 20 seconds brings it back to soft
Doesn't seem to, I have done in dozens of times and it is still like new in that it isn't falling to bits or leaving residue